NO335831B1 - Apparatus and method for determining and imaging borehole resistivity using capacitive coupling - Google Patents

Description

Background for the invention

1. Technical area

The present invention generally relates to the search for hydrocarbons which entails electrical investigations in a borehole that penetrates a foundation formation. More particularly, the invention relates to very localized borehole investigations by using the introduction and measurement of individual measuring currents injected into the wall of a borehole by capacitive coupling of electrodes on a probe which is moved along the borehole inside the basic formation.

2. Technical background

Electric borehole logging is well known and many devices and various techniques have been described for this purpose. In general, there are two categories of devices used in electrical logging devices. In the first category, a measuring electrode (current source or sink) is used in connection with a diffuse return electrode (such as a probe body). A measuring current flows in a circuit that connects a current source to the measuring electrode, through the base formation to the return electrode and back to the current source in the probe. In inductive measuring probes, an antenna in the measuring instrument induces a current flow in the base form ion. The magnitude of the induced current is detected by using either the same antenna or a separate receiving antenna. The present invention belongs to the first category.

There are several modes of operation that can be used: in one, the current at the measuring electrode is kept constant and a voltage is measured, while according to the other mode, the voltage to the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to keep the voltage measured at a monitoring electrode constant, then the current is inversely proportional to the resistivity in the underlying formation being investigated. On the other hand, it is desirable that if this current is kept constant, the voltage measured at a monitoring electrode is proportional to the resistivity in the base formation being examined. Ohm's law teaches that if both current and voltage vary, the resistivity of the base formation is proportional to the ratio of the voltage to the current.

Birdwell (US Patent No. 3,365,658) describes the use of a focused electrode to determine the resistivity of subsurface formations. A measuring current is emitted from a central measuring electrode into adjacent ground formations. This measuring current is focused into a relatively narrow current beam outwards from the borehole by using a focusing current given off by nearby focusing electrodes placed near the measuring electrode and on each side of it. Ajam et al. (US patent no. 4,122,387) describes a device where simultaneous logs are taken at different lateral distances through a formation from a borehole by means of protective electrode systems placed on a probe which is lowered into the borehole in a logging cable. A single oscillator controls the frequency of two formation streams flowing through the formation at the desired different lateral depths from the borehole. The armature of the logging cable acts as current return for one of the protective electrode systems, and a cable electrode in a cable-electrode unit immediately above the logging probe acts as current return for the other protective electrode system. Two embodiments are also described for measuring reference voltages between electrodes in the cable electrode unit and the protective electrode systems.

Techniques for the investigation of bedrock formations with groups of measuring electrodes have been proposed. See e.g. US Patent No. 2,930,969 to Baker, Canadian Patent No. 685,727 to Mann et al., US Patent No. 4,468,623 to Gianzero and US Patent No. 5,502,686 to Dory et al. The Baker patent proposed a number of electrodes each formed of buttons electrically connected by means of flexible wires with buttons and wires baked into the surface of a collapsible tube. The Mann patent proposes a group of small electrode buttons either mounted on a probe or a sidewall pad, each in turn introducing a separate, measurable current for an electrical survey of the bedrock formation. The electrode buttons are placed in a horizontal plane with circumferential distances between electrodes and a device for sequential excitation and measurement of a measuring current from the electrodes is described.

The Gianzero patent describes probe-mounted sidewall pads, each with a number of small measuring electrodes from which individually measurable measuring currents are injected against the wall of the borehole. The measuring electrodes are arranged in a group where the measuring electrodes are positioned at intervals along at least one circumferential direction (around the borehole axis) in order to inject measuring currents into the borehole wall segments which overlap each other to a predetermined degree when the probe is moved along the borehole. The measuring electrodes are made small to enable a detailed electrical survey over circumferentially adjacent segments of the borehole to provide indications of the stratigraphy of the formation near the borehole wall, as well as fractures and their orientations. According to one technique, a spatially closed loop group of measuring electrodes can be arranged around a central electrode where the group is used to detect the spatial pattern of the electrical energy injected by the central electrode. In another embodiment, a linear array of measuring electrodes is provided to inject a current flow into the formation over a circumferential, effectively adjacent segment of the borehole. Discrete parts of the current flow are separately measurable to obtain a number of measurement signals that are representative of the current density from the group and from which a detailed electrical image of a circumferentially continuous segment of the borehole wall can be derived as the probe is moved along the borehole. In another embodiment of a group of measuring electrodes, they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity anomalies.

The Dory patent describes the use of an acoustic sensor in combination with pad-mounted electrodes, the use of the acoustic sensors making it possible to fill in the gaps in the acquired image by using pad-mounted electrodes, due to the fact that in large diameter boreholes, the pads will not necessarily provide complete coverage of the borehole.

The previously known devices, which are contact devices, are sensitive to the effects of borehole roughness; the currents flowing from the electrodes depend on good contact between the electrode and the borehole wall. If the borehole wall is uneven, the contact and current from the electrodes will be uneven, resulting in inaccurate imaging of the borehole. Another disadvantage is the relatively shallow examination depth caused by the use of measuring electrodes at the same potential as the pad and the resulting divergence of the measuring currents.

Yet another disadvantage of using contact devices that inject electrical currents into a borehole occurs when oil-based muds are used in drilling. Oil-based muds must be used when drilling through water-soluble formations. An increasing number of today's exploration prospects lie below the salt layer. Besides reducing the electrical contact between the logging probe and the formation, invasion of porous formations by a resistive, oil-based mud greatly reduces the effectiveness of previously known resistivity imaging devices and wireline-based devices for determining formation resistivity. This problem will not be alleviated by

use of focusing electrodes.

It would be desirable to have a device and a method for determining formation resistivity that is relatively insensitive to borehole roughness and that can be used with either water-based or oil-based muds. The present invention satisfies this need.

Summary of the invention

The present invention is a device and a method for use in a borehole containing a mainly non-conductive fluid, to provide a resistivity parameter for a basic formation that is penetrated by the borehole. At least one measuring electrode is capacitively coupled to the base formation through the non-conductive fluid. A measuring current is led into the formation, and by measuring the current in the electrode and its potential, the resistivity can be determined. A number of measuring electrodes can be used. With a group of measuring electrodes, a resistivity image of the formation can be obtained.

In one embodiment of the invention, the measurement current is a modulated electric current with a carrier frequency chosen to have low impedance due to the capacitive coupling. An isolator is arranged on the logging probe to minimize crosstalk between the current from a current source and the measurement signals. Focusing and protection electrodes can be used with the measuring electrodes. The logging probe can be transported by means of a cable or form part of a downhole device transported on a drill pipe.

In an MWD embodiment of the invention, many options are available for placement of the measuring electrodes. They can be on a stabilizing device, a non-rotating sleeve or on a pad. An expansion device can be arranged to hold the measuring electrode at a specified distance from the borehole wall. When directional sensors are arranged, a downhole processor can provide resistivity images without the need to have electrode groups with a large number of electrodes.

In an alternative embodiment of the invention, a modification has been made to make it possible to use the device with water-based sludge. The measuring electrode is capacitively connected to the source of modulated electric current.

The invention also provides for taking multi-frequency measurements. When such multi-frequency measurements are taken, frequency focusing can be used to determine formation resistivity.

Brief description of the figures

The present invention will be best understood with reference to the following figures where like reference numbers refer to like components, and where: Fig. 1 is a circuit diagram representing a formation resistivity device according to the present invention. Fig. 2 shows a comparison between signals that are representative of the measuring current and the voltage for the circuit in fig. 1, for a sinusoidal excitation signal at 1 kHz. Fig. 3 shows a comparison between signals that are representative of the measuring current and the voltage for the circuit in fig. 1, for a sinusoidal excitation signal at 10 kHz. Fig. 4 shows a comparison between signals that are representative of the measuring current and the voltage for the circuit in Fig. 1, for a square wave excitation at 10 kHz. Fig. 5 (prior art) shows a schematic illustration of a previously known imaging probe in a borehole. Fig. 6 illustrates a model used to derive the impedance of an imaging probe. Figures 7a-7f illustrate the impedance for a measuring electrode at a frequency of 1 kHz. Figures 8a-8f illustrate the impedance for a measuring electrode at

a frequency of 10 kHz.

Fig. 9 shows the imaging probe according to the invention suspended in a borehole. Fig. 10 is a mechanical, schematic sketch of the imaging probe.

Fig. 10A is a detailed sketch of an electrode pad.

Fig. 11 is a circuit diagram showing the principles of operation of the probe. Figures 12a and 12b show a comparison between a previously known modulated signal and an inversely modulated signal according to the present invention. Fig. 13 is a circuit diagram of the probe used with a conductive borehole fluid. Fig. 14 illustrates an alternative embodiment of an electrode pad. Fig. 15 (prior art) is a schematic illustration of a drilling system. Fig. 16 is a schematic illustration of the invention where resistivity measurements are taken at different azimuth values. Fig. 17 illustrates the pads on a non-rotating sleeve used for resistivity measurements.

Detailed description of the invention

In order to obtain a correct understanding of the present invention, reference is made to figures 1-17. The description of the present invention first begins with a discussion of the concepts in connection with capacitive coupling which are used in the present invention. A multi-frequency probe is then discussed, followed by a discussion of embodiments of the invention where resistivity images of the borehole can be obtained.

Fig. 1 is a circuit diagram illustrating the methodology behind devices for measuring formation resistivity. A measuring electrode, sketched at 3, injects a measuring current into a formation denoted by 7, which has a resistivity Rt. This current is supplied by a current source 1. The current from the formation returns (not shown) through a return electrode (earth) denoted by 7. A voltage drop 11 across a resistor 10 in the circuit is usually used as an indication of the measurement current. By measuring the voltage drop 13 between the measuring electrode and the return electrode, information is derived about the impedance that the current encounters between the measuring electrode and earth.

This impedance includes, as mentioned above, the desired formation resistivity Rt. In addition, there is also an impedance 5 between the measuring electrode 3 and the formation 7. In water-based (conductive) muds, this impedance is almost completely resistive, and is caused by the mud cake and possible invasion of borehole fluid into the formation. In oil-based (non-conductive) muds, however, the impedance between the measuring electrode 3 and the formation 7 is mainly capacitive, denoted by a capacitance Mc. This capacitance manifests itself in a phase shift between the measuring current signal and the voltage drop from the measuring electrode to earth. This can be seen in fig. 2 which shows a phase shift between the signals 11' and 13' for a sinusoidal current of 1 kHz. This frequency is typical for previously known devices for measuring formation resistivity. The curves in fig. 2 is normalized independently to emphasize the phase rotation; in reality, there can be several orders of magnitude differences between the two signals.

Reference is now made to fig. 3 where the signals 11" and 13" are a sinusoidal current of 10 kHz. The phase shift between the two signals is shown to be much smaller. This is due to the fact that at the higher frequency of 10 kHz the effect of the capacitance is less than at 1 kHz. This suggests that by using higher frequencies, it will be possible to obtain signals indicating the formation resistivity. This is confirmed in fig. 4 showing the signals 11'" and 13"' for a square wave excitation at 10 kHz. As can be seen, both signals rise and fall almost immediately; this is due to the fact that a square wave consists of a bunch of high frequencies that are largely unaffected by the capacitance of the sludge. The use of higher frequencies forms the basis of the present invention, as described below.

Fig. 5 is a schematic illustration of a part of a previously known imaging probe suitable for use with the method according to the present invention. A borehole 51 is shown which is filled with a borehole fluid (drilling mud). A mud cake 53 is formed between the borehole fluid and the formation 55. The probe comprises one or more measuring electrodes 59 carried on a conductive pad 57. In the illustration only two electrodes are shown. As discussed below, the actual number of electrodes can be much higher, and they can be arranged in a group. The electrodes 59 are separated from each other by an insulator 61. To simplify the illustration, further insulation between the electrodes 59 and the pad 51 is not shown.

In previously known imaging probes, the pad acts as a protective electrode and is held at a potential related to the potential of the measuring electrodes. As known to those skilled in the art, the current from the measuring electrodes, due to the presence of the protective electrode and the current flowing into the formation from this, flows in current paths as shown at I, and is prevented from spreading due to the focusing current F from the protective electrode. Optionally, additional focusing electrodes can be used (not shown). Focusing electrodes are known in the art, but a particular embodiment utilizing focused electrodes for a resistivity imaging probe is discussed below. The current that flows from the measuring electrode is related to the potential V and the impedance of the electrical circuit in which the measuring currents flow.

When a device such as shown in fig. 5, is used with a water-based drilling mud, the impedance of the mud and mud cake is relatively small compared to the impedance of the formation. As experts in the field will understand, the formation impedance at the frequencies used in previously known devices is mainly resistive, and from a knowledge of the potential V and the measuring current I, the formation resistivity can be derived.

In oil-based sludge, on the other hand, the measured impedance of individual measuring electrodes is strongly dependent on the sludge cake parameters. In addition, an oil film on the pad surface can completely eliminate the electrical contact between the pad and the formation.

The size of a measuring electrode is connected to the probe's spatial resolution. Usually the measuring electrode is in the range from 1 to 2 mm, which creates a very large earth resistivity. A measuring electrode of 2 mm on a typical pad device has e.g. an earth resistance of 10,000 ohms in a formation with resistance 1 Q-m or 10 MQ in a formation with 1,000 Q-m. This illustrates the technical challenge of producing a high-resolution image in a resistive environment.

There are several possible ways to overcome the physical limitations of direct current (DC) imaging in oil-based muds. One solution that has been used is to change the composition of oil-based sludge to increase the sludge cake's conductivity. The present invention is based on increasing the frequency to produce capacitive coupling between the pad and the formation.

Reference is now made to fig. 6 where the impedance of the measuring electrode is derived. We consider a model consisting of two conducting layers 103, 105 enclosed between an insulating half-space at the top 101 and a perfect conductor at the bottom 107. From the upper boundary, a uniform current is injected with the surface density, Js. A measuring electrode of any shape can be examined by cutting out a suitable area 109 from the plane of injection. The upper half-space 101 represents a borehole filled with oil-based mud. The conductor 107 at the bottom is a power outlet. In reality, at a certain distance, the streamlines diverge depending on the focusing conditions. This gives a final value for the measuring electrode's K-factor. To simplify the modelling, we introduce a parallel current flow. We can change the K-factor by replacing the current return (perfect conductor) at different distances from the borehole. It is well known that the K-factor for a cylindrical volume with a cross-section S and length L is defined by the following equation:

where S(l) is the cross-sectional area at a distance 1 along the current path. The mud cake 103 is characterized by wood conductivity ai, permittivity Si and thickness hi, likewise the formation 105 is characterized by wood conductivity CT2, a permittivity £2 and a thickness h2. The complex conductivities of the mud cake and the formation are given by respectively

where©=27if (where f is the frequency) .

Denoting the electric field by Ei and E2i respectively the mud cake and the formation, and by denoting the potential difference between the measuring electrode and the current return (ground in Fig. 1) by V, the following equations result:

This gives: By introducing electrode impedance, we finally obtain

The first term on the right-hand side of equation (5) represents the impedance of the mud cake, while the second term represents the impedance of the formation. At low frequencies (co—>-0), the measured impedance depends mainly on the mud cake conductivity and the formation conductivity, i.e. it does not depend on the dielectric constant of the mud cake and the formation. If, however, the mud cake is oil-based (the mud cake is resistive), then the measured impedance can be so great that it will be practically impossible to inject any current into the formation.

Fig. 5 indicates that we can reduce the sludge cake impedance by increasing the frequency oo. This can be done by choosing the frequency so that

When the mud cake impedance is reduced, we must also keep the frequency so that the second term in equation (5) is mainly dependent on the formation conductivity02. This leads to the condition By combining equations (6) and (7), the following results are obtained

In an oil-based sludge, both inequalities in equation (6) must be satisfied because ai «02. Under these conditions, equation (5) can be written in the form:

Equation (9) can be written in the form where 9?(Z) and 3(Z) are the real and imaginary (phase and quadrature) parts of the impedance given by

The following points can be noted about equation (11) (the real part of the impedance): 1. The first term depends on formation conductivity and does not include dielectric permittivity. It represents

accurate resistivity reading in the absence of sludge cake.

2. The second paragraph contains only mud cake properties. It is important that this is inversely proportional to the second power of the frequency. 3. The second paragraph can be eliminated in two different ways. The first way is to use a high frequency. The other way to eliminate the second term is by combining measurements at two different frequencies. This is given by the following equation:

Reference is now made to equation (12) where the following points should be noted about the quadrature component of the impedance. 1. With increasing frequency, the formation contribution (the second term) becomes more significant. 2. Although dominant, the formation signal retains the dependence on the formation dielectric constant. This introduces unwanted uncertainty into the interpretation process. 3. Due to equation (8), the quadrature component usually becomes small compared to the phase component.

The above-mentioned points are illustrated in figures 7-8 which show exact relationships derived from equation (5). Calculations were made for an electrode radius of 2 mm, a K-factor of 12,000 rn-1 and a relative dielectric constant of 10 for both the mud and the formation. The relative dielectric constant is the ratio between the permittivity of a medium and the permittivity in free space.

Reference is now made to fig. 7a where the abscissa is the formation resistivity in fim and the ordinate is 5R(Z). Values are plotted for a frequency of 1 kHz. Three curves are shown for mud cake resistivities at 10 kQm, 100 kQm and 1,000 kQm and a mud cake thickness of 0.1 mm. As can be seen, 9?(Z) is not only dependent on the formation resistivity, but also on the resistivity of the mud cake.

Fig. 7b is similar to fig. 7a except that the sludge cake thickness is 0.5 mm. The difference between fig. 7b and fig. 7a shows that 9?(Z) also depends on the sludge cake thickness. Fig. 7c is a plot of the absolute value of the electrode impedance for a sludge cake thickness of 0.1 mm.

Reference is now made to fig. 7d where a plot of the double frequency impedance determined by equation (13) for a sludge cake thickness of 0.1 mm is shown. The double frequency values were obtained by using measurements at 1 kHz and 2 kHz respectively. Fig. 7e shows the results of the double frequency measurements for a sludge cake thickness of 0.2 mm. Fig. 7f finally shows a plot of the relationship between 9?(Z) and 3(Z).

Figures 7a-7f thus show why measurements taken using conventional resistivity imaging probes do not work well in connection with oil-based muds: the measured impedance for audio frequency signals is dependent on many other factors than the formation resistivity.

Reference is now made to Figures 8a-8f where a completely different picture emerges. The figures are similar to figures 7a-7f with the significant difference that the operating frequency is now 1 MHz (compared to 1 kHz in figures 7a-7f). For a relatively thin mud cake (Fig. 8a), 5R(Z) is mainly dependent on the formation resistivity, except for extremely conductive formations where some dependence on the mud cake resistivity can be noticed. The effect is more noticeable for a thicker sludge cake (0.5 mm in Fig. 8b). The amplitude of the impedance (Fig. 8c) shows little variation with the mud cake resistivity, but exhibits a non-linear dependence on the formation resistivity. The double frequency measurement (fig. 8d, 8e) shows that the measured impedance is mainly independent of mud cake thickness and resistivity and furthermore exhibits the desired property of being linearly related to the formation resistivity. The present invention takes advantage of the fact that at high frequencies (^1MHz or thereabouts) the effect of the sludge cake and sludge impedance can be ignored for all practical purposes.

The double frequency solution given by equation (13) is a special case of multi-frequency focusing. In an alternative embodiment of the invention, measurements are taken at a number of frequencies ©i, ©2, ©3... ©m. As described in US Patent No. 5,703,773 to Tabarovsky et al., the contents of which are hereby incorporated by reference in their entirety, the response at multiple frequencies can be expressed using a Taylor series expansion of the form:

In a preferred embodiment of the invention, the number m of frequencies © is equal to 10. By using the measurements at the m frequencies, the magnitude so,S1/2,S3/2 is determined. In equation 12, n is the number of terms in the Taylor series development. This can be any number less than or equal to m. The coefficient s3/2i©<3/2->term (© is the square of k, the wave number) is generated by the primary field and is relatively unaffected by any inhomogeneities in the medium that surrounds the logging instrument, i.e. that it mainly responds to the formation parameters and not to the borehole and the invaded zone. The coefficient S3/2 for the oo<3/2-> term actually reacts to the formation parameters as if there were no borehole in the formation. This frequency focusing method has been shown to provide reliable, consistent results even when there is significant invasion of the formation by borehole fluids. In one embodiment of the invention, a processor controls the signal generator to provide a measurement current at a number of frequencies. The processor then performs a frequency focusing of the apparent conductivity at the number of frequencies to obtain the coefficients S3/2. This is then used as an estimate for the formation conductivity.

Reference is now made to embodiments of the present invention which are suitable for resistivity imaging. Fig. 9 shows an imaging probe 110 suspended in a borehole 112 that penetrates bedrock formations such as 113, from a suitable cable 114 passing over a disc 116 mounted on a drilling rig 118. By industry standard, the cable 114 includes an armature and seven conductors for transmission of commands to the probe and to receive data back from the probe, as well as power to the probe. The probe 110 is raised and lowered by means of a hoisting device 120. An electronics module 122 on the surface 123 transmits the necessary operating commands down the hole, and receives in return data that can be recorded on an archiving storage medium of any desired type for simultaneous or later treatment. The data can be transmitted in analogue or digital form. The data processors, such as a suitable computer 124, can be arranged to perform data analysis in the field in real time, or the recorded data can be sent to a processing center, or both for post-processing of the data.

Figures 10a and 10b are schematic, external sketches of an imaging system for the side wall of a borehole. The probe 110 comprising the imaging system includes resistivity groups 126 and preferably a mud cell 130 and a circumferential acoustic televiewer 132. The electronics modules 128 and 138 may be located at suitable positions in the system and not necessarily in the indicated positions. The components may be mounted on a mandrel 134 in a conventional, well-known manner. The outer diameter of the unit is about 12.5 cm (5 inches) and it is about 4.5 m (15 feet) long. An orientation module 136, which includes a magnetometer and an accelerometer or an identical guidance system, can be mounted above the imaging units 126 and 132. The upper part 138 of the probe 110 contains a telemetry module for sampling, digitizing and transmitting the data samples from the various components up through the hole to the surface electronics 122 in a conventional manner. If acoustic data is collected, it is preferably digitized, although in an alternative arrangement the data may be retained in analog form for transmission to the surface where it is later digitized by the surface electronics 122.

In fig. 10a, three resistivity groups 126 are also shown (a fourth group is hidden in the illustration). Reference is now made to figures 10a and 10b where each group includes measuring electrodes 141a, 141b, ... 141n for injection of electric currents into the formation, focusing electrodes 143a, 143b for horizontal focusing of the electric currents from the measuring electrodes and focusing electrodes 145a, 145b for vertical focusing of the electrical currents from the measuring electrodes. "Vertical" conventionally refers to the direction along the axis of the borehole, and "horizontal" refers to a plane perpendicular to the vertical.

In a preferred embodiment of the invention, the measuring electrodes have a rectangular shape and are oriented with the long dimension of the rectangle parallel to the probe axis. Other electrode configurations are discussed below with reference to FIG. 14. For the purpose of simplifying the illustration, insulation around the measuring electrodes and focusing electrodes to electrically isolate them from the probe body is not shown.

Other embodiments of the invention may be used in measurement-while-drilling (MWD), logging-while-drilling (LWD) or logging-while-retrieval (LWT) operations. The sensor unit can be used on a mainly non-rotating pad as described in US Patent No. 6,173,793 to Thompson et al., which has the same applicant as the present application and the contents of which are hereby incorporated by reference in their entirety. The sensor unit according to the present invention can also be used with rotating sensors as described in Thompson. These embodiments are described below with reference to figures 15-17. The sensor unit can also be used on a non-rotating sleeve such as that described in US patent no. 6,247,542 to Kruspe et al., the contents of which are hereby incorporated by reference in their entirety.

For a 5 inch (12.5 cm) diameter unit, each pad cannot be more than about 4.0 inches (10 cm) wide. The pads are attached to extendable arms, such as 142. Hydraulic or spring-loaded caliper arm actuators (not shown) of any well-known type bring the pads and their electrodes against the borehole sidewall for resistivity measurements. In addition, the extendable caliper arms 142 provide the appropriate measurement of the borehole diameter as known in the art. Using time-division multiplexing, the voltage drop and current flow are measured between a common electrode on the probe and the respective electrodes on each array to provide a measure of sidewall resistivity (or its inverse, conductivity) as a function of azimuth.

Reference is now made to fig. 11 where a circuit diagram showing the principles of operation of the probe is given. A source of electrical power 151 produces an electrical current which is delivered to the measuring electrodes. In one embodiment of the invention, the device is intended for use with oil-based drilling mud and the capacitor 157 outlines the capacitive coupling between a measuring electrode, such as 141a in fig. 10b, and the formation 113 in fig. 9. The electric current flows through the formation which has an equivalent impedance equal to Zf, and returns to the current source 151 through an equivalent capacitor 159 which represents the connection between the formation and the diffuse return electrode, typically the probe body. The measurement of the voltage drop across a resistor 153 is used as an indication of the current flowing to a measuring electrode. Other methods for measuring the current in the measuring electrode can also be used. Such methods will be known to those skilled in the art and are not discussed here. In a preferred embodiment of the invention, the value of the resistor 153 is equal to lkQ. The impedance of the rest of the return path in the probe body can be ignored.

Reference is still made to fig. 11 where a voltage detector 161 measures the voltage difference between the measuring electrode and the diffuse return electrode and controls the current at the current generator to maintain a constant voltage. In this case, the output of the current measurement circuit serves as a measurement signal. Alternatively (not shown), the output of the current measurement circuit 155 is used to maintain a constant current, and the output of the voltage detector is used as a measurement signal. As a further alternative, both the voltage detected by the voltage detector 161 and the current measured by the current measurement circuit 155 are used as measurement signals.

Selection of the size of the measuring electrode and the operating frequency is based on several considerations. An important consideration is that the impedance of the formation must be mainly resistive at the operating frequency so that the currents in the measuring electrode indicate the formation resistivity and are mainly unaffected by its dielectric constant. Based on typical values for the dielectric constant of the formation, as described in US Patent No. 5,811,973 issued to Meyer et al., the operating frequency should be less than 4 MHz. As mentioned above, a preferred embodiment of the present invention uses a measurement current at a frequency of 1 MHz. Another consideration is that the impedance (ie the resistance) in the formation must be greater than the impedance of the rest of the circuit in fig. 11. Another consideration is the desired resolution of the probe. A reasonable resolution for a usable imaging probe is about 3 mm in the horizontal and vertical directions.

The impedance of the equivalent capacitance 159 and the probe body can be ignored at a 1 MHz since the equivalent capacitor has a huge area compared to the size of the probe. The capacitance at 157 is a function of the dielectric constant of the borehole fluid, the area of the electrode and the distance between the electrode and the borehole wall. Formation resistivities encountered in practice can lie in the range between 0.2 Q-m and 20,000 Q-m. As mentioned above and as discussed below, the present invention makes use of focusing electrodes so that the effective dimensions of the formation sampled by an electrode are smaller than the actual physical size of the electrodes. Based on these considerations and the requirement that a number of the electrodes must fit on a single pad according to a preferred embodiment of the invention, as shown in figures 10a, 10b, the individual measuring electrodes 141a, 141b, ... 141n have a width of 8 mm and a length of between 20-30 mm. This makes it possible to have eight electrodes on a single pad. The corresponding value of the capacitance 157 is then typically between 1 pF and 100 pF. At the lower value, the impedance of the capacitance 107 at 1 MHz is about 160 kQ and at the highest value about 1.6 kQ.

In the present device, the focusing electrodes 145a, 145b are of some importance as they perform a significant amount of focusing. By designating the potential of the measuring electrodes 141a, 141b ... by V, the electrodes 145a, 145b are maintained at a potential equal to V + 5. The pad body is maintained at a voltage V ± e. The pad acts as a protective electrode and prevents divergence of the measuring current until the flow has penetrated a certain distance into the formation. This makes it possible to take deeper readings. A typical value of the voltage V is 5 volts, while a typical value of 5 and s is 500 microvolts and 100 microvolts, where s is less than 5. Since little focusing is required in the horizontal direction,

side focusing electrodes 143a, 143b held at substantially V volts. Those skilled in the art will realize that the device can also function if all the voltages are reversed, in which case the above-mentioned voltages as typical values will be voltage magnitudes.

With the potentials of the measuring electrodes, focusing electrodes and pads as discussed above, the current from the current source 151 of FIG. 11 be focused down to square blocks with sides equal to about 8 mm. The operating frequency of the present device is typically 1 MHz, compared to an operating frequency of 1.1 kHz for the device in the '431 application.

Those skilled in the field will realize that a significant crosstalk will normally be generated between the current flowing in the measuring electrodes from the electronics module 138 and the measuring signal or measuring signals that return from the measuring electrodes that carry information about the voltages and/or currents in the electrodes. The measuring electrodes are preferably isolated from the electronics module by means of an insulator section, such as 137, which is preferably between 2.5 feet (75 cm) and 15 feet (4.5 m) long. Crosstalk between conductors (not shown) over such distances will be quite large at an operating frequency of 1 MHz and overwhelm the measurement signal or signals indicating the formation resistivity.

This problem is taken into account in the present invention by modulating the current output from the generator at 1.1 kHz. The result is that the current that propagates down through the conductors in the insulator section and into the formation is a 1 MHz current modulated at 1.1 kHz. A demodulator (not shown) is arranged in the voltage measurement circuit so that the return signal to the electronics module 138 is a signal at 1.1 kHz. This makes it possible to use substantially the same circuit configuration as in previously known devices designed to substantially suppress crosstalk.

In order to further reduce the effects of crosstalk, instead of conventional amplitude modulation of the currents, an inverse modulation is used. Conventional

amplitude modulation is given by a current i(t)

where C0mer is the frequency (1.1 kHz) of the modulating signal and ooc is the carrier frequency (1 MHz). The inverse modulation according to the present invention uses a modulation of the shape

where a is small compared to 1. The result is that the current output from the generator 151 is mainly at 1 MHz with an amplitude close to one at all times. This makes the cross talk mainly independent of the size of the measuring current. Essentially the same result can be achieved in alternative embodiments of the invention by using frequency or phase modulation of the 1 MHz carrier signal.

Figures 12a and 12b show a comparison between a previously known modulated signal and an inversely modulated signal according to the present invention. A carrier signal 161 having a carrier frequency has its amplitude modulated by a modulation signal 143 with a lower frequency. As can be seen, the amplitude level of the modulated signal goes to zero every time the modulating signal goes to zero at times such as 165. An inversely modulated signal is shown in fig. 12b with a carrier signal 161' and a modulation signal 163'. This modulated signal always has a significant current flow. The advantage of using such an inversely modulated signal is that the crosstalk is largely unaffected by the level of the modulating signal.

In an alternative embodiment of the invention, the measurement signal or measurement signals are sent through an optical fiber. When an optical fiber is used for the purpose, there will be no crosstalk between the current transported through the insulator section and the measurement signal. Modulation of the current is therefore not necessary.

In an alternative embodiment of the invention, the principles described above are used when the measuring electrodes are not part of an electrode group. With a single electrode, measurements indicating the resistivity of the formation can be obtained. With a number of azimuthally distributed electrodes, such output measurements can be processed by using previously known methods, such as those used in drop meters, to provide information regarding the formation's drop in relation to the borehole. Combined with measurements of the borehole orientation and probe surface orientation, such relative dip information can be further processed to provide estimates of the formations' absolute dip.

Another embodiment of the present invention can be used with water-based sludges. The equivalent circuit for this embodiment is shown in Fig. 13. It is identical to fig. 11 except that the gap between the measuring electrode and the formation is a conductive gap denoted by points 209-211 and a return gap denoted by 219-221. A further capacitor 207 may be incorporated in the circuit. The operation of the device is essentially unchanged from that used for non-conductive sludge. The conductive paths through the mud short-circuit any effect of the capacitance of the probe's distance from the wall.

Such an arrangement has been used in the past in connection with contact electrodes for resistivity measurements or resistivity imaging devices. The function of an internal capacitor in such previously known circuits has been solely for the purpose of blocking any spurious currents originating from sources outside the measuring circuit from entering the amplifiers and distorting the operation of the previously known device. Other methods have also been used to compensate for such unwanted currents. The particular embodiment which uses an external capacitor constructed from the instrument's electrode plate, the conductive base formation plate and the dielectric constant of the drilling mud, with high frequency, modulated measuring currents as used in the present invention and as outlined in fig. 13, however, has not previously been used.

The resolution of the devices described above is essentially equal to the dimensions of the focused current at a depth where the current from the measuring electrode has the smallest dimensions. Those skilled in the art will appreciate that if lower resolution can be accepted, the focusing electrodes can be eliminated. In such a device, the measuring current beam is only protected or limited to flow mainly outwards from the surface of the measuring electrode, as in previously known, non-focused devices with conductive sludge, in that the pad (or protective electrode) is kept at substantially the same voltage as the measuring electrode.

Alternatively, other designs of the electrodes on a measuring pad can also be used. Fig. 14 shows an arrangement where five circular measuring electrodes 303a, 303b, ... 303e are arranged on a pad 301. Each measuring electrode is surrounded by an associated focusing electrode 305a, 305b, ... 305e with insulation 307a, 307b, ... 307th between these. To simplify the illustration, the insulation between the protective electrodes and the pad 301 is not shown.

The invention has further been described with reference to log probes which are intended to be transported in a cable. However, the method according to the present invention can also be used with probes for measurement-while-drilling (MWD), or probes for logging-while-drilling (LWD), each of which can be transported in a drill string or a coiled pipe (CT).

Fig. 15 shows a schematic diagram of a drilling system 410 with a drilling unit 4 90 shown arranged in a drill hole 42 6 for drilling the drill hole. The drilling rig 410 includes a conventional tower 411 erected on a deck 412 which supports a rotary table 414 which is rotated by a main drive device, such as an electric motor (not shown) at a desired rotational speed. The drill string 420 includes a drill pipe 422 that extends down from the rotary table 414 into the borehole 426. The drill bit 450 attached to the end of the drill string breaks up the geological formations as it is rotated to drill the borehole 42 6. The drill string 420 is connected to a hoist 430 via a Kelly connection 421, a swivel 428 and a line 429 through a disc 423. During drilling operations, the hoist 430 is operated to control the weight of the drill bit, which is an important parameter affecting the rate of penetration. The operation of the elevators is well known in the area and is therefore not described in detail here.

During drilling operations, a suitable drilling fluid 431 from a mud pit (source) 432 is circulated under pressure through the drill string by means of a mud pump 434. The drilling fluid passes from the mud pump 434 into the drill string 420 via a pressure equalization device 436, a fluid line 328 and the Kelly connection 421. The drilling fluid 431 comes out at the bottom of the drill hole 451 through an opening in the drill bit 450. The drilling fluid 431 circulates up through the hole through the annulus 427 between the drill string 420 and the drill hole 426, and returns to the mud pit 432 via a return line 435. A sensor Si which is preferably placed in the line 438, provides information of the fluid flow rate. A torsion sensor S2 on the surface and a sensor S3 connected to the drill string 420 respectively provide information about the torsion and rotation speed of the drill string. In addition, a sensor (not shown) associated with the line 429 is used to provide the hook load to the drill string 420.

In one embodiment of the invention, the drill bit 450 is rotated by simply rotating the drill pipe 452. In another embodiment of the invention, a downhole motor 455 (mud motor) is arranged in the drilling device 490 to rotate the drill bit 450 and the drill pipe 422 is rotated in the usual way for to supplement the rotation effect, if necessary, and to influence changes in the drilling direction.

In the embodiment of fig. 15, the mud motor 455 is connected to the drill bit 450 via a drive shaft (not shown) arranged in a bearing unit 457. The mud motor rotates the drill bit 450 when the drilling fluid 431 passes through the mud motor 455 under pressure. The bearing unit 457 supports the radial and axial forces on the drill bit. A stabilizer 458 connected to the bearing unit 457 acts as a centralizing unit for the lower part of the mud motor unit.

In one embodiment of the invention, a drilling sensor module 459 is arranged near the drill bit 450. The drilling sensor module contains sensors, circuits and processing programs and algorithms regarding the dynamic drilling parameters. Such parameters preferably include drill bit impact, wedging/slipping of the drilling unit, back rotation, torsion, impact, borehole and annulus pressure, acceleration measurements and other measurements of the drill bit condition. The borehole sensor module processes the sensor information and transmits it to the control unit 440 on the surface via a suitable telemetry system 472.

Fig. 16 shows an embodiment of the invention where sensors mounted on stabilizers for a drilling unit are used to determine the resistivity of the formation. One or more of the stabilizers 1033 is provided with a recess 1035 in which a sensor module 1054 is inserted. Each sensor module 1054 has one or more measurement electrodes 1056 for injecting measurement currents into the formation, as described above. As discussed above, the body of the sensor module is held at approximately the same potential as the measuring electrode to operate as a protective electrode. Optionally, focusing electrodes may be arranged as discussed above.

In a measurement-while-drilling application, there is usually a small gap between the stabilizer and the borehole wall (not shown): the diameter of the drill bit (not shown) transported in the drill pipe 1040 is larger than the outer diameter defined by the stabilizers. The operation of the stabilizers will be known to experts in the field and will not be described in more detail here. Used in conjunction with a non-conductive downhole fluid, the gap determines the capacitance 107 discussed above. If necessary, extendable arms (not shown) may be provided to keep the gap within acceptable limits. Used in conjunction with a conducting borehole fluid, the size of the gap is not critical. An electronics module 1052 at a suitable location is arranged to process the data collected by the sensors 1056.

Fig. 17 illustrates the arrangement of the sensor pads on a non-rotating sleeve. This is similar to a sensor arrangement described by Thompson, although other designs can also be used. Shown is drill pipe 1260 with a non-rotating sleeve 12 62 mounted thereon. Pads 12 64 with one or more measuring electrodes 1301 are attached to sleeve 1262. The mechanism for moving the pads out into contact with the borehole, whether hydraulic, a spring mechanism or another mechanism, is not shown. The shaft 1260 is provided with stabilizer ribs 1303 to control the direction of the bore.

Data can be collected by using the design in either fig. 16 or fig. 17 while the well is being drilled and the drill string and the measuring electrodes on it are rotating. In an MWD application, the telemetry capability is extremely limited and consequently much of the processing is done downhole. Processing of the data in the present invention is carried out by using the methods described in Thompson et al. The resistivity measurements are taken simultaneously with measurements taken using an orientation sensor (not shown) on the drilling unit. When the resistivity sensor rotates in the borehole while being moved together with the drill bit, it traces out a spiral path with known depths and azimuth values. The depths are determined either from data remotely measured from the surface or by using at least two axially separated measuring electrodes to provide a penetration rate. According to one embodiment of the invention, the downhole processor uses the depth information from the downhole telemetry and sums all the data within a specified depth and azimuth sampling interval to improve the S/N ratio and to reduce the amount of data to be stored. A typical depth sampling interval would be 1 inch, and a typical azimuth sampling interval would be 15°. Another method to reduce the amount of stored data would be to discard redundant samples within the depth and azimuth sampling interval. Further details of the treatment method can be found in what is described by Thompson et al.

Although the preceding description is directed to the preferred embodiments of the invention, various modifications will be obvious to those skilled in the art. It is intended that all variations within the framework of the attached patent claims shall be covered by the preceding description.

Claims (44)

1. Measurement-while-drilling (MWD) device in a downhole device (BHA) for use in a wellbore to obtain a resistivity parameter for a bedrock formation penetrated by the wellbore, the wellbore containing a substantially non-conductive fluid, characterized in that the device comprises: (a) at least one measuring electrode, transported on and rotating with a drill string, where said measuring electrode is capacitively connected to the base formation through the non-conducting fluid, where the at least one measuring electrode conducts a modulated measuring current into the formation, and ( b) a device that responds to at least one of (i) the current in the at least one measuring electrode, and (ii) a voltage for the at least one measuring electrode to produce a measuring signal representative of the resistivity parameter. 2. MWD device according to claim 1, where the at least one measuring electrode further comprises a number of measuring electrodes, and where the resistivity parameter further comprises a resistivity image. 3. MWD device according to claim 1, where the at least one measuring electrode further comprises a group of measuring electrodes. 4. MWD device according to one of claims 1-3, further comprising a demodulator to demodulate the measurement signal and produce a demodulated signal from it, and an insulator section to isolate the at least one measurement electrode from the source of the measurement current. 5. MWD device according to one of claims 1-4, further comprising at least one focusing electrode in the vicinity of the at least one measuring electrode, the at least one focusing electrode focusing the measuring current into the formation. 6. MWD device according to claim 5, further comprising a protection device to maintain focusing of the measuring current in the formation. 7. MWD device according to claim 1, where the at least one measuring electrode is carried on the body of a sensor module which is deployed in a recess on a stabilizer for the bottom hole device (BHA). 8. MWD device according to claim 7, where the at least one measuring electrode comprises at least two measuring electrodes separated from each other in the bottom hole device's (BHA's) axial direction. 9. MWD device according to claim 7, where the at least one measuring electrode is electrically isolated from the body of the sensor module, and where the body is held at substantially the same potential as the at least one measuring electrode. 10. MWD DEVICE according to claim 1, further comprising an extension device for moving the at least one measuring electrode to maintain a specified distance between the at least one measuring electrode and the wall of the borehole. 11. MWD device according to claim 1, further comprising: (i) a telemetry device for receiving depth information from a control unit on the surface, (ii) a direction sensor for taking measurements related to the orientation of the at least one measuring electrode, and (iii) a processor to determine the resistivity parameter from the measurements taken by the direction sensor, the depth information and the measurement signal. 12. MWD device according to claim 1, further comprising a source for a modulated electric current connected to the at least one measuring electrode. 13. MWD device according to claim 12, where the modulated electric current has a carrier frequency and a modulation frequency which is significantly less than the carrier frequency. 14. MWD device according to claim 13, further comprising an insulator section between the current source and the at least one measuring electrode, the insulator section comprising conductors which lead the modulated current to the measuring electrode and the demodulated measuring signal from the at least one measuring electrode. 15. MWD device according to claim 1, wherein the measuring current has a frequency selected to make an impedance caused by a dielectric constant of the substantially non-conducting fluid to be significantly less than a resistivity of the non-conducting fluid. 16. MWD device according to claim 15, where the at least one measurement electrode is transported on and isolated from a conductive pad, where the conductive pad is maintained at a potential sufficient to maintain focusing of the measurement current into the formation. 17. MWD device according to claim 15 or 16, where the at least one measuring electrode comprises a group of measuring electrodes. 18. MWD device according to claim 15 or 16, wherein the frequency is further selected to make an impedance caused by a dielectric constant of the formation significantly smaller than a resistivity of the formation. 19. MWD device according to one of claims 15-18, further comprising a processor for determining the formation's resistivity parameter from the measurement signal. 20. MWD device according to claim 6 or 15, where the at least one measuring electrode is held at a first electrical potential, where the at least one focusing electrode is held at another potential that has a magnitude greater than the magnitude of the first potential, and the protective device is maintained at a third electrical potential substantially equal to the first electrical potential. 21. Method for obtaining a resistivity parameter for a base formation penetrated by a borehole containing mainly non-conductive fluid, characterized by the following steps: (a) transporting at least one measurement electrode on a rotating part of a measurement-while-drilling (MWD) -apparatus inside a borehole, (b) using at least one measuring electrode capacitively coupled to the underlying formation through the non-conductive fluid to pass a modulated measuring current into the formation, and (c) using at least one of (i) the current in the at least one measuring electrode, and (ii) a voltage for the at least one measuring electrode to produce a measuring signal representative of the resistivity parameter. 22. Method according to claim 21, where the at least one measuring electrode further comprises a number of measuring electrodes, and where the resistivity parameter further comprises a resistivity image. 23. Method according to claim 21, where the at least one measuring electrode further comprises a group of measuring electrodes. 24. Method according to one of claims 21-23, further comprising the following steps: (i) using a demodulator to demodulate the measurement signal and produce a demodulated signal from it, and (ii) using an insulator section to isolate the at least one measurement electrode from a source for the measurement current. 25. Method according to one of claims 21-24, further comprising: using at least one focusing electrode in the vicinity of the at least one measuring electrode to focus the measuring current into the formation. 26. Method according to claim 2 5, further comprising: using a protection device to maintain focusing of the measuring current into the formation. 27. Method according to one of claims 21-26, further comprising: transporting the at least one measuring electrode into the borehole on a bottom hole device (BHA) arranged on a drill string. 28. Method according to claim 27, where the at least one measuring electrode is carried on the body of a sensor module which is deployed in a recess on a stabilizer in the bottom hole device (BHA). 29. Method according to claim 27, where the at least one measuring electrode comprises at least two measuring electrodes separated from each other in the axial direction on the BHA. 30. Method according to claim 28, where the at least one measuring electrode is electrically isolated from the body of the sensor module, and where the body is kept at substantially the same potential as the at least one measuring electrode. 31. Method according to claim 2 8, further comprising: using an extension device to move the at least one measuring electrode to maintain a specified distance between the at least one measuring electrode and a wall in the borehole. 32. Method according to claim 2 8, further comprising: (i) using a telemetry device on the BHA to receive depth information from a downhole control unit, (ii) using a direction sensor on the BHA to take measurements related to the orientation of the at least one measuring electrode, and (iii) using a processor to determine the resistivity parameter from the measurements taken using the direction sensor, the depth information and the measurement signal. 33. Method according to claim 28, further comprising: operatively connecting a source for the modulated electric current to the at least one measuring electrode. 34. Method according to claim 33, where the modulated electric current has a carrier frequency and a modulation frequency which is significantly less than the carrier frequency. 35. Method according to claim 34, further comprising: using an insulator section between the current source and the at least one measuring electrode to isolate the at least one measuring electrode from the source of the modulated current. 36. The method of claim 21, further comprising: selecting a frequency for the measurement current to make an impedance caused by a dielectric constant of the substantially non-conductive fluid significantly less than a resistivity of the non-conductive fluid. 37. Method according to claim 3 6, where the at least one measuring electrode is carried on and isolated from a conductive pad, the conductive pad being held at a potential sufficient to maintain focusing of the measuring current into the formation. 38. Method according to one of claims 3 6-37, where the at least one measuring electrode comprises a group of measuring electrodes. 39. Method according to one of claims 3 6-38, further comprising: selecting the frequency to make an impedance caused by a dielectric constant of the formation significantly less than a resistivity of the formation. 40. Method according to one of claims 36-39, further comprising a processor for determining the resistivity parameter of the formation from the measurement signal. 41. Method according to claim 3 6, further comprising: (i) repeating steps (a) and (b) according to claim 30 at at least one further frequency, and (ii) frequency focusing apparent conductivities derived from the measured signals at said frequency and the at least one additional frequency. 42. Method according to claim 41, where the frequency focusing further comprises: representing each measurement signal by a Taylor series expansion. 43. Method according to claim 42, where the resistivity parameter is related to a coefficient for a co<3/2-> term in the Taylor series expansion. 44. Method according to claim 2 6, where the at least one measuring electrode is held at a first electrical potential, the at least one focusing electrode is held at a second potential that has a magnitude greater than the magnitude of the first potential, and the protective device is held at a third electric potential which is substantially equal to the first electric potential.