RU2614853C2 - Method of inductive logging from cased wells and device for its implementation - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: low-frequency electromagnetic field is excited in the environment with the help of harmonic currents in the system of two coaxial transmitter coils with opposing moments of different sizes conveyed within the well studied, and host rock electrical resistance value is determined. Location of the system of two transmitting coils and their moments are chosen so that the measured current -antiphase squaring of the magnetic induction (JmBz) axial component, created by the current in the transmitting coils, placed in a conductive casing string in non-conductive medium, was compensated (close to zero). If the host medium is conductive (rocks), the JmBz component of the magnetic induction will be uncompensated (different from zero) by the induction field in the conducting medium. The magnitude and frequency of the JmBz component behavior determine the value of the host rock electrical resistance.

EFFECT: improved accuracy of medium resistance determination in the cased well annulus during geophysical studies.

2 cl, 4 dwg

Description

The proposed method and device relate to the field of geophysical research on alternating current excited by an inductive method in a cased field exploration well, and are designed to determine the electrical resistivity of the surrounding rocks in the annulus.

The field of predominant application is the study of the geoelectric properties of the formations of the host rocks behind the casing of the well, productive for hydrocarbons.

A known method of electrical logging proposed by Alpin L.M. in 1939 [1] and developed in the works of A. Kaufman. [2, 3], and the INTECH-NEC equipment implementing this method (NLP GERS OJSC), Tver, EKOS-31-7M (NPPTT Geofizika LLC, Pyatigorsk), SCREEN (Tyumenpromgeofizika PGO CJSC) [ 4] and CHFR (Schlumberger) [5], in which the resistance of the host rocks behind the casing of the well is determined by measuring the voltage drop from the electric current flowing through the iron pipe of the well using measuring electrodes in direct contact with the inner surface of the pipe.

The electrical logging method has significant disadvantages.

The electric logging method [1, 7] uses a probe consisting of current and measuring electrodes. During the study, the device through the borehole supply electrode creates a current in the casing string that propagates through the string and closes to the surface electrode of the opposite sign. Since the typical rock resistance is approximately 10 ⁸ times greater than the resistance of the steel casing, the main current component (more than 99.99%) flows through the metal string, while a small part (less than 0.01%) flows through the rocks - the step potential of this particular current component is measured by several (from 3 to 4) receiving electrodes in contact with the casing. The measured potential difference is an extremely small value and is in the range of the first nanovolts and their fractions (10 ^-9 V) [7].

The widespread use of this technology is limited: firstly, by the requirement of good contact between the supply and receiving electrodes and the column, which is very difficult to achieve in the wells of the old foundation, when there are significant violations of the integrity of the column, especially in the perforation interval; secondly, the possibility of conducting only point-by-point measurements with fixing the block of measuring sensors in the wellbore up to 15 minutes at each point, which significantly increases the research time. This does not take into account the resistance values of the cementation zone and a number of transition zones on the path of the current flowing from the supply electrodes in the well to the electrode located on the surface of the earth.

In the method [8], which relates to ground based methods of inductive electrical prospecting, the medium under investigation is excited by an alternating magnetic field created by a current flowing in coaxial horizontal non-earthed loops connected in opposite directions. The geometric dimensions and the number of turns of the loops are chosen so that the vertical component of the magnetic induction at the point of their common center at the lower frequency is compensated (equal to zero). Measuring the values of the real and imaginary components of the vertical component of magnetic induction at a number of higher frequencies, the resistivity and parameters of the vertical section are determined by their frequency characteristics.

The main advantage of the method [8] is the determination of the resistivity and parameters of the vertical section according to the results of measurements performed under the reduced (compensated) effect of induction currents in a horizontal near-surface well-conducting layer located directly near the measurement point, which significantly affects the quality of the obtained measurement results .

The closest technical solution is the method of induction logging [6], taken by us as a prototype method. In the prototype method [6], the resistance of the host rocks behind the casing of the well is determined by subtracting two measurements of the axial component of the magnetic induction created by a harmonic current of different frequencies in alternatively connected two coaxial generator inductance coils located above and below the measuring coils at several frequencies ranging from 0.001 Hz and 20 Hz (p. 2).

In the prototype method, each of the two measurements of the axial component of the magnetic induction created by the harmonic current in two vertical generator coils connected in series, contains three terms decreasing in order of magnitude: the first is the field created by the current of the generator coil in a homogeneous medium (primary magnetic current field) the second is the field from the conductive casing and the third is the field from the conductive host rocks. The contribution of the last term is much smaller than that of the conductive casing because of the high contrast of their resistivities and significantly less than the first term - the primary magnetic field created by the current of the generator coil.

A significant disadvantage of the prototype method is the small value of the useful (abnormal) part of the magnetic field contained in each of two consecutive measurements of the field of current from the generator coils and the magnitude of their difference. The determination of the resistivity of rocks by this method will lead to large errors.

The novelty of the proposed method is seen in the fact that measurements of the magnetic field generated by the currents of an external source are carried out under conditions when the influence of the conductive casing string is compensated (close to zero).

The purpose of the proposed technical solution is to increase the accuracy of determining the resistivity of the enclosing rocks in the annulus of cased holes by compensating for the effect of the conductive casing string.

This goal is achieved by the fact that in the induction logging method, an electromagnetic field is excited in the environment using an alternating harmonic current in a system of two coaxial generator coils with opposing moments of different sizes moving along the well under study, and the antiphase current value of the axial component of magnetic induction at a number of frequencies measured using a measuring coil located at different distances from the generator coils, determine the value of the specific electron Cesky resistance of enclosing rocks.

In FIG. 1 shows a structural diagram of a device with which the proposed method is implemented.

The device comprises a generator device 1, a current sensor 2, two emitters - a generator coil 3 with a moment M _Z1 and a generator coil 4 with a moment M _Z2 , placed coaxially and connected in opposite series to the generator device, a magnetic induction measuring coil 5, a signal amplifier 6, analog-to-digital phase-sensitive meter 7 and a recording device 8. Moments of coils 3 M _Z1 and 4 M _{Z2 are} opposite and directed parallel to the axis of the well. The moment of the measuring coil of magnetic induction 5 is directed along the axis of the well.

The proposed method is implemented as follows. An electromagnetic field in the surrounding space is created by a harmonic current with a force J and frequency ω flowing in the generator coils 3 and 4 with counterpropagating moments parallel to the axis of the well Z, using the generator device 1. The output voltage from the measuring coil 5 is fed through an signal amplifier 6 to an analog a digital converter 7, in which, using the signal of the current sensor 2, the amplitude and phase of the axial component of the magnetic induction are determined and then to the device 8, which records the magnitude of the antiphase current the axial component of the magnetic induction JmBz.

The distances L ₁ and L ₂ from the generator coils 3 and 4 to the measuring coil 5 and the values of their counter moments M _Z1 and M _{Z2 are} selected so that the measured value of the antiphase current of the axial component of the magnetic induction JmBz of the device placed in a casing in a non-conductive medium was compensated (close to zero).

Compensation for the effect of the conductive casing string is carried out during preliminary calibration of the device placed inside the casing string in the air or in the interval of a cased well with high resistance rocks (with resistivity> 100 Ohm⋅m). By placing the installation in the well with the casing being studied, due to the induction and the occurrence of eddy currents in the host conducting medium, the measured value of JmBz will be different from zero.

In FIG. 2-4 presents materials explaining the principle of implementation of the proposed technical solution.

To determine the electrical resistivity of the rocks in the annulus of the cased hole, an asymmetric installation of two coaxial generator coils with different angular directions M _Z1 ≠ -M _{Z2 is used} . At the measuring point N, located on the same axis at different distances L ₁ ≠ L ₂ from the generator coils, the axial component of the magnetic induction is measured, which is antiphase to the current JmBz (Fig. 1). The moments M _Z1 and M _Z2 , as well as the distances L ₁ and L _{2 are} selected so that the measured value of the antiphase current of the axial component of the magnetic induction JmBz of the device placed inside the casing in the air is compensated (equal to zero).

Calculation parameters: current strength J = 1 A, coil radii 3 and 4 0.05 m, spacing L ₁ = 0.1 m and L ₂ = 0.5 m, casing: outer diameter D = 0.15 m, specific electrical resistance ρ = 5⋅10 ^-6 Ohm⋅m, thickness h = 0.01 m (longitudinal conductivity S = h / ρ≈2⋅10 ³ cm); the specific electrical resistivity of the host rocks ρ is from 5 to 50 Ohm⋅m (curve code), the space of the well where the downhole tool is located is a dielectric (ρ = ∞ Ohm⋅m), the frequency range is ƒ = 0.1 ÷ 10 kHz.

The differences in the amplitudes of the frequency curves JmBz in FIG. 2 are due to different electrical resistivities of the enclosing rocks in the annulus of the cased hole. The frequencies corresponding to the extreme values of JmBz are determined only by the characteristics of the casing: diameter D, thickness h and its specific electrical resistance. As can be seen from FIG. 2, the proposed method using an asymmetric installation allows you to determine the electrical resistivity of the surrounding rocks behind the casing of the well from the extreme values of the axial component of the magnetic induction JmBz, measured at a number of frequencies.

To determine the horizontal boundaries of the media with different electrical resistivities through the casing of the well, a symmetric installation of two coaxial generator coils with equal moments of the opposite direction M _Z1 = -M _{Z2 is used} . At the measuring point N, located on the same axis at an equal distance L ₁ = L ₂ from the generator coils, the axial component of the magnetic induction is measured, which is out of phase with the current JmBz (Fig. 1). A feature of the field created by the symmetric feed unit is the absence of the Bz component of magnetic induction, measured on the axis of the well, from electrical inhomogeneities in the form of cylindrical-inhomogeneous layers due to the symmetry of the electromagnetic field, since the magnetic field at point N created by the current in the first coil is compensated counter magnetic field created by the current in the second coil with a counter moment.

In the presence of horizontal interfaces between media with different resistivities, moving the installation along the well from a medium with resistivity ρ ₀ to a medium with resistivity ρ ₁ leads to field uncompensation near the media boundary and the appearance of a magnetic field component JmBz, the magnitude and direction of which depends on the moments M _z1 and M _z2 , frequency f, separation 2L and media contrasts in electrical resistivity ρ ₁ / ρ ₀ .

In FIG. Figure 3 shows the JmBz curves along the Z axis intersecting the horizontal contact of rocks with different resistivities. Installation parameters: L ₁ = L ₂ = L = 0.5 m, frequencies f = 0.7, 1 kHz, moduli of the moments of the contours | Μ _z1 | = | Μ _z2 | = 1 A * m ² ; the values of specific electrical resistances of rocks ρ ₀ = 10 Ohm⋅m, ρ ₁ = 50 Ohm⋅m. From the graphs shown in FIG. 3, it follows that the contact of media with different electrical resistivity can be determined by the position of the minimum point JmBz taking into account the half-spacing of the installation L. With an inverse ratio of electrical resistivity of the media, the JmBz curves have opposite signs, i.e. a view mirrored relative to the interface. As can be seen from FIG. 3, the proposed method using a symmetric feed unit allows determining horizontal rock boundaries with different specific electrical resistances behind the casing string from the behavior of the axial component of magnetic induction JmBz measured along the borehole axis.

In FIG. Figure 4 presents the results of physical modeling using current in a generator coil with a diameter of D = 90 mm and the number of turns N = 90 with measuring EMF in closed conductive circuits of various radii when placing the coil inside a metal titanium pipe with a specific electrical resistance ρ = 6.7⋅10 ^{- 7} Ohm⋅m with a diameter of 147 mm with a wall thickness h = 37 mm (dashed lines) and without it (solid lines). From the graphs of FIG. 4 it follows that behind a conducting pipe (dashed lines) with high longitudinal conductivity S = h / ρ≈5.5⋅10 ⁴ cm in closed loops with radii of 7.5; fifteen; 21; 28 and 35 cm, the magnitude of the emf at frequencies f = 1 and 5 kHz is significantly weakened compared to the emf without a pipe (solid lines), and at the upper frequency it is an order of magnitude, but not completely shielded.

The essence of the claimed invention is expressed in the aggregate of essential features sufficient to achieve a technical result, which is expressed in increasing the accuracy of determining the resistance of the host rocks in the annular space of cased wells.

The claimed combination of essential features is in direct causal connection with the achieved result. Analysis of the current level of technology has shown that the proposed solution meets the criteria of "novelty" and "inventive step" and can be industrially implemented using existing technical means.

Information sources

1. Alpin L.M. Method for cased hole electric logging. USSR AS No. 5626, 11/30/1939

2. Kaufman Α.Α. The Electrical Field in a Borehole with a Casing. Geophysics 55, No. 1 (1990). P. 29-38.

3. Kaufman A.A. and Wightman W.E. A Transmission-Line Model for Electrical Logging Through Casing. Geophysics. No. 12, 1993. P. 1739-1747.

4. Axelrod S.M. Measurement of rock resistance through the casing (based on foreign literature). NTV "Logger", vol. 75, Tver, 2000, pp. 125-140.

5. Investigation of the bottomhole zone. Schlumberger. Oil Review, 2002, Volume 7, No. 2.

6. Vail, III; William B. Methods and Apparatus For Induction Logging in Cased Boreholes. U.S. Patent No. 4,748,415. May 31, 1988.

7. Electric logging through the casing. Implementation Experience. Feofilov D.T., Bulatov A.V., Shkvarok I.R. // Neftegaz, vol. 1, 2008. http://www.neftepixel.ru/node/193.

8. Chistoserdov BM, Manchekov AI, Baidikov S.V. Patent No. 2230341. The method of induction vertical sensing. BI No. 16, 2004, 8 pp.

Claims (2)

1. The method of induction logging in cased wells, which consists in creating a low-frequency electromagnetic field with harmonic current in two coaxial generator coils moving along the well under study, and measuring the axial component of magnetic induction on the axis of the well, characterized in that the field is excited while the current is turned on in generator coils with opposing moments of different sizes located at different distances from the measuring coil, and the magnitude of the measured thyroid-phase current of the axial component of magnetic induction (JmBz), measured at a number of frequencies with the compensated effect of the conductive casing string, determine the value of the electrical resistivity of the surrounding rocks. 2. A device for conducting induction logging in cased hole conditions, containing two generator coils placed coaxially and coaxially and connected in opposite series to the generator device; an axial measuring coil, a current sensor, a signal amplifier, an analog-to-digital phase-sensitive meter of amplitude and phase of magnetic induction and a recording device for out-of-phase current of the quadrature of the axial component of magnetic induction, characterized in that the magnetic moments of the generator coils and the distance to the measuring coil are selected so that the antiphase current quadrature of the axial component of magnetic induction (JmBz) in the casing of the conductive string in a non-conductive medium was compensated ka to zero), and a conductive medium enclosing the casing will differ from zero by inducing electric field in a conductive medium.

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