JP2545484B2 - Perforation measurement and interpretation of NMR characteristics of formations - Google Patents

Description

TECHNICAL FIELD The present invention relates to apparatus and techniques for making nuclear magnetic resonance (NMR) measurements in boreholes and boreholes, and to the magnetic field of the formation traversed by the boreholes. It relates to a method of determining a physical characteristic.

PRIOR ART Over the past decades, numerous attempts have been made to use the principles of nuclear magnetic resonance to log wells in oil exploration, but have not always yielded satisfactory results. It is recognized that any particle in the formation that has a magnetic spin, such as a nucleus, a proton or an electron, will tend to match the magnetic field applied to the formation. Such a magnetic field causes the Earth's magnetic field B with an intensity of about 0.5 gauss in the area of the Earth where perforations are usually made.
It may be naturally occurring, as in case _E. For any given particle in the formation,
It is affected by local magnetic fields associated with nearby magnetic particles, other paramagnetic materials, and layers of ions that typically match the pore walls of some types of formations, such as shale. receive. These localized magnetic fields tend to be non-uniform, while the Earth's magnetic field is relatively uniform.

Hydrocarbons generated in the hydrogen nuclei (protons) of water and in the pores of rocks produce NMR signals that are distinct from any signals induced in other rock constituents. The population of such nuclei with a net magnetization tends to match any applied magnetic field, such as B _E.

When a second magnetic field B ₁ across B _E is applied on the protons by the logging electromagnet, the protons are aligned with the vector sum of B _E and B ₁ after a sufficient polarization time t _pol has elapsed. To do. When the polarizing magnetic field B ₁ is then switched off, the protons around the B _E vector at a characteristic Larmor frequency (rotational speed) ω _L , which depends on the strength of the Earth's magnetic field B _E and the gyromagnetic constant of the particles. Will tend to precession. A hydrogen nucleus that precesses around a magnetic field B _E of 0.5 Gauss has a characteristic frequency of about 2 kHz. When a group of populations of hydrogen nuclei are precessed in phase, the combined magnetic field of all the protons produces a detectable oscillating voltage in the receiver coil. Since the magnetic moment of each proton causes inhomogeneity of the magnetic field, precessing protons tend to lose phase coherence over time, and the specific time constant in that case is the transverse relaxation time or spin- It is called the spin relaxation time T ₂ . Further, the non-uniformity of the magnetic field is also caused by other physical phenomena as described above, and therefore the observed phase shift relaxation time T ₂ ^* is usually shorter than T ₂ . Perforation magnetic resonance measurements of the type described above are commercially available as part of the NML ™ service of Schlumberger Technology Corporation of Houston, Texas, USA. This device can measure the Free Induction Decay of hydrogen nuclei in the formation fluid, and can obtain the parameters T ₁ and T ₂ ^* . It does not measure the lateral relaxation time T ₂ . The basic components, operation and interpretation of commercially available logging equipment used in NML services are described in RC Herrick,
SH Couturie, DL Best co-authored "An improved magnetic logging system and its application to geological evaluation (An Improved
Nuclear Magnetism Logging System and its A
"Pplication to Formation Evaluation)" at the 54th Annual Fall Technical Conference and Association of Petroleum Engineers (AIME, 23-26 September 1979, Las Vegas, NV).
(Dallas, Texas).

It is possible to apply other sequences of magnetic fields to the population of protons or protons in the formation to measure other properties. For example, when an alternating current pulse having a frequency f is passed through the transmitter coil, a static magnetic field is generated.
An oscillating polarization field B ₁ is generated perpendicular to B ₀ , and the population of protons that precess at the Larmor frequency equal to f tends to match B ₁ at a given angle.
At the end of this pulse, when B ₁ is removed, the aligned protons experience vertical torque and precess around the B ₀ vector. After a characteristic time called the longitudinal relaxation time or spin-lattice relaxation time T ₁ , the protons relax to thermal equilibrium, where a weighted proportion of the protons are aligned in the B ₀ direction. Various other sequences of applied magnetic fields can be used, such as TC Farrar and
ED Becker, "Pulse and Fourier Transform Nuclear Mag
netic Resonance) ", Academic Press Press, New York (1971), Chapter 2, pp. 18-33 can be used.

While it is possible to make accurate measurements of the NMR properties of rock samples in the laboratory, making equivalent measurements in boreholes or drillings would result in temperatures in the hundreds of degrees Fahrenheit and pressures in the thousands of psi and equipment. All are severely exacerbated by the harsh environmental conditions that must be contained within a cylindrical volume that is only a few inches in diameter.

One of the earliest NMR logging devices is shown in U.S. Pat. No. 3,289,072 (NA Schuster) issued Nov. 29, 1966. A strong electromagnet is used to expose a water or oil sample to a given magnetic field. The RF coil produces an oscillating second magnetic field that produces proton nuclear magnetic resonances in the sample and similar proton resonances in the adjacent formations. Schuster uses a multi-pole electromagnet mounted in a wall-engaging pad or a larger electromagnet mounted in a logging sonde to generate a static magnetic field B _0. Is proposed. Sc
Huster also proposes other forms of electromagnets and sensing RF coils, such as in US Pat. No. 3,083,335 issued Mar. 26, 1963, where the coils are two bar magnets. Is located in the gap between the two opposite poles of. In this case, the magnetic field lines of the coil intersect perpendicularly the magnetic field of the bar magnet, which is the optimum angle for inducing nuclear magnetic precession.

US Pat. No. 3,667,035 (CP Slichter), issued May 30, 1972, was located in the gap between two coaxially aligned bar magnets and the opposite poles of these magnets.
A similar configuration consisting of an RF coil is shown. In the present specification, the term "bar magnet" means any magnet having only one N pole and one S pole facing in opposite directions, which is a permanent magnet. Or an electromagnet. Slichter composition and Schuster
Both configurations use an electromagnet, which has the inconvenience of having to send large DC currents to the logging sonde over thousands of feet of long electrical cable.

U.S. Pat. No. 3,528,000 (HF Schwede), issued September 8, 1970, uses NMR in FIGS. 8 and 9.
1 illustrates one type of logging device, where a permanent magnet produces a fixed strength first magnetic field and an induction coil produces an oscillating magnetic field of varying frequency over a selected range. Since the first magnetic field is generated by two opposite poles (one N and the other S) located side by side, the magnetic field is not uniform, and the spatial gradient of the magnetic field is all over the formation. Is obviously not zero in terms of. Furthermore, the first and second magnetic fields intersect not only in the formation, but also in the borehole, so that the protons that make up the water or hydrocarbons in the borehole fluid contribute to the signal sensed by the RF coil, Obviously, if a true formation measurement is desired, it must be removed either electronically or by chemically treating the drilling fluid.

For example, US Pat. No. 3,593 issued Aug. 3, 1971.
As shown in No. 7,681 (WB Huckabay), other NMs in which a sensing coil is located in the gap between the magnets by coaxially aligning permanent bar magnets in the logging sonde
An R logging device has been proposed.

Another permanent magnet form is described in US Pat. No. 4,350,955 (JA Jackson) issued Sep. 21, 1982, in which case the two poles of two magnets are between the same poles. Two permanent bar magnets are coaxially aligned so that the RF sensing coil is located in the gap. Similarly, 1984 12
UK Patent Application No. 2,141,236-A filed and published on March 12
Shows a similar arrangement with coaxially arranged bar magnets with the sensing coil located in the gap between the magnets. This type of arrangement produces an annular region of uniform magnetic field in which nuclear magnetic resonance measurements can be made.
However, these devices can be adversely affected by signals from the drilling fluid in large or deviated drillings where the device tends to lean against one wall of the drilling hole. If the device is configured to generate an annular region far away from the device body, the magnetic field so formed will be fairly weak and the resulting signal will be significantly weak. Such an arrangement further requires that the sensing coil antenna be surrounded by a structure that does not block the oscillating electromagnetic waves of the measurement signal. For example, glass fibers and other non-metallic materials are commonly used, but such structurally weakened links reduce the structural integrity of the device and their usefulness under harsh drilling conditions. It should be markedly reduced.

NMR measurements of particles other than hydrogen nuclei with magnetic spin have also been proposed. U.S. Pat. No. 3,439,260 (GJ Benn et al.) Issued April 15, 1969 discloses, for example, a technique for measuring magnetic resonance of carbon-13 nuclei in a formation.

Other representative U.S. patents issued for NMR logging equipment and techniques include U.S. Pat.No. 3,042,855 (R.
JS Brown), US Pat. No. 3,508,438 (RP
Alger et al.), U.S. Pat. No. 3,483,465 (JH B
aker, Jr.), U.S. Pat.No. 3,505,438 (RP Alger
et al.), US Pat. No. 3,538,429 (JH Baker, J
r.) and U.S. Pat. No. 4,035,718 (RN Chandler).

Each of the conventional NMR logging devices as described above has practical problems. All of these are subject to the fundamental difficulty of making these kinds of delicate measurements under the typical temperature, pressure and physically challenging conditions of well logging in an oil well. I have to deal with it. In addition, the concentration of hydrogen nuclei in the borehole is higher than in any rock formation, so that the unwanted NMR signal generated in the borehole is significantly higher than the signal from the surrounding formation. There is. To alleviate these problems, the drilling fluid is treated with a paramagnetic material, such as magnetite, and the treated fluid is circulated through the drilling before the logging operation, resulting in It is known in the art to reduce the relaxation time of hydrogen nuclei in the pores so that they do not affect the NMR measurements. Pretreatment of such drilling fluids is expensive and time consuming. Such pretreatment may also introduce the same chemicals through the perforations into the adjacent permeable formation, thus distorting the measurement results.

The high power currents flowing through the logging device tend to unavoidably destroy various electronic components such as switches, especially under high temperature environmental conditions in the borehole, thus requiring a strong electromagnet. It is recognized that such an NMR logging device has a tendency to lack reliability. All conventional logging devices have a sonde or pad body, to enable detection of AC signals.
For example, it has been required to be composed of glass fibers or non-metallic materials such as synthetic rubber or Teflon.
These materials are significantly weaker than the alloy metals typically used in making other types of logging devices. This is one of the reasons for its relative unpopularity in the industry due to the inability to use strong metallic superlattice structures when manufacturing NMR logging devices.

Conventional NMR logging equipment is typically referred to as the "dead time" after interruption of the polarizing magnetic field pulse and before the transmitter coil has been sufficiently dampened to allow measurements to be taken. It takes 20-30 ms. During this dead time, considerable information on magnetic relaxation is inevitably lost and the S / N ratio is significantly degraded.

The commercially available NMR logging device is not capable of directly measuring the spin-spin relaxation time T ₂ .
Instead, existing commercial logging devices use free-fluid index (FFI) and observable phase shift relaxation time T ₂
^* Measure (also called free induction decay time constant).
For example, CH Neuman and RJS Brown, “Application of Nuclear Magnetic Logging for Geologic Evaluation (Applications
of Nuclear Mhnetism Logging to Formation Eva
luation) ", Journal of Petroleum Technology, December 1982, pages 2853-2860, and various other useful sources of information such as those described in Herrick et al., supra. It is possible to use the well logging interpretation technique.

Interpretation The basic purpose of making NMR measurements within a borehole is to obtain information on the formation components around the borehole. Such information is typically related to the magnetic relaxation times of hydrogen nuclei in water and hydrocarbons, but when it is also related to other physical parameters or other particles in the formation. There is also.

It is known that the population of hydrogen nuclei or protons (protons) relaxes in several modes after the application and removal of the polarizing magnetic field, as described in the above-mentioned document. As described in Herrick et al., The NML device applies a strong DC-polarizing magnetic field, B _PO, to align the proton spins approximately perpendicular to the Earth's magnetic field, B _E. During "normal mode" period of operation of the device, the polarizing magnetic field, is applied over almost five times the period T _1, to polarize the protons of the formation saturation. After blocking a large polarization current (power of the order of 1 KW), and after the coil has completely dumped (requiring about 25 ms), a measuring circuit is coupled to the coil and the earth's magnetic field B _E
Detects a signal induced in the coil by a population of protons in the formation that freely precesses around. This free induction decay signal is referred to as the "FID" signal, which has a frequency equal to the Larmor frequency ω _L in the earth's magnetic field of about 2 kHz, as shown in FIG. The basic NMR measurement is thus the induced voltage in a conductive coil located in the bore. The amplitude, frequency and phase of this voltage signal are correlated with secondary parameters to determine the magnetic properties of the measured population of protons,
That is, information on T ₁ , T ₂ , T ₂ ^* is obtained. These useful secondary parameters are linked to interpretation methods to obtain the desired information such as measured formation drainage, fluid porosity, viscosity, permeability, etc.

Referring to FIG. 20, it is known in the art to measure the decay envelope of a 2 kHz free induction decay signal and then extrapolate the envelope to the initial time t ₀ when the polarization pulse B _P is blocked. The voltage amplitude at the initial time t ₀ , when multiplied by a calibration constant that depends on the design parameters of the particular device, produces a free fluid index (FFI) called the free fluid porosity (Φ _f ). The FID decay curve is
It is known to be related to the observable phase shift time constant T ₂ ^* .

The NML device can also be used in "T ₁ quiescent mode" operation, in which multiple unsaturated polarization pulses are applied to the formation, while the logging device is used at its measured depth within the borehole. It stays stationary at. Referring to FIG. 21, the free induction decay signal is measured and, as previously described, the decay envelope is estimated to extrapolate the formation magnetization at the initial time t ₀ when the polarizing pulse was shut off,
And this is done sequentially for each period a, b, c, d of polarization. The larger the polarization period, the more stratum protons are aligned with the polarizing field B _P and the higher the net magnetization obtained. These polarization pulses should be of the unsaturated type. Because the maximum magnetization M
This is because the FID curve following (t ₀ ) and the saturation pulse is the same and redundant. Then, the extrapolated value M (t ₀ ) of the magnetization immediately after the magnetization for polarization is shut off is plotted against the polarization period to form a T ₁ buildup curve. The curve has a characteristic time constant equal to the longitudinal relaxation time T ₁ . In conventional interpretation methods, T ₁ is usually T ₁
The build-up curve has been estimated by fitting it to an exponential decay function using a commercially available "least squares fit" computer program.

Although only four points are shown in FIG. 21, it is known to make at most eight separate sets of FID measurements and obtain corresponding values of maximum magnetization M (t). . The motivation for extracting T ₁ is, for example, A. Timur
"Pulsed Nuclear Magnetic Resonance Study of Porosity, Moving Fluid and Permeability of Sandstone"
Studies of Porosity, Movable Fluid and Perme
ability of Sand Stones) ", Journal of
It is possible to use some known correlation between k and T ₁ to determine the fluid flow permeability k, as described in the article by Petroleum Technology, June 1969, pages 775-786. It is based on what is desired.

The S / N ratio of each measured T ₂ ^* decay signal is not very high, so the first 20-
It is understood that coupled with the problem that the proton precession during the 30 ms period cannot be measured, it can introduce considerable error in the estimate of the magnetization at t ₀ . . The t ₁ build-up curve obtained as a result of fitting the data may have large errors and the extracted parameter T ₁ tends to be unreliable.

For many of the reasons mentioned above, conventional logging equipment and interpretation methods are capable of characterizing formations with sufficient accuracy and reliability to be completely acceptable in the industry. It wasn't something.

Aim The present invention has been made in view of the above points, solves the above-mentioned drawbacks of the prior art, and determines the magnetic characteristics of the formation more accurately and more reliably than the conventional art. It is an object of the present invention to provide an improved device and method capable of doing the above.

Another object of the invention is to determine the nuclear magnetic relaxation time, free fluid porosity, permeability, and associated pore fluid properties of a formation traversed by a borehole. An apparatus and method are provided.

Yet another object of the present invention is to be able to accurately measure formation properties without the need to pretreat the drilling fluid with a magnetic material, and under reliable drilling conditions. An object of the present invention is to provide a perforation apparatus for magnetic resonance measurement, which can operate reproducibly and can be composed of a strong metallic substance.

Yet another object of the present invention is an NMR measurement that has a simple and robust configuration and that can be easily and conveniently tested, calibrated and used when logging perforations. To provide a magnet configuration for the.

It is still another object of the present invention to provide a magnetic resonance logging apparatus capable of directly measuring the longitudinal relaxation time T ₂ of a formation traversed by drilling.

Yet another object of the present invention is to provide an improved method for determining magnetic permeability and similar parameters from measured free induction decay signals of an NMR logging device.

Yet another object of the present invention is to provide an improved method for determining the magnetic relaxation time of a measured population of particles in the formation around a perforation.

Structure According to the present invention, there is provided a device for producing a static and substantially uniform magnetic field focused into the formation on one side of a logging device. By directing and forming a magnetic field coupled by a magnet having a predetermined configuration, a predetermined region is generated at a position distant from the magnet, and the spatial magnetic field gradient substantially disappears in that region. It ensures that the magnetic field is highly uniform over the region. In the preferred form, these magnets are mounted to a skid or logging pad so that the static magnetic field is directed through the face of the pad into the adjacent formation and is substantially uniform. An area of strong magnetic field is usually located within a volume of the formation behind the mudcake layer which lines the perforated wall. Therefore, it is possible to apply or "focus" a uniform magnetic field several hundred times stronger than the earth's magnetic field on a given volume of the formation.

In one aspect of the invention, the RF antenna is mounted on the outside of the metallic construct of the logging device, so that the device body is provided with a signal, especially It acts as a natural shield for strong resonant signals from the drilling fluid. In a preferred form, the antenna is configured to focus its signal radially outward from the pad surface into a predetermined volume of the formation with a uniform magnetic field, whereupon the perforation effect distorts the measured signal. It further reduces the occurrence. As a result of this unique feature of the present invention, it is possible, unlike the prior art, for the logging device to be constructed from a strong metal alloy, and the present measurement technique actually favors the shielding effect of a metal sonde. It is used to improve the S / N ratio of NMR measurements. It is not necessary to pretreat the drilling fluid with a paramagnetic chemistry, as the drilling effect is eliminated by the double polymerized configuration.

According to a preferred embodiment of the present invention, an elongated groove antenna is provided on the pad surface, which is parallel to the drilling axis and has a substantially uniform static magnetic field in the adjacent formation. And are parallel to the volume. By superimposing a uniform static magnetic field volume geometry with the pattern of the RF field from the antenna, it is possible to create nearly optimal resonance conditions. In the preferred embodiment, the static magnetic field is directed radially into the volume, while the RF field (magnetic field) is directed circumferentially, thus for a uniform static magnetic field within the examination volume. The vertical direction. The length of this groove antenna is preferably approximately equal to the length of the examination volume.

According to the present invention there is provided a method and apparatus for measuring magnetic resonance in the formation surrounding a perforation with fast pulses, in particular it is possible to directly measure the spin-spin relaxation time T _2. It is possible to determine the characteristics of the formation.

According to another aspect of the invention, the transmitting antenna is also used for receiving magnetic resonance signals and very quickly attenuates the ringing current generated in the antenna after shutting off the power. A special circuit is used for this. This special circuit, called a Q-switch, attenuates the polarizing antenna current about 1000 times faster than conventional devices and can inject multiple pulses in succession into the formation during a short period of time. Is.

By significantly increasing the number of measurement cycles per unit time, the present invention provides that the logging device (1) increases the S / N of the overall measurement data set, with a faster logging rate or continuous. (2) to reduce the NMR measurement time for nuclei to diffuse in the pores of rocks and to reduce the undesired magnetic effect due to such diffusion. It is possible.

According to another aspect of the invention, an additional device is provided for pre-polarizing a given formation volume before the main magnetic contour reaches near the formation. This pre-polarizing field is preferably much stronger than that of the main magnet form and acts to increase the population of protons aligned in the B ₀ vector direction, and thus the magnetic precession signal level. Is further increased.

According to yet another aspect of the invention, a small current is introduced in the vicinity of the measuring device to change the static magnetic field during part of the measuring cycle, spoiling the signals from these local areas. Make it invalid. In the preferred embodiment, this small current flows through a wire configured as a loop that covers the antenna aperture and is mounted parallel to the surface that engages the wall of the logging device. This arrangement acts to significantly reduce or eliminate the resonant signal generated by the drilling mud or mudcake immediately adjacent to the antenna surface, thus substantially reducing unwanted signals. The spatial extent and magnetic effects of these field (field) inhomogeneities can be carefully controlled by choosing the spacing, current, and other relevant dimensions of adjacent sectors of wire. In a different aspect of the invention, the wire or its equivalent can be used to generate a magnetic field gradient extending into the measured resonant volume, which is another useful magnetic measurement. It is possible to do.

In another aspect of the invention, for example, free induction decay signals, pulse sequence magnetisation curves, and other magnetic resonance measurements, etc., are used to determine values of formation perforated fluid properties such as permeability. A method is provided for interpreting various measurement data.

Examples Hereinafter, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Referring to the drawings, and particularly to FIG. 1, a borehole or bore 10 is shown adjacent to formations 11, 12 to be characterized. Within the perforation 10 is located a logging tool or logging device 13 which is connected via wire lines 8 to a device 7 at the surface of the earth. The logging device 13 preferably has a face 14 which is configured for intimate contact with the perforation wall with a minimum gap. The logging device 13 further has a retractable arm 15, and the arm 15 is
When actuated, the body of the logging device 13 is pressed against the perforated wall during the logging operation so that the one side 14 is pressed against the wall surface.

In the preferred embodiment shown in FIG. 1, the logging device 13 is shown as a single piece, but the logging device is obviously a separate piece, such as a cartridge, a sonde, or a skid. And the logging device can be combined with other logging devices as will be apparent to those skilled in the art. Similarly, wireline 8
Is a preferred form of physical support and communication link for the present invention, although other alternatives can be used and the present invention does not require a wireline, for example. Such remote system configurations can be used to incorporate within the drill stem.

Formations 11 and 12 consist of formation type, porosity,
It has individual properties such as permeability and petroleum content, which can be determined from measurements taken by the logging device. A mudcake layer 16 is typically deposited on the perforated walls of formations 11, 12 by depositing on the perforated walls by the natural permeation of perforating fluid filtrate into the formation. To be done.

In the preferred embodiment shown in FIG. 1, a logging device 13
Has a magnet array 17 and an antenna 18 positioned between the magnet array 17 and the wall engaging surface 14. Magnet array 17
Generates a static magnetic field B ₀ in the entire area surrounding the logging device 13. The antenna 18 has an oscillating magnetic field B ₁ at the selected time.
Which is focused into formation 12,
And it is superposed on the static magnetic field B ₀ in the part of the formation facing the surface 14. The inspection volume 9 (that is, the predetermined volume to be inspected) of the present logging device shown by the dotted line in FIG.
Located immediately before 14 is a vertically elongated region within which the magnetic field generated by the magnet array 17 is substantially uniform and its spatial gradient is approximately zero.

The pre-polarizing magnet 19 shown by the dotted line can be located directly above the array 17 in a modified embodiment of the invention, the modification of which will be described separately.

The logging device 13 magnetically tilts the nuclear spins of the particles in the formation 12 with a pulse of an oscillating magnetic field B ₁ and then within a stationary, uniform magnetic field B ₀ within the examination volume for a predetermined time period. The measurement is made by detecting the precession of the tilted particles at. As shown in FIG. 1, this inspection volume 9 does not overlap with the surface of the wall engaging surface 14 as in conventional inspection devices, and may also overlap the mud cake 16 on the perforated wall. Absent.

In the pulse echo type measurement, for example, Fa described above is used.
RF as detailed in the rrar and Becker reference.
A pulse of current is passed through the antenna 18 and the RF magnetic field
A pulse of B ₁ is generated, in which case the RF frequency is selected to resonate only the hydrogen nuclei exposed to a static magnetic field equal to the magnetic field B ₀ in the examination volume. The signal induced in the antenna 18 following the RF pulse represents a measure of nuclear magnetic precession and collapse in the examination volume of a drilling fluid, mud cake, or formation with different magnetic field strengths B ₀ . Undesired effects from the surrounding area are automatically eliminated.

In developing the preferred embodiment of the present invention, we sought to optimize the signal to noise (S / N) ratio of the measurement process. As will be appreciated by one of skill in the art, the following description is useful for understanding the key parameters to be considered in constructing the preferred embodiment of the present invention.

Referring to FIGS. 2 and 3, when considering the signal strength of nuclear magnetic resonance measurements performed by logging device 13 in adjacent formations 12, the interaction of antenna 18 with the magnetic moment of the formation is It is useful to treat it as part of a single 4-terminal network to which the reciprocity theorem can be applied. A network 20 having an input impedance Z ₀ comprises a lossless matching circuit 21 and an RF probe or antenna 22 shown simply as a series connection of a resistance and an inductance. When an oscillating current I _{1 of} frequency ω flows through the RF probe 22, it produces an oscillating magnetic field B ₁ in the formation, which includes the area within the test loop 23. The voltage induced in the test loop 23 as a result of applying the current I ₁ is given by:

V ₂ = Z ₂₁ I ₁ = −iωA · B ₁ (1) Note that Z ₂₁ is the cross impedance between the antenna current and the voltage induced in the test loop 23.

It is assumed that a current I ₂ is applied to this test loop and a voltage V ₁ represented by the following equation is induced at the terminal of the matching circuit 20.

V ₁ = Z ₁₂ I ₂ (2) Note that Z ₁₂ is the cross impedance between the test loop current and the voltage induced in the antenna 22. Applying the reciprocity theorem, the following equation is obtained because Z ₁₂ = Z ₂₁ .

Since the magnetic moment of the loop 23 is m = I ₂ A, the magnetization dM of the volume dV is dM = I ₂ AdV. The net signal from all formations can be derived from the above, which can be expressed as:

The integral is taken with respect to the volume satisfying the resonance condition B ₀ = ω / γ. In order to optimize the measured signal response of the present NMR device, it can be assumed that the NMR examination volume has an area A _R and a length L and that the integrand is constant within this volume. It is possible. In practice, such an assumption turned out to be a very good approximation. Substituting ω = γB ₀ and M = χB ₀ / μ ₀ in the above equation yields the following equation: It should be noted that γ is the rotational magnetic ratio, μ ₀ is the magnetic permeability of free space, and χ is the nuclear magnetic susceptibility of protons in the formation.

Here, it is assumed that it is desired to estimate the size of the area A _R to the actual magnet forms such as the form of the magnet array 17 shown in Figure 1. As will be described later, the magnet array 17 generates a static magnetic field B _0, and the saddle point at the center of the uniform magnetic field area is shown as the inspection volume 9 in FIG.
This field (magnetic field) strength can be approximated by Taylor expansion as shown in the following equation.

B ₀ (x, which is the deviation of the static magnetic field from the value at the center
y) −B ₀ (0,0) is not larger than half the magnitude of the RF magnetic field B ₁ , the resonance condition in pulsed NMR is satisfied. Therefore, the area of the resonance region is approximated by the following equation. It is possible to

It is assumed that the area has a square cross section. It should be understood that such and other approximations are made to more simply explain the principles underlying the present invention, and that other more rigorous derivations are possible. The above equation (5) for the measured NMR signal
Substituting for, we obtain

It was measured as is clear from the qualitative relationship of the above equation (8).
The NMR signal can be made better or worse by varying the various parameters given. However, it is not sufficient to increase the signal level, relative to the signal V _S, it is highly desirable to maintain the thermal noise level as low as possible. The root mean square thermal noise is expressed by the following equation.

V _N = (4 _K TZ ₀ Δf) ^1/2 (9) where Z ₀ is the input impedance of the matching circuit 20 (nominal 50Ω), κ is the Boltzmann constant, and Δf is the measurement bandwidth. , and the it is aligned with the bandwidth .gamma.B ₁ / 2.pi. population of resonance particles in the formation. Therefore, the ratio of the peak signal to the root mean square noise is obtained:

In equation (10), the power P ₁ supplied to the antenna is
It is substituted with respect to current using the relationship ^{_{P 1 = I 1 2 Z 0}} /2. Further, using the conventional definition of Curie's law susceptibility in NKS units, we have

Finally, the following equation for the signal-to-noise (S / N) ratio is obtained.

In the above equation (12), the first bracketed portion depends on environmental condition parameters such as the proton spin density N of the fluid, the porosity Φ, and the absolute temperature T.
The parameters in the other terms are, for example, Planck's constant (for 2Π) h and the nuclear spin I with a value of 1/2 for the proton. The second bracketed section contains design parameters for optimizing the performance of the logging device. For example, the high static magnetic field B ₀ is squared, and it is possible to significantly improve the S / N ratio.

Furthermore, the part related to the "Q" of the antenna (B ₁ / P ₁ ^1/2 )
It turns out that ^3/2 should be large. However, the S / N cannot be increased infinitely by increasing B ₁ / P ₁ ^1/2 . Since equation (10) and (12), the bandwidth is as it only effective if it is limited by .gamma.B ₁ / 2.pi., for example because the case for Q <B ₀ / B _1. On the other hand, when Q> B ₀ / B ₁ , the bandwidth is ΔF = γB ₀ / 2Π Q, and the equation (12) is as follows.

B ₁ / P for the set antenna geometry and measurement point
It is known that the amount of ₁ ^1/2 is directly proportional to Q ^1/2 . As can be seen from the above, further reduction of the antenna loss beyond the point of Q> B ₀ / B ₁ is
It does not increase N.

Referring to FIGS. 1 and 3, the magnet array 17 has three
It is composed of individual samarium-cobalt permanent magnets 24, 25, 26, which are mounted in a metal alloy body 27 parallel to each other. The magnets 24, 25, 26 extend longitudinally of the bore and are 12 inches long in the preferred embodiment. The poles of these magnets appear on two opposite ends of the slab magnet, rather than on the smallest face of the slab, which is commonly considered the end of a bar magnet, and each of the first In both FIG. And FIG. 3, it points to the left and right. Therefore, in the formation 12, the magnetic field B ₀ surrounding these magnets remains substantially constant along the longitudinal direction of the drilling axis.

The magnets 24, 25, 26 should be as strong as practical and should be able to withstand physical shock without disassembling. These samarium-cobalt magnets are preferably housed in a robust brass casing to prevent debris from flying off when the magnet is chipped or destroyed. These magnets are commercially available and typically have a residual magnetic flux density of 10,500 Gauss. As will be apparent to those skilled in the art, other magnets could be substituted for this samarium cobalt magnet, and the slab magnets could have different sizes than those shown in the preferred embodiment. is there.

It is desirable to use elongated or elongated slab magnets to generate a constant static magnetic field in formation 12 for a significant distance L along the z coordinate parallel to the drilling axis. L
A large value improves S / N and facilitates continuous logging or logging operations along the z coordinate. However, these magnets may be too long to cause the logging device 13 to be structurally awkward or to cause excessive spacing between the mating surface 14 and the perforated wall in the flush zone. Should not be.

The magnets 24 and 26 are mounted symmetrically on both sides of the body 27, and their north poles face the same direction. Magnet 25
Are located parallel to and between these two magnets, but with their north poles facing away from the magnets 24,26. The magnet 25 is relative to the magnets 24 and 26 and has an engaging surface 1.
It is slightly shifted from 4. As shown in FIG. 3, the north poles of the magnets 24, 26 are oriented toward the engagement surface 14 of the logging device, while the north pole of the magnet 25 is oriented away from the engagement surface 14. There is. However, this form can obviously be inverted, and still with similar results.

Referring to FIGS. 4 to 6, the magnet array is formed by directing the two north poles of the magnets 24 and 26 to the engagement surface 14 and forming the stratum 12 at a position beyond that.
17 appears like a magnetic pole N at large distances.
However, the polarity of magnet 25 is reversed so that the magnetic field is substantially altered at near and intermediate distances to formation 12. At intermediate distances, this preferred magnet array 17 configuration produces interesting and significant magnetic field anomalies within the unique defined volume just prior to the logging device surface 14. As shown in more detail in FIGS. 5-7, there is a well-defined volume in which the magnetic field is substantially constant, in which the spatial gradient of B ₀ substantially disappears. There is. This is the main resonance region for NMR measurement and corresponds to the inspection volume shown in FIG.

FIG. 8 shows vector arrows along the magnetic field lines within the cross-sectional area about 1 inch from the surface 14 of the logging device, the length of each vector arrow being proportional to the magnetic flux strength and The direction of the vector arrow corresponds to the magnetic field line. The magnetic field B ₀ is emitted radially or radially into the formation and is very uniform over this area and has a substantially constant magnetic field strength of about 232 Gauss.

Although the maximum magnetic field homogeneity volume is centered about a point about 4/5 inches away from the wall engaging surface 14, this substantially uniform magnetic field volume is relative to the magnets 24, 26. Magnet 2
Depending on the relative positioning, spacing and field strength of 5, it is possible to shift shorter or larger distances into the formation. Furthermore, in another embodiment of the present invention, in order to change the inspection depth depending on the situation,
It is possible to shift the magnet 25 in the main body 27 during the operation period of the present logging device. For example, if the formation is highly corrugated or there is an extremely thick mud cake, the operator can shift the position of the magnet 25 to the right by a predetermined distance as shown in FIG. Therefore, the examination volume 9 is shifted deeper into the formation, and it is possible to avoid collecting unwanted resonance signals from the mudcake. However,
In the illustrated preferred embodiment, the location of the resulting inspection volume 9 is already such as to avoid substantial overlap with the relatively thin mudcake layer of typical perforations. The magnets 24, 25 and 26 are mounted at fixed positions.

Referring again to FIGS. 5-7, it can be seen that the size of the examination volume 9 depends on the nature of the measurement taken and on the strength of the RF pulses transmitted by the antenna 18, as will be described below. Is.

On its front face 14, the metal body 27 has a semi-cylindrical cavity or slot 28, which faces the formation which is engaged by the engagement face 14. cavity
28 is adapted to receive an RF antenna 18, as described below. However, as already specified, the antenna
18 is located outside the metal body 27 of the logging device,
Moreover, it is automatically shielded from electrode-like communication with the area of the perforations present on the rear side of the body 27 or with other areas of the formation in the direction interrupted by the body 27. Therefore, the antenna 18 responds only to magnetic fields emanating from the front of the wall-engaging surface 14, eg, from the mud cake or muddy contacting the surface 14 in the vicinity of the antenna 18 or from within the formation 12. By using the relative geometric positioning of the antenna and the metal body 27, it is possible to minimize unwanted effects on the signal that would be difficult to remove by other means. In the preferred embodiment, the body 27 is composed of a metal alloy exterior body and is fixedly attached to an internal metal support and includes an antenna including a circuit, a magnet array 17, a hydraulic system of the arm 15, and the like.
It encloses most of the components of this logging system except for 18. Also, the main body 27, the overall configuration is sufficiently strong,
It can also consist of a combination of other materials, for example composite resins, steel, etc., as long as the magnetic field of the magnetic array 17 can penetrate and enter the adjacent formation 12.

The antenna 18 is used as an RF transmitter for generating a polarization magnetic field in the formation 12, and for a receiver for detecting a coherent magnetic signal generated from a proton that precesses immediately after the polarization magnetic field is terminated. Also used as an antenna. The antenna 18 may consist of one or more current carrying loops that are very efficient at generating magnetic fields in the formation. It preferably consists of a current loop generating an oscillating magnetic field B ₁ in the examination volume perpendicular to B ₀ . Other current loop orientations may be useful in other embodiments of the invention having a static magnetic field B ₀ different from that of the preferred magnet array 17.

The antenna 18 is attached to the body 27 and is fitted in the slot 28. Its efficiency can ideally be maximized if the current density in the slots 28 is made uniform. In practice, it is difficult to obtain optimum antenna efficiency due to various electromagnetic parasitic effects such as "skin effect", mutual induction effect between individual current loops, and electrical effect within individual conductors. Is. The preferred antenna 18 in accordance with the present invention comprises a single current loop in the form of a groove or slot, as shown in FIG.

Referring to FIG. 9, the antenna 18 is a highly conductive semi-cylindrical cavity or groove 29, end plates 30 and 31, and
It has an antenna element 32 which is parallel to and centrally located with respect to the semi-cylindrical groove 29 and which extends from one end plate 30 to the other end plate 31. Groove 2
9, the end plates 30, 31, and the antenna element 32 are all
It is preferably constructed from heavy gauge copper which has a very low electrical resistance. The antenna element 32 is insulated from the end plate 30 by a non-conductive bushing 33,
It is also connected to an electrical mount 34 opposite the end plate 30. The antenna element 32 is attached at its other end to the other end plate 31, so that current can freely pass through it between the groove 29 and the antenna element 32. The electrical mount 34 is shown in FIG. 9 connected to a circuit that includes an amplifier 35 and a detector 36. All connections at antenna 18 are soldered or silver brazed to ensure low resistive losses.

The RF antenna 18 can be driven by the amplifier 35 during a specified time period during which it is RF
Acts as an antenna transmitter. On the other hand, at other specified times, the antenna 18 is electrically connected to the detector 36, during which time it acts as an RF receiving antenna. In some operating modes, the antenna 18
May be required to act alternately as transmitters or receivers at very rapid frequencies.

The space between the groove 29 and the antenna element 32 is preferably filled with a non-conductive substance 37 having a high magnetic permeability. Ferrite material is preferably used to increase antenna sensitivity. Several tuning capacitors 38 are connected between the base of the antenna element 32 and the groove 29, their capacitance is selected to generate an LC circuit, the resonance frequency of which is the Larmor frequency ω _L = γB. It is ₀ .

Referring to FIG. 10, the relative dimensions of antenna 18 should be selected to maximize antenna efficiency. The slot element radius R should be as large as practicable and the R-r spacing is r
Should be maximal under conditions where is not too small to increase the antenna impedance excessively. It has been found that for a 12-inch groove antenna without ferrite filling, setting R = 0.75 inches and R = 0.2 inches produces optimum efficiency. R = 0.75
Ferrite-filled groove antennas with dimensions of inches and r = 0.3 inches have been found to be optimal.
The length L of this antenna can be the same as the length of the magnet array 17, which is 12 inches in the preferred embodiment, but the antenna 18 is preferably formed by the magnet array 17 in the formation. It is about the same length as the resonance region generated therein, which is about 4 to 8 inches long.

The magnetic field B ₁ generated by the antenna 18 is an oscillating magnetic field with a frequency f equal to the resonance frequency of hydrogen nuclei in the sensitive volume of the formation, whose static magnetic field is about 232 gauss. Therefore, f = (γB ₀ ) /2Π=1.0 MHz. B ₁
Has a direct effect on the bandwidth of precessing nuclei resonated by B ₁ according to the bandwidth formula:

The Δf = ( _Y B ₁ ) / 2n static magnetic field B ₀ is about 232 Gauss in the preferred embodiment, and the magnetic field strength B ₁ produced by the antenna at 1 inch is 3 Gauss and is therefore desired. The bandwidth of B ₀ occurring in the volume of is also 3 Gauss.

The strength of the B ₁ magnetic field in a sensitive volume affects the S / N ratio (Equation 12) according to B ₁ / P ₁ ^1/2 . Note that B ₁ is perpendicular to the static magnetic field B ₀ . FIG. 13 shows a plot of the magnitude of B ₁ / P ₁ ^1/2 in front of a 12 inch groove antenna, where P ₁ is the power applied to the 50 Ω impedance matching circuit of antenna 18, And B ₁ is the circumferentially polarized component of the radiated magnetic field. As can be seen, the magnetic field strength is quite constant at 1 inch over a longitudinal distance of about 8 inches.

Referring to FIG. 3, FIG. 10 and FIG. 11, the antenna 18
Is installed in the slot 28 and is coated with a wear resistant plate 39 composed of a non-conductive wear resistant material to protect the ferrite material 37 and the antenna element 32. It is preferred to provide a thin conductive wire 40 either below or within the wear resistant plate 39, which substantially fills the antenna aperture. The wires 40 are preferably arranged in a loop and the spacing S between the wire segments is about 1/2 inch. However, such dimensions can be modified if it is desired to spoil magnetic resonance in localized regions of greater or lesser thickness. During the measurement cycle, at selected intervals, a small DC current is passed through the wire 40, producing a locally non-uniform magnetic field B _2, which is approximately equal to the interval S between the segments of the wire 40. Extends to formation 12 over a distance. Within this region of local magnetic field inhomogeneity, during some part of the measurement cycle, the nuclear magnetic precession is disrupted and otherwise the resonance conditions generated by the interaction of B ₀ and B ₁ Is substantially changed. The wire 40 constitutes one form of means for locally producing an inhomogeneous magnetic field, although other embodiments may be used. For example, multiple wires, coils, or conductive grids can be used.

In the case of the preferred embodiment shown in FIGS. 1 to 8, it is particularly advantageous to spot the resonant magnetic field conditions in the mudcake region. Because a typical mud cake has a high concentration of hydrogen nuclei, which may be strongly resonant with the applied RF pulse from antenna 18. The mudcake that is adjacent to the antenna 18 may be subjected to a stronger RF magnetic field B ₁ than the inspection volume 9 in the formation 12 and thus strongly polarized by B ₁ . Further, as shown in FIGS. 6 and 7, there is a high slope point within 1/2 inch of surface 14 where the B ₀ field strength is equal to the resonance frequency of 232 Gauss.

By applying a non-uniform magnetic field B ₂ in the mudcake region and spoiling the resonance conditions therein, the unwanted NMR contributions from the mudcake are eliminated.

The wire 40 produces a magnetic field B ₂ with a high spatial gradient dB ₂ / dx, while it can be used to make magnetic field gradient NMR measurements in the formation 12. In this case, it is desirable to set the relevant dimensions of the wire 40 such that the region of measured NMR resonance overlaps the gradient field B ₂ .

The logging device 13 preferentially directs both the static magnetic field B ₀ and the oscillating magnetic field B ₁ , that is, “focusing”.
By doing so, a special inspection volume Q is formed to perform the measurement in a single direction. Inspection volume Q and logging device surface 14
By applying an additional localized magnetic field B ₂ in the region between and which eliminates the resonance in that region, the measurement is performed with the signal generated from within the mudcake region of the hole effectively removed. It is possible. Furthermore, since the measurement range (sensitivity distance) of the logging device 13 is quite limited, during the testing or calibration of the logging device, the logging device can be removed in order to eliminate magnetic effects in the environment. Can be enclosed within a calibration cell with reasonable dimensions. As a result, the effective use of the logging device 13 for oil well logging or logging operations is simplified.

Electronics The electronics for the present logging device 13 can be mounted on the body 27 or provided in a separate cartridge or sonde. The preferred circuit shown schematically in FIG. 12 operates in three modes: transmit mode, dump mode, and receive mode.

In transmit mode, the circuit 41 produces a large power of about 1 kW at a frequency on the order of 1 MHz for a short, precisely synchronized period, shutting off this current very quickly within about 10 microseconds, Any signal or noise in this power circuit must be isolated from coupling with other sensing circuits in the logging device 13.

Referring to FIG. 12, this transmitter circuit has a 20 MHz oscillator 42 and a synthesizer 43 for generating a sine wave signal having a frequency of 20 MHz + f, where f is the desired operating frequency of the logging device. Both oscillators have clock 44
Are linked by and are always kept in sync. The output of the oscillator 42 is supplied to the phase shifter 45, which is controlled by the timing generator 46. The phase shifter 45 is capable of producing 0 °, 90 °, 180 °, 270 ° phase shifts, as desired by the operator of the logging device, and is a Meiboom-Gil.
ill) It is possible to generate these phase shifts according to the conditions of various measurement techniques such as sequence. This phase-shifted signal passes through gate 47 and then 20 from synthesizer 43 in mixer 48.
Combined with MHz + f signal. This combined signal is fed through a low-pal filter 49 which allows only signals of frequency f to pass. Mixer 48 also has other frequency components at its output, but these higher frequency components are filtered out. The desired operating signal at frequency f can be turned on and off at any time by appropriate control of gate 47 by timing generator 46. The timing generator 46
A 40 MHz synthesizer maintains precise synchronization with the clock. Furthermore, the oscillators 42, 43 having a higher frequency can be kept in the operating state at all times without the risk of adversely affecting the detection circuit operating at the frequency f.

The f signal, which retains the shifted phase information, is passed to an amplitude modulator 50, which adjusts the amplitude to change the signal into the desired pulse shape. Both modulator 50 and gate 47, along with other components in circuit 41, provide timing generator 46.
, While the timing generator 46 is controlled by the computer 52. Referring to FIGS. 14-15, a typical pulse formed by modulator 50 and gate 47 has a first short time interval t (1) where the amplitude is Is increased by the modulator 50 and has a second short time interval t (2),
During that period the amplitude has not been increased and has a third short time interval t (3) during which the amplitude has been increased and the phase of the signal has been inverted. This third period of phase reversal may not be necessary if the shaped pulse has already been properly dumped by the Q-switch described below. The increased amplitude during period t (1) contributes to reducing the time it takes to operate RF antenna 18, while period t (3).
The inverted phase signal therein contributes to deactivating antenna 18 at the end of the pulse. Thus, the resulting pulse of the magnetic field B ₁ radiated into the formation 12 has a shape that is more like a square pulse.

The pulse signal from the amplitude modulator 50 is amplified by the power amplifier 35 capable of outputting about 1.2 kW without causing distortion in the signal shape. The signal is then fed to the amplifier if not activated during receive mode.
It passes through the extender 53 which prevents low level noise of about 10V or less leaked from the 35. The amplified pulse is then fed to the RF antenna 18 from which the magnetic field is
A pulse of B ₁ is emitted, causing nuclear spins in the formation to resonate. During the transmit pulse, the RF antenna 18 receives an oscillating magnetic signal of nuclear spin precession.

As mentioned above, the receiving system of the RF probe and matching circuit is designed to have a high Q to maximize the S / N ratio. In such high-Q systems, the antenna tends to ring undesirably for long periods of time and produces unwanted magnetic spin tilting behavior in the formation. If the antenna is allowed to ring out of control, the bandwidth of the transmitted magnetic field pulse may be substantially reduced. In order to minimize this antenna ringing problem, a Q switch
54 is connected in a line between extender 53 and antenna 18 as a suitable means of dampening the antenna ringing very quickly at the end of the transmitted pulse. The Q-switch 54 closes the circuit at the appropriate time, which changes the impedance of the RF probe system (including the RF antenna 18) so that the system is critically damped and the ringing energy is Dissipated quickly.

During the receive mode of operation, the Q-switch 54 is switched off, and signals from the precessing nuclei are received by the RF antenna 18 and passed through the duplexer 55 to the receiver amplifier 56. The duplexer 55
Protects the receiver amplifier 56 from high power pulses passing from the extender 53 to the RF antenna 18 during the transmit and damping modes. During receive mode,
The duplexer 55 effectively acts as a simple 50Ω cable connecting the antenna 18 to the receiver amplifier 56. The detected and amplified signal is then fed into a two-phase sensitive detector 57.
Which also receives a reference signal which controls the frequency sensitivity of the detector 57.

A reference signal having a frequency f is generated by mixing the respective outputs of 20 MHz and 20 MHz + f from the oscillator 42 and the synthesizer 43 in a second mixer 58, and then extracting a low frequency component f via a low pass filter 59. To be done. The output of low pass filter 59 has a sinusoidal form at frequency f and is used as a reference signal for both detector 57 and pulse generator 60. Pulse generator
60 is a timing generator that generates an appropriate pulse shape in response to a control and trigger signal from the computer 52.
Trigger 46.

The detector 57 responds to this reference signal and the formation NMR signal from the receiver amplifier 56 with a frequency f from the desired inspection volume.
, And supplies it to the digitizer 82. The digitized signal is then delivered to computer 52 for desired processing and / or formatting by an operator.

Q-Switch Referring to FIG. 16, a Q-switch 54 has two symmetrical circuits 61 and 62 shown in the upper and lower halves of the drawing. The purpose of the Q switch 54 is to parallel the capacitor 38 (see FIG. 9) in the antenna circuit in parallel with the capacitor 38 (see FIG. 9) according to the formula R _C = 0.5 (L / C) ^1/2 in order to critically attenuate the circuit. Is to introduce a resistive element into. For example, L = 1.1
× 10 ^-7 H and C = 2.3 × 10 ^-7 F, operating frequency is 1
Assuming MHz, the additional resistance needed to critically dampen antenna 18 is R _C =
It is 0.36Ω.

Q-switch 54 uses two field effect transistors (FETs) 63, 64 connected in series, which, when closed, provide a resistance of about 0.36Ω. Two fets
Are required because each one can provide resistance to high voltages of only one polarity due to the effective internal diode 67, and the ringing of the antennas 18 is a bipolar oscillating voltage. That's why. The resistance required to critically dump the antenna 18 is these FETs 63,
If it is larger than the resistance value of 64, it is possible to insert an additional resistance element in series.

When the FETs 63 and 64 are switched and opened, this circuit
It is an open circuit and does not affect the antenna circuit at all. Therefore, the Q switch 54 is
During the transmission mode period and the reception mode period (in the figure), it is in the open state, and in FIG. 14 and FIG.
It is closed during the damping mode shown as (3).

The rest of the circuit in FIG. 16 has means for gated the FETs 63, 64 into the on and off states while keeping the noise introduced into the antenna circuit to a minimum. Circuit 61,6
Since the two are basically the same, only one of them will be described below.

In the transmit mode, during the RF pulse, the 20nF capacitor 65 and the collector of the intermediate stage transistor 66 are charged via a line having a diode 67, a resistor 68 (51Ω) and a diode 69, the diode 67 being It is part of the FET 64 of the lower symmetrical circuit 62. The base of transistor 66 is connected to an optical coupler 70,
70 is controlled by a signal on line 71 from timing generator 46. This optical coupler 70 is preferably
Used with the present NMR logging device. Because it makes it possible to ensure the separation between the switching signal and the high voltage on the antenna.

The emitter of the intermediate-stage transistor 66 is connected to the gate 72 of the FET 63 via the diode 77. Gate 72
Is further connected to a 1k resistor 73 and a 8.2nF capacitor 74, which are connected to a source line 75, forming an RC coupled high pass filter between gate 72 and source line 75. are doing. This R-C filter is
It is ensured that the source-to-gate voltage never goes to an excessive level and gives a self turn-off time constant during the transmission of the RF pulse. Zener diode 76 in parallel with 20 nF capacitor 65 prevents optical coupler 70 from being damaged by excessive voltage.

Timing generator during damping mode
The signal from 46 is passed through line 71 to activate optical coupler 70 and charge the base of intermediate stage transistor 66. Transistor 66 is turned on, applying a voltage to gate 72, rendering FET 63 conductive and providing damping of the antenna circuit.

During operation of the logging device 13, the operator enters into the computer 52 information regarding the type of measurement sequence to be performed. The computer 52 then sets the sequence of electronic steps required for the device to carry out the measurement sequence. Computer 52
Controls the timing generator 46,
Sends control signals to the various components of circuit 41 to control the pulse height, length, frequency, relative phase of successive pulses, receive mode duration, frequency, and their timing for polarization.

The high-Q antenna 18 can be quickly dumped or attenuated after transmitting a 1 kW RF pulse, so that the logging device 13 resonates the target formation 12 with a large number of successive pulses in a short time. ) Is possible. The dead time between the transmitted pulse and the start of receive mode is about 25 microseconds, which is about 1000 times shorter than the dead time of conventional commercially available logging equipment. Using a peak power of 100 W, the pulse can have a duration of about 40 microseconds and can have at most 1000 pulses in a measurement cycle lasting 1 second.

In addition to the previously known NMR measurement as being performed by existing commercially available logging device described supra and literature, a car in order to measure the transverse relaxation time T ₂ - parcel (Carr- Purcell) type measurements can also be performed. This sequence is also commonly known as the 180 ° -90 ° sequence, where these angles are the tipsing experienced by the protons that precess during the measurement process. Represents the angle of. Other measurement sequences that can be performed by this device include, for example:
There is the Meiboom-Gill sequence, which is described in Ferrar and Becker, supra. In addition, CG McDonald and JS Leigh, J
r. Co-authored “A New Method for measuring longitudinal relaxation”
for Measuring Longitudinel Relaxation) ", Journal of Magnetic Resonance, Vol. 9, 3
Pages 58-362, 90 of the type described in the 1973 literature.
There is a ° -τ-90 ° sequence, which measures the longitudinal relaxation time T ₁ . Furthermore, in order to effectively test the magnetic properties of the formation, it is possible to devise additional types of perforated NMR measurements according to the present invention.

Pre-polarization When each measurement of the logging device 13 is used to continuously perform logging operation without stopping the logging device, the S / N is further improved beyond the above description. To do so, it is preferable to use another embodiment. Referring to FIG. 1, the pre-polarizing magnet 19 is the main magnet array.
Above the location of 17 is incorporated into the logging device 13 which is used by the magnet array 17 to make measurements.
It is to magnetically polarize the formation 11 before reaching near. The magnetic field of the polarizing magnet 19 is similar in orientation to that of the magnet array 17, but is preferably stronger and able to polarize a larger population of nuclei. It is something. As the logging device is moved up through the borehole, the magnet array 17 moves closer to the formation 11 and emits RF pulses. However, due to the pre-polarization, a larger number of nuclei are aligned with the magnetic field B ₀ of the magnet array 17 in a substantially uniform magnetic field volume,
And a corresponding larger signal is generated.

This pre-polarized magnet is preferably a magnet array having a form as shown in FIG. 17, and in the case of the illustrated example,
It has magnets 90, 91, 92, which are arranged with the same poles pointing in the same direction to maximize the magnetic field generated in the formation. Obviously, other combinations of magnets could produce similar magnetic fields, and a single magnet could be used instead of the magnet array as shown. Referring to FIGS. 18 and 19, the magnetic field B _P of the pre-polarizing magnet 19 adversely affects the S / N or bandwidth of the NMR measurement that depends only on the static magnetic field B ₀ existing at the time of measurement. Without, it can be substantially less uniform than the magnetic field of the magnet array 17.

It goes without saying that these preferred embodiments can be modified in various forms by those skilled in the art without departing from the scope of the present invention. For example, the pre-polarization magnet need not be configured in the form of a magnet as shown in FIG. 17, but it could be a single magnet or other array configuration. A slab magnet is used because it has a slim shape that tends to minimize demagnetization effects and is relatively easy to assemble. Since the magnets are large and have a very high energy density, the ease of handling and assembly of the magnets has significant advantages, and the simple described herein. The composition is
It was determined to be advantageous from various points of view. However, it is also possible to use other types of magnets according to the invention.

If static measurements are desired, differential sticking and other dynamic perforation effects can cause the logging device 13 to stick. Therefore, when performing such static measurements, it is desirable to provide means for disengaging the logging device engagement surface 14 from the perforation wall after completing the measurement cycle at a predetermined depth. For example, the two detaching pistons 93, 94 shown by dotted lines in FIG. 1 are hydraulically operated to retract the arm 15 to the retracted position, and then the logging device 13 is forcibly detached from the boring wall. It is possible.

Method of Interpretation Using the apparatus described above, eg porosity, pore size,
Nuclear magnetic resonance measurements in the formation around the perforations can be made to determine formation characteristics such as fluid flow permeability. Various conventionally known methods of interpreting NMR logging data can be used in the apparatus of the present invention.

The traditional way of interpreting measured NMR decay data is to
It centered around deriving rock formation properties such as ₁ and then linking them with properties of other formations of interest, such as permeability k. It was impossible to directly link the decay time constant T ₂ ^* observed by the conventional logging device to k. because,
This is because T ₂ ^* depends on various magnetic interferences between the perforation and the logging device, the environment that masks the true decay time constant of hydrogen nuclei in the pore fluid of the formation.

Referring to FIG. 20, the FID waveform or signal 101 represents the signal measured by a conventional logging device operated in either "continuous mode" or "quiescent mode", and the collapse envelope 102 is , Obtained from the peak value of this FID signal. There is no the signal is received during the period of "dead time" of approximately 20-30 milliseconds following the end of each polarization pulse at time t _0. FIG. 21 shows four such collapse envelopes 102a, b, c, d, and the FID signal 101
_{Are the} periods t _pol (a), t _pol (b), t _pol (c), t, respectively.
Measured after an unsaturated polarization pulse with _pol (d). Note that the envelope 102 in Figure 21 is
Shows a number of zig-zag points that represent noise in the signal, while FIG. 20 is a simplified schematic diagram that does not show the effects of noise so the FID envelope 102 appears smooth. Is shown.

In one conventional method of determining the longitudinal relaxation time T1, an exponential decay fit curve 103 (see FIG. 21) of the form:

A (t _dec ) = A ₀ exp (−t _dec / T ₂ ^* ) (A) where t _dec is the decay time, and A ₀ is the time T ₀ during which the polarization pulse is initially shaft off. Is the maximum amplitude of the free induction decay signal at and A (t _dec ) is the measured amplitude. This fitting method extracts the values of A ₀ and T ₂ ^* that produce the best fit, that is, the optimum fitting, and the resulting values of A ₀ are shown as FFI points in FIGS. 20 and 21. , And the extrapolated vertical axis intersection at time t _dec (0). In the traditional way,
A second step is required, in which case the extrapolated value is
As shown, the amplitude A (t _pol ) versus polarization time t _pol is plotted in a graph, which results in a T ₁ buildup curve 104. Finally, the T ₁ build-up curve 104
To another exponential decay display, typically having the form ΣA ₀ (1-e ^{−t / T} _1i ), to extract the value for T ₁ .

In contrast to the previously known conventional interpretation techniques described above, the preferred method of the present invention simultaneously provides the measured data of the NMR decay mode and that aspect that occurs from the change in the degree of initial polarization prior to the decay. Fit a model that integrally describes changes. This preferred method gives a simultaneous fit of the amplitude A measured by minimizing the amount of fit of the form

ΣΣ [A (t _pol , t _dec ) −A _REPR (t _pol , t _dec )]
² (B) Note that this display allows the amplitude A _REPR to depend on both t _dec and t _pol simultaneously. Referring to FIG. 23, a plurality of FID envelopes 102a, 102b, 102c, 102d, etc. are shown, each of which is measured as in FIGS. 20 and 21 above, but these The envelopes of A are not individually or separately fitted to the mathematical representation, and the process of obtaining an intermediate extrapolated _{value of} A (t ₀ ) for each envelope as is done in the prior art. It is not something that does. Thus, in the preferred method, the fitting is performed in a single step, as compared to the two-step process of the prior art.

The amplitude and T ₁ parameters in the model that give the best fit or best fit to the measured data are extracted and used directly to interpret the data. This best fit is preferably obtained by a least squares fitting computer program that performs an iterative numerical minimization of the squared fit amount (also known as the fit error). For example, a program known in the art as "ZXSSQ" is commercially available from IMSL Company of Houston, Tex., Which program can be used. This fit is achieved simultaneously in a single step (“global” approach) with respect to both the NMR decay response and the differential response due to different initial polarizations (t _pol ), so the extracted parameters A _i and T _1i are fairly stable and less affected by signal noise.

Three representative examples are described below, which show the preferred global method of the present invention. Amplitude and T ₁
The general formula for these representations, including the parameters of, is:

A (t _pol , t _dec ) = ΣA _i [i-exp (-t _pol / t _1i ] exp (-t _dec / T _2i ^* )
(C) where t _pol is the polarization period of the formation that tends to tip or tilt the magnetic spins of the measured population of particles before measuring the decay signal, and T _1i is the i-th species of the measured particle. That is, it is the longitudinal relaxation time of the component, and T _2i ^* is the observed relaxation time of the i-th species or component.

In particular, as a first preferred indication of the global approach of the present invention, the measured amplitude of FID signal 101 is represented by the sum of two exponential terms as:

In this two-index representation, the stratum has two kinds of protons (protons), and each type has individual decay components with maximum amplitude A ₀ , longitudinal relaxation time T ₁ and transverse relaxation time T ₂ ^*. It is based on the assumption that. The additional letters i = 1,2 indicate two components, one characterized by a short time constant for T ₁ , T ₂ ^* , and the other by a long time constant. Characterized From this condition, the observed decay time constant T ₂ ^* becomes dependent on the polarization period t _pol . Therefore, in this global method, T ₂ ^* is t
_{It will be appreciated that} the envelopes 102a, 102b, 102c, 102d, etc., which are functions of _pol , may not be parallel and have the same curvature in FIG. Furthermore, by fitting all collapse envelopes simultaneously using the parameters of A _i and T _1i without using an intermediate fitting step (as in the prior art), the method of the present invention fits. It is possible to obtain parameter values that are more stable and less affected by noise in the data.

In a second preferred representation according to the invention, the six parameters of equation (D) are _{assigned the} corresponding values of T _1i for each T _2i ^* via the common parameter T _inh defined by:
It is reduced to five by assuming that it is related to.

The parameter T _inh is a time constant that describes the additional decay mechanism resulting from the magnetic field inhomogeneities common to both species of the representation. Therefore, the expression of the formula (D) is reduced to the following formula.

_{A (t pol · t dec)} = exp (-t dec / T inh) [As {1-exp (-t pol / T 1S)} exp (-t dec / T 1S) + A L {1-exp (- t _pol / T _1L )} exp (−t _dec / T _1L )]
(F) Using these five parameter displays, A _s , A _L , T _1S , T
It is possible to obtain a relatively stable value of _{1 L} , which can give information about the measured permeability or other desired properties of the formation. Often, this five-parameter model fits the data as well as the first model with six parameters.

Other representations can be used for this global fitting method, which further reduces the number of parameters and in doing so increases the reliability of the estimation results when processing data with relatively low signal / noise. Improve. In particular, the third representation with only four fitting parameters is given by

A (n, t _dec ) = exp (−t _dec / T _inh ) A ₀ [exp {− (t _dec / T ₁ ) ^α } −exp {− ((t _dec + t _pol ) / T ₁ ) ^α }] (G) In order to obtain a complete description of NMR decay, four parameters, A, T ₁ , a, T _inh, are used. In this model, the longitudinal relaxation behavior is assumed to be the longitudinal relaxation behavior described by the "stretch exponential" form expressed by the following equation.

A (t _pol ) = A ₀ exp {−t _pol / T ₁ ) ^α (H) This form is T _2i ^* and T as in the second model.
_It occurs when nuclear magnetism is composed of an infinite number of individual exponential distributions from a large number of individual species of magnetic particles, provided that _1i and 1 are interrelated. This "stretched exponential" representation can, in some cases, extract relaxation time parameters with the same accuracy as other models, but is more sensitive to problems arising from low signal / noise in the data. It has the advantage of having great flexibility. According to the knowledge of the present inventors, this extension type exponent is 2 in spite of a smaller parameter term.
It describes the rock T ₁ data measured in the laboratory somewhat better than the index representation.

After extracting the values of relaxation times T ₁ , T ₂ and / or T ₂ ^* ,
It is usually desirable to use these values to determine other properties of the measured formation, such as liquid permeability k. Several methods for determining k are known in the art and therefore will not be described here. For example, Seevers' proposal uses a permeability estimation equation of the form k ∝ΦT ² , where T = (T _b · T ₁ ) / (T _b −T ₁ ), and
T _b is the longitudinal relaxation time of the bulk fluid when there is no pore surface effect. This is Seevers, DO, “A Nuclear Magnetic Met for determining the permeability of sandstone.
hod for Determining the Permeability of Sand
stones) ”, Society of Professionals
Well Log Analyst Transactions, 19
1966, described in the paper L. On the other hand, Timu
r, A "Pulsed Nuclear Magnetic Resonance Study of Sandstone Porosity, Movable Fluid, and Permeability
ance Studies of Porosity, Moveable Fluid, and P
ermeability of Sandstones) ”, Journal of Petroleum Technology, 1969, pp. 775-786, an estimation formula represented by k∝ΦΣA _i T ² is proposed.

However, according to the method of the present invention, the longitudinal relaxation time T ₁
The value of is preferably used in a permeability estimation formula in the form of k∝Φ ^m T _1i ⁿ , where the index m is about 4 and n is about 2. For example, when used in connection with the stretch exponential notation, the optimal estimation formula for the measured sample of formation rock is m = s ₂ = 4.61 and n = S ₁ = 2.09.
Also, the global method of the present invention, when used in connection with the preferred estimation equation defined herein, provides a significantly better determination of the measured rock permeability compared to conventional methods. .

Another aspect of the present invention, hereinafter referred to as the "integration method", is the formation in the case where it is desired to minimize the measurement time (e.g. to allow continuous logging operations). It relates to the case of determining the characteristics. In such a case, it is desirable to minimize the number of parameters to be extracted from the obtained data for the reason described below. In the preferred form, referring to FIG. 24, the amplitude values (representing the magnetization) are integrated with respect to the time axis t _pol to obtain a single integrated quantity with units of ΦT.

In the T ₁ buildup curve 104 or equivalent representation of the NMR decay response, the curve 104 is integrated to obtain the area above the T ₁ buildup curve, which is equal to the area below the corresponding decay curve. This integral quantity has a unit equivalent to ΦT ₁ ,
Used in conjunction with an appropriate estimation formula to obtain a measure of permeability. Preferably, this integral quantity is squared to obtain (ΦT ₁ ) ² and then multiplied by (Φ _t ) ^m−2 . It should be noted that Φ _t is the porosity measured using other known perforation logging devices, such as are commonly known as neutron porosity devices. As mentioned above, other estimation formulas can be used as well. This integrated amount is the low S /
A more stable display of N data. This is because it is less responsive to the exact shape of curve 104 and therefore less responsive to the errors introduced by mathematical fitting of NMR data with a significant level of noise.

The integration of curve 104 can be accomplished by a variety of methods apparent to those of ordinary skill in the art.
FIG. 24B shows an example of the integral obtained by simply adding different products of the form ΣA _i · Δt _pol . Similarly, the measured NMR decay signal is integrated over decay time,
It is possible to obtain integral quantities with information representing formation properties that are relatively immune to signal noise or other subtle features of NMR decay.

Although the amplitude axis is scaled by the logarithm of A and the time axis is scaled linearly, this method of interpretation is not intended to be limited to a particular display of the data. In another embodiment, the method can be implemented with a modified scale for amplitude and time. For example, the time axis can be scaled by the square root or other transformations.

The global fitting method described in relation to the FID signal obtained by the conventional logging device is the pulse type of the present invention.
It can be applied to the data obtained by the NMR apparatus. For example, the Car-Purcell (Carr-Pu) shown in FIG.
A pulsed spin-echo measurement, such as a rcell) sequence, produces a series of pulses whose successive peaks are T ₂
Define a relaxation curve. Having several such sequences measured with each sequence having a different degree of initial polarization (t _pol ), it is possible, according to the invention, to determine the longitudinal relaxation time T ₁ . Therefore, it is possible to extract the amplitude and T ₁ values using any one of the preferred representations of the global fitting method, and use these extracted values to determine other characteristics of the measured formation. It is possible to decide.

Similarly, if it is desired to minimize the measurement time, the integration method of the invention can also be used to better interpret the measurement signal obtained from the device of the invention. If the data set has a relatively low S / N, the permeability is determined in response to the area obtained by integrating on the basis of the echo signal and in the manner already described, as shown in FIG. 25. Is desirable.

As described above, the NMR logging apparatus of the present invention generates a static and uniform magnetic field in the inspection volume, and resonates the spin of protons in the inspection volume. ) A vibration pulse composed of an RF magnetic field is superimposed. By continuously applying the pulse B ₁ according to the Carr-Purcell sequence, the logging device 13,
Direct measurement of NMR decays with a T ₂ time constant, and without obtaining intermediate measurements of the T ₂ ^* decay signal as required with conventional logging equipment, Pore fluid properties can be measured directly.

Since there exists a method of using the information of T ₁ to determine other formation characteristics, an NMR decay signal with a characteristic T ₂ time constant is measured, and such a measured value is used by a transducer F _T. T ₁
It is the preferred method of the present invention to convert to a hypothetical value of and then use the value of T ₁ to determine the permeability k or equivalent property of the formation to be measured. According to the knowledge of the inventors, laboratory measurements of rock samples from different wells show that T ₁ = C ×, as illustrated in FIGS. 26 and 27.
(T ₂ ) holds and C is a constant for each well.
Thus, for example, using the Carr-Purcell sequence, determining the proportionality constant C, and then continuously logging the rest of the perforations to obtain a fast measurement of T ₂ ,
It is desirable to obtain static measurements of both T ₁ and T ₂ at a given depth in the well. The value of T ₂ is converted to the value of T ₁ by multiplying by the constant C. Finally, the value of T ₁ is used in conjunction with one of the preferred methods of the invention or another equivalent method to determine the permeability of the formation at each of the corresponding depths of the T ₂ measurement.

To the knowledge of the inventors, at low NMR frequencies, the values of T ₁ and T ₂ converge, so that the data of T ₂ measured at a sufficiently low measurement frequency should be considered as a virtual set of data.
It is desirable to determine the permeability by making it equal to the T ₁ data and applying the measured T ₂ data directly to the T ₁ estimation equation. Moreover, other T ₁ and T ₂ related quantities can be similarly converted. For example, referring to FIG. 25, the integrated area of the Carr-Purcell sequence 105 was calculated and used in place of the integrated area of the T ₁ buildup curve to determine the permeability, as described above. To do.

It should be noted that although the preferred methods of the present invention have been described for the most part in terms of determining magnetic permeability, these methods are traversed by perforations such as fluid viscosity, water saturation and porosity. Of course, it can likewise be used to determine other useful properties of the rock formations being laid.

Although specific embodiments of the present invention have been described above in detail, the present invention should not be limited to these specific examples, and various modifications can be made without departing from the technical scope of the present invention. Of course, it is possible.

[Brief description of drawings]

FIG. 1 shows an NM located in a borehole for making measurements around a formation constructed according to one embodiment of the present invention.
R is a schematic side view showing a logging device, FIG. 2 is a conceptual schematic diagram showing a measuring system of the logging device of FIG. 1, and FIG. 3 is a preferred embodiment of the present invention shown in FIG. FIG. 4 is an enlarged schematic cross-sectional plan view of the magnet array used, FIG. 4 is a schematic diagram showing the magnetic field lines represented by the arrows around the magnet array of FIG. 3 and 4 are schematic sectional views showing the magnetic field B ₀ isointensity lines of the magnet array shown in FIGS. 3 and 4, and FIG. 6 shows the inspection region of the preferred embodiment shown in FIG. 1 and FIGS. 232 as in Figure 5
A schematic diagram showing a Gaussian magnetic field B ₀ isointensity line, FIG. 7 is a schematic sectional view showing a magnetic field B ₀ isointensity line in the inspection region shown in FIG. 6, and FIG. 8 is an inspection of FIG. Schematic explanatory diagram of magnetic force lines showing the vector direction and magnitude of B ₀ in the region,
FIG. 10 is a schematic perspective view showing an antenna constructed according to an embodiment of the present invention, FIG. 10 is an enlarged schematic sectional side view of the antenna shown in FIG. 9, and FIG. 11 is an embodiment of the present invention. FIG. 12 is an enlarged schematic front view showing a wall engaging surface of the logging device shown in FIG. 1 partially broken to show an antenna and a meandering wire based on FIG. FIG. 13 is a block diagram showing a circuit which is preferably housed inside, and FIG. 13 shows the electric power P ₁ supplied to the preferred embodiment of the antenna of the present invention as shown in FIG.
FIG. 14 is a graph showing the ratio B ₁ divided by the square root of, FIG. 14 is a schematic diagram showing the signal input to the amplifier shown in FIGS. 9 and 12, and FIG. 15 is generated by the antenna of FIG. Schematic diagram showing the pulse shape of an exemplary oscillating magnetic field B ₁
FIG. 16 is a circuit diagram showing a Q switch constructed according to an embodiment of the present invention, and FIG. 17 is a pre-polarization unit having three slab magnets constructed according to another embodiment of the present invention. FIG. 18 is an enlarged schematic cross-sectional plan view showing the magnet, FIG. 18 is a schematic view showing magnetic field lines represented by short arrows in the inspection area in front of the pre-polarization magnet in FIG. 17, and FIG. 19 is in the inspection area in FIG. Fig. 20 is a schematic diagram showing isomagnetic field lines, Fig. 20 is a simplified schematic FID signal based on the conventional interpretation method and its collapse envelope, and Fig. 21 is a semilog based on the conventional interpretation method. 4 sets of FID signals plotted on a scale (not shown)
22 is an explanatory diagram showing four curves representing the collapse envelope of
The figure is extrapolated to the initial values and M based on the conventional interpretation method.
Multiple sets of T ₁ buildup curves in the (t _pol ) plane
FIG. 23 is an illustration showing a three-dimensional representation of the FID signal, FIG. 23 is an illustration showing eight sets of FID positive peak signals and their corresponding global fitting curves according to the preferred interpretation method of the present invention, FIGS. 24A and 24A. FIG. 24B is an explanatory view showing the T ₁ buildup curve as in FIG. 22 and showing the integration of the curve based on another preferred interpretation method of the present invention, and FIG. 25 is shown in FIGS. FIG. 26 is an illustration showing a series of spin-echo measurements that can be performed by the preferred apparatus of the present invention shown in the figure and showing the interpretation of the curve based on another preferred interpretation method of the present invention. Of actual rock samples from different formations traversed by an exemplary well
Laboratory measurements of T ₁ and T ₂ are shown and each data point is due to internal magnetic inhomogeneity in the rock sample T ₂
Fig. 27 shows a straight line representing the relative error in the measurements of Fig. 27, which is similar to Fig. 26 but of actual rock samples from different formations traversed by the second exemplary well. illustration showing the measured values in the laboratory of T ₁ and T _2, a. (Explanation of symbols) 10: Borehole (drilling) 11, 12: Formation 13: Logging device 14: Engagement surface 15: Arm 17: Magnet array 18: Antenna 19: Pre-polarization magnet 21: Matching circuit 22: RF probe ( Antenna) 23: Test loop 27: Main body 28: Cavity (slot) 29: Groove (trough) 30, 31: End plate 32: Antenna element 35: Amplifier 36: Detector

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masafumi Fukuhara 1-11-16 Chiyoda Sagamihara-shi, Kanagawa Yamagami Mansion 203 (72) Inventor Abdlerman Sezjiner Connecticut, United States 06804, Brookfield, Stony Hill Village 139 ( 72) Inventor Wen Xi Chu, United States, Illinois 61820, Cyan Payne, Stanford Drive 2510 (72) Inventor William E. Kenyon United States, Connecticut 06877, Ridgefield, Bayberry Hill Road 19 (72) Inventor Peter Eye. Day USA, California 92686, Yorba Linda, Grandview Avenue 5321 (72) Inventor Max Lipsicus USA, Connecticut 06896, Riding, Old Riding Road 132 (56) Reference JP-A-1-152348 (JP, A) JP 62-282290 (JP, A) US Patent 3483465 (US, A)

Claims (26)

(57) [Claims] 1. An apparatus for inspecting the characteristics of a formation traversed by perforation, comprising an elongated main body (13) adapted to move in the longitudinal direction in the perforation, and carried by the main body. And having a magnetic field direction that is substantially uniform within the examination volume (9) of the formation in a predetermined one-sided direction of the body and extends substantially orthogonal to the longitudinal axis of the body. A magnet means (17) arranged to generate a static magnetic field, and a magnetic precession of a particle group, which is carried in the main body, generates an oscillating magnetic field in the inspection volume and which is carried in the main body. Antenna means operable to detect a signal representing (1
8) and, wherein the antenna means is located offset from the longitudinal axis of the main body in the direction of the one side of the main body and is provided outside the main body, and the main body is An inspection apparatus, wherein one side of the main body is configured to be capable of contacting a wall of the perforation. 2. The inspection apparatus according to claim 1, wherein the main body is formed of a substantially non-magnetic metallic housing, and the antenna means is carried outside the housing. 3. The magnetic means according to claim 1 or 2, wherein the magnet means is configured to generate a static magnetic field having a substantially flat magnetic field amplitude gradient in the examination volume of the formation. Inspection device. 4. The magnet means according to any one of claims 1 to 3, wherein the magnet means is arranged such that the examination volume is elongated along an axis parallel to the perforations. And an inspection device which is spaced from the body by at least the expected thickness of the mudcake on the perforated wall. 5. The magnet means according to any one of claims 1 to 4, wherein the magnet means is substantially uniformly magnetized and extends substantially orthogonal to the longitudinal axis of the body. An inspection apparatus having a bar magnet (24) having an existing magnetization direction. 6. The first bar magnet (24) according to any one of claims 1 to 5, wherein the magnet means includes a magnetic pole directed to the one side of the main body, and the one side of the main body. An inspection device having a second bar magnet (26) having magnetic poles of the same polarity directed to. 7. The inspection apparatus according to claim 6, wherein the first and second bar magnets are substantially elongated in a direction parallel to the hole and are parallel to each other. 8. The first and second bar magnets according to claim 7, wherein the first and second bar magnets are separated from each other in the main body and are arranged substantially symmetrically on both sides of a longitudinal axis of the main body. An inspection device characterized by the above. 9. A third magnet according to any one of claims 6 to 8, wherein a third magnet is positioned between the first and second magnets, and the third magnet is directed to the one side of the main body. And an inspection device having magnetic poles of opposite polarities. 10. The method according to claim 1, wherein the pre-polarizing means (19) is oriented parallel to the static magnetic field of the magnet means and is displaced longitudinally from the magnet means. An inspection apparatus characterized by generating another static magnetic field positioned at a predetermined position. 11. The inspection apparatus according to claim 1, wherein the frequency of the oscillating magnetic field is selectively changed. 12. The magnetic field direction of the oscillating magnetic field in the examination volume is substantially orthogonal to the magnetic field direction of the static magnetic field of the magnet means according to any one of claims 1 to 11. An inspection device characterized by the above. 13. A conductive recess (29) according to any one of claims 1 to 12, wherein a conductive recess (29) is formed in said antenna means facing said perforation wall on said one side of said body. An inspection apparatus, wherein a conductive element (32) is provided in the recess. 14. The conductive rod according to claim 13, wherein the conductive recess is a substantially semi-cylindrical recess, and the conductive element is coaxially positioned in the semi-cylindrical recess. An inspection device characterized by having. 15. The conductive recess and the conductive element according to claim 13 or 14, wherein the conductive recess and the conductive element are substantially elongated, parallel to each other and parallel to a longitudinal axis of the body, and The inspection apparatus characterized in that the space between the conductive recess and the conductive element is substantially focused by a non-conductive substance (37) having a high magnetic permeability. 16. The antenna means according to any one of claims 1 to 15 for generating the oscillating magnetic field at a frequency close to the NMR precession motion frequency of the particle group in the examination volume of the formation. Circuit means (42, 43, 48, 50, 35) for driving the antenna means, and Q switching means (54) for rapidly switching the Q value of the antenna means from a very high value to a low value Inspection device characterized by being 17. Inspection according to claim 16, characterized in that the Q-switching means comprises a field-effect transistor (63 or 64) and an optical / electronic means (70) for switching the transistor on and off. apparatus. 18. A space according to any one of claims 1 to 17, which is equal to the distance between the body and the inspection volume of the formation, with one side of the body abutting the wall of the perforation. Means (40) for generating an additional local magnetic field are provided in the region near the body extending to the depth, thereby changing the nuclear magnetic resonance of the group of particles in the region. Inspection device. 19. The method of claim 18, wherein the means for generating the additional local magnetic field is located between the antenna means and the perforation wall and is approximately equal to the depth of the region between wire segments. An inspection apparatus comprising: a plurality of wire segments (40) having intervals (S); and means for generating an electric current in the wire segments. 20. Inspection device according to any one of claims 1 to 19, characterized in that said body is moved in the perforation by means comprising a wireline cable (8). 21. Magnetic inspection of a group of particles in a formation traversed by a bore using an inspection device having an elongated body (13) adapted to move in the bore along a longitudinal axis. In a method of inspecting properties, a static magnetic field is generated that is directed radially in a test volume (9) of the formation in one direction of the body and has a substantially uniform magnetic field line, and vibrates the test volume. Irradiating with a magnetic field, a method of detecting an NMR signal at the Larmor frequency of the particle group under the influence of the substantially uniform static magnetic field in the examination volume, the irradiation and detection comprising: A method characterized by using an antenna means (18) that is positioned offset from the longitudinal axis of the body to one side with one side in contact with the perforated wall. 22. The method of claim 21, wherein the static magnetic field is generated in the examination volume with a substantially flat magnetic field amplitude gradient. 23. The method according to claim 21, wherein the magnetic field direction of the oscillating magnetic field is substantially perpendicular to the magnetic field direction of the static magnetic field. 24. The Q value of the antenna means according to any one of claims 21 to 23, in order to drive the antenna means to generate the oscillating magnetic field and to detect the NMR signal. A method characterized by rapidly switching from a very high value to a low value. 25. The method according to any one of claims 21 to 24, wherein an additional local magnetic field is generated in the region between the body and the examination volume, whereby particle groups within the region are generated. Changing the nuclear magnetic resonance of the. 26. A body according to any one of claims 21 to 25, wherein the body is moved longitudinally in the bore and is oriented parallel to the static magnetic field, but in the direction of movement of the body. A prepolarizing the examination volume by displacing the static magnetic field in the longitudinal direction to generate another static magnetic field.