RU2065957C1 - Method for measuring electrochemical properties of formations adjacent to hole - Google Patents

Abstract

FIELD: mining and petroleum industries. SUBSTANCE: specific resistance of adjacent geological formations is measured from inside of fixed hole. Stationary measurements at predetermined depth are carried out by initial measuring step and subsequent first and second compensation steps correspondingly leading to higher accuracy of measurements. According to the method, sent from casing string to geological formation is first alternating current of first frequency, and second alternating current of second frequency is delivered through casing string. Possible is simultaneous realization of measuring step and both compensation steps needed for obtaining accurate results during displacement of device in vertical direction in fixed hole. EFFECT: high efficiency. 6 dwg

Description

 The invention relates to the oil industry and is intended to measure resistivity and other electrochemical parameters through the casing.

 A known method for determining conductivity in a formation having a cased hole, having two pairs of voltage measuring electrodes that are in contact with the casing, and which has a calibration device for calibrating variations in the thickness of the casing and errors in the placement of the electrodes (1).

 However, the known method does not exclude all types of "errors of the second kind."

 A known method for measuring the electrical resistivity of geological formations through a metal drill pipe or casing (2), in which the voltage is measured by a plurality of electrodes inside a cased hole that touch the wall of the well. However, in this method, the difference between the two pairs of electrodes for direct measurement of electronic properties in the immediate vicinity of the formations is not recorded.

 A known method of electrical measurements of casing strings of wells (3), based on the measurement by means of two pairs of voltage measuring electrodes, which are forcedly in contact with the inner surface of the casing string. However, this method does not provide for any suitable calibration device for calibrating thickness variations or electrode placement errors.

 The closest analogue to the present invention is a method for measuring the electrochemical properties of formations adjacent to a well surrounded by a casing string, comprising supplying a first sinusoidal alternating current to the casing string at a first input point in proximity to a specific part of the formation to be tested, measuring this current in a first input the point where a portion of the first current is supplied to the formation along the length of the casing string of the well, measuring stresses between a plurality of discrete pairs, consisting of at least three points measured vertically along the well casing, determining the part of the casing adjacent to a specific part of the formation to be tested, the first resistance being defined as the resistance between the first and second measuring points of three measuring points, and the second resistance is defined as the resistance between the second and third measuring points from these three measurement points, and the measurement of the electrochemical parameter of the formation according to the differential voltage D (4).

 However, this method only eliminates first-order errors and does not eliminate second-order errors.

 The aim of the present invention is to improve the accuracy of measurements by compensating for second-order errors and making measurements during the vertical movement of the measuring device in the well.

 To this end, in a method for measuring the electrochemical properties of formations adjacent to a well surrounded by a casing string, comprising supplying a first sinusoidal alternating current to the casing string at a first reference point near a specific part of the formation to be tested, measuring this current at the first input point, where part of the first current is supplied to the formation along the length of the casing of the well, measuring stresses between a plurality of discrete pairs consisting of at least three measurement points vertically along the well’s casing, determining the part of the well’s casing adjacent to the specific part of the formation to be tested, the first resistance being determined between the first and second measuring points from these three measuring points, and the second resistance being determined between the second and third measuring points from of these three measurement points and the measurement of the electrochemical parameter of the formation by the values of the differential voltages, simultaneously with the supply of the first sinusoidal alternating current, p give a second sinusoidal alternating current at the second entry point to the casing of the well, through this part of the casing determined by these spaced points of voltage measurement and to the first output point on the casing, and this second input point and this first output point are located on opposite sides of this parts of the casing defined by these spaced points of voltage measurement, and measure the magnitude of the differential stresses between the sets of discrete pairs of these three measurement points, ra carried along the casing, and also determine the magnitude of the differential voltages between this set of discrete pairs of these measurement points spaced along the casing, and using this determination of the values of the differential voltage, the first compensation determination is made from the first and second resistances and the second current measured at the second input the casing string, and the second compensation determination from the first and second resistance and from the part of the first current entering the formation in which fissioning electrochemical parameter of time-varying values of differential voltages, including first and second compensation determination along the well casing.

The invention is illustrated in the drawing, where in Fig.1 a section of a preferred embodiment of the invention, referred to as a "casing resistivity measuring instrument" (TCRT), in Fig.2 is a graph of the dependence of DI on Z, characterizing the sensitivity of the tool to various formations, in Fig.1 . 3 is a sectional view of a preferred embodiment of the invention illustrating a method for measuring V _o ; FIG. 4 is a sectional view of an embodiment of the invention that has voltage measuring electrodes spaced apart at various distances; FIG. 5 is a sectional view of an embodiment of the invention in which the electrodes are spaced apart at various distances and which illustrates a method for measuring V _o , and FIG. 6 is a sectional view of an embodiment of the invention that is adapted to take measurements during vertical movement in a cased well.

 In FIG. 1 shows a typical cased well in an oil field. The well 2 is fixed by a casing 4, which is held by cement 6 in the rock formation 8. In the vicinity of the cased well there is an oil reservoir 10.

 The casing may or may not extend electrically to ground 12. The signal voltage generator 14 supplies alternating current voltage via cable 16 to the power amplifier 18. The signal generator is the main voltage source, which includes both relatively simple devices, such as an oscillator, and relatively sophisticated electronics, similar to an arbitrary waveform generator. A power amplifier 18 is used to supply alternating current through an insulated wire 20 to electrode A, which is in electrical contact with the casing. This current can be returned to the power amplifier via cable 22 over two different circuits. If the SWI switch is connected to an electrode B, which is electrically grounded on the surface of the earth, then the current from the power amplifier mainly passes through cable 20 to electrode A and then returns through the casing and cement layer, through the rock formation to electrode B, and finally through cable 22 to a power amplifier. In this case, most of the current passes through the earth. Alternatively, if the switch is connected to an insulated cable 24, which, in turn, is connected to an electrode G, which is in electrical contact with the casing, then the current mainly passes from electrode A to electrode G through the casing, returning to the power amplifier by cable 22. In this case, a very weak current flows through the ground.

The electrodes C, D and E are in electrical contact with the inner surface of the casing. Generally speaking, the current flowing through the casing depends on the position. For example, current I flows down to electrode C, current ID flows down to electrode D, and current I _E flows down to electrode E. In general, therefore, there is a voltage difference VI of electrodes C and D, which is amplified by a differential amplifier 26. Voltage difference V2 the electrodes and D and E are also amplified by the amplifier 28. When the switches SW2 and SW3 are set to the "closed" position, as illustrated in the figure, the output signals of the amplifiers 26 and 28 are subtracted differentially on the amplifier 30. The voltage from the amplifier 30 is supplied to the surface via cable 32, per phase Sensitivity detector 34. Phase-sensitive detector receives a reference signal from the signal generator by cable 36. In addition, the digital gain controller 38 digitally controls the gain of the amplifier 28 by sending a command via cable 40 in a borehole. The gain controller 38 is also able to switch the input wires of the amplifier 20 on command, thereby actually changing the output polarity of the signal coming from the amplifier 28 for certain types of measurements.

 The total current passing to electrode A measures element 42. In the preferred embodiment shown in FIG. 1, the alternating current is a symmetrical sine wave, and therefore, in preferred embodiment 1 is the O-peak value of the alternating current passing to electrode A (O the peak value of the sine wave is 1/3 of the amplitude of the sine wave).

In general, when switch SW1 is connected to electrode B, current flows through the formation. For example, the current ΔI passes into the formation along the length 2L between the electrodes C and E. However, if the switch SW1 is connected to the cable 24 and therefore to the electrode F, then the current does not flow through the formation to the electrode B. In this case, I _C I _D I _E , since substantially the current ΔI passes through the formation.

 It should be noted that if the switch is connected to electrode B, then the current tends to pass through the formation, and not along the well casing. Calculations show that with a 7-inch outer diameter of the casing and 1/2 inch wall thickness, if the formation resistivity is I Ohm-meter and the formation is homogeneous, approximately half of the current will break away from the casing and go into the formation along a section of 320 meters casing string. For a homogeneous formation with a resistivity of 10 ohm-meter, this length will be 1040 meters.

 One embodiment of the invention provides a preferred method of operation for the device described above: The first step in measuring the formation resistivity is to “balance” the tool. The switch SW1 is connected to the cable 24 and therefore to the electrode F. Then, an alternating voltage current is passed from the electrode A to the electrode F through the casing of the well. Although a very insignificant current leaves the formation, the voltages V1 and V2 are generally speaking different due to variations in the thickness of the casing, inaccurate placement of the electrodes and many other factors. However, the amplifier 28 is tuned by the gain controller so that the differential voltage V3 becomes zero. (Amplifier 28 can also be equipped with phase-balancing electronics if it is necessary to provide zero at any given operating frequency). Therefore, if the electrodes are then left in the same place after balancing to zero, spurious effects, such as variations in the thickness of the casing, will not affect subsequent measurements.

 If then switch SW1 is connected to electrode B, the signal generator drives a power amplifier that conducts current to electrode A, which is in electrical contact with the inner surface of the well casing. Here, a current with an amplitude of 1 ampere to 30 amperes with a frequency of 1 Hz is supplied to the casing, as a rule. Low-frequency operation is limited to electrochemical effects similar to the polarization phenomenon and the invention can be used at frequencies up to 0.1 Hz, obtaining the correct results of measurements of resistivity. High-frequency operation is limited by the surface effects inherent in the casing, and for measuring resistance in accordance with the invention, the upper limit frequency is probably a frequency of 20 Hz. Then, current is passed through the casing up and down from electrode A and some of the current passes through the cement saturated with drill water surrounding the casing and through different resistivity zones surrounding the casing. The current then returns to the earth's surface through electrode B.

 Thus, when operating in the preferred manner described above, first the instrument is balanced on the zero signal by the output amplifier 30 when the switch SW1 is connected to the cable 24 and then the deviation of this signal from zero when the switch SW1 is connected to the electrode B serves as a measure of current leakage in formation. This way of working does not automatically exclude all "second-order measurement errors." An improved method of operation is described in the last section of this application, moreover, this method automatically eliminates these "second-order measurement errors."

In FIG. Figure 2 shows the measurement of the differential current ΔI, going into the formation, for different depths Z inside a well cased with steel pipes. Z means the depth at which the electrode D in FIG. 1 is located. It should be noted that when voltage is applied to the electrode A and the switch SW1 is connected to the electrode B, a radially symmetric electric field appears, applied to the formation, which is approximately perpendicular to the casing. This electric field creates outward currents similar to the current ΔI in FIG. 1, which are inversely proportional to the formation resistivity. Therefore, we should expect discontinuities in the current ΔI at the junction of zones of different resistivities, in particular, at the oil-water and oil-gas boundaries. For example, curve (a) in FIG. 2 illustrates the result for a homogeneous formation with resistivity ρ ₁ . Curve (c) shows the deviation from curve (a) when the formation intersects with resistivity ρ ₂ and thickness T ₂ , where ρ _{2 is} less than ρ ₁ . And curve (c) illustrates the opposite situation when the formation intersects with a resistivity of ρ ₃ that is greater than ρ ₁ and a thickness of T ₃ . Obviously, in such circumstances, ΔI _{3 is} less than ΔI ₁ , which is less than ΔI ₂ .

 Figure 3 illustrates in detail a method for measuring a parameter.

Electrodes A, B, C and E and F are defined with reference to Fig. 1. All elements digitized by positions 2 through 40 are already defined with reference to Fig. 1. In Fig. 3, the thickness of the casing is indicated by τ ₁ , the thickness of the cement layer is τ ₂ and the diameter of the casing is d. Switches SW1, SW2 and SW3 are also defined in fig. 1. Additionally, Fig. 3 shows electrode C, which is the reference electrode for voltage measurement and which is in electrical contact with the earth's surface. This electrode is used as a reference electrode and transmits negligible current to avoid measurement errors induced by current.

 Additionally, in FIG. Figure 3 shows switch SW4, which allows cable 24 to be connected to one of three positions: and an open circuit, to electrode G or to the upper end of the well casing. Additionally, FIG. 3 shows switches SW5, SW6, and SW7 that can be set to the illustrated positions. (The device shown in Fig. 3 can be used in exactly the same way as shown in Fig. 1, provided that the switch SW2, SW5, SW6 and SW7 are set to the positions opposite to those in which they are shown in Fig. 3 and if switch SW4 is in the open circuit position).

If the switches SW2, SW5, SW6 and SW7 are set as shown in FIG. 3, then the value of V _o can be measured. When a given current I flows to electrode A, the potential of this casing point rises relative to the zero potential of a hypothetical point infinitely remote from the casing. In the interval of the casing between the electrodes C6 D and E in figure 3 there is an average potential relative to an infinitely distant reference point. However, the potential measured between only the electrodes E and C is approximately equal to V _o provided that the distance between the electrodes A, C, D and E is less than a certain critical distance, let it be 10 meters, and provided that the electrode C is at a distance exceeding some other critical distance from the casing, for example, 10 meters from the well casing. The output signal of the amplifier 28 depends on the voltage difference across the electrode E and the other input of the amplifier to which cable 24 is connected. When switch SW1 is connected to electrode B and switch SW4 is connected to electrode G, cable 24 is at substantially the same potential as electrodes G and V _o can be measured by a phase-sensitive detector 34. In many cases, switch SW4 can be connected to the upper end of the casing, which also allows measurement if the electrode A is below a certain "critical depth" .

 To make the description of the invention accurate, let's say that electrode A is the upper conductive electrode that is in electrical contact with the inner surface of the casing, electrode B is the conductive electrode that is in electrical contact with the earth’s surface, the electrodes C, D and E are measuring voltage with electrodes that are in electrical contact with the inner surface of the casing, electrode F is the lower conductive electrode that is in electrical contact with the inner the casing surface, and the electrode C is a reference electrode in measuring the voltage, which is in electrical contact with the ground.

Further, V _o is called the local casing potential. An example of an electronic differential device is a combination of amplifiers 26, 28 and 30. The differential current flowing into the formation and to be measured is denoted by ΔI. The differential voltage is the voltage at the output of the amplifier 30 (see figure 1), when the switch SW1 is connected to the electrode B and all other switches are in the positions shown in the figure.

Figure 4 is almost identical to Figure 1, except that the electrodes C and D are separated by a distance L ₁ , the electrodes D and E are separated by a distance L ₂ , the electrodes A and C are separated by a distance L ₃ , the electrodes E and F by a distance L ₄ . Additionally, r ₁ is the radial distance of the electrode B from the casing. And Z is the depth from the surface of the earth to electrode D.

FIG. 5 is almost identical to FIG. 3, except that the distances L ₁ , L ₂ , L ₃ , L ₄ , r ₁ and Z are also clearly indicated here. Additionally, r ₂ is defined as the radial distance from the casing to the electrode C. As will be clear from the subsequent analysis, the invention will work well if L ₁ and L _{2 are} not equal. For various types of measurements, the distances L ₃ and L ₄ do not have a significant effect, provided that they do not significantly exceed L ₁ and L ₂ .

 Below is a description of an improved method of operation of a preferred embodiment of the invention with reference to figure 1. Using this “preferred mode of operation” described below, the device can be brought into one of three states: (1) “sensitive measuring state”, (2) “preferred zero state”, (3) “preferred calibration state”.

These three states defined by the preferred mode of operation of the present invention are defined as follows:
(1) The preferred measurement state of the device of FIG. 1 is determined by the following configuration: SW1 is connected to electrode B, switch SW2 is closed and switch SW3 is closed.

 (2) The preferred zero state of the device of FIG. 1 is determined by the following configuration: the switch SW1 is connected to the cable conductor 24 and therefore is electrically connected to the electrode F, the switch SW2 is closed and the switch SW3 is closed.

 (3) The preferred calibration state of the device shown in FIG. 1 is determined by the following configuration, the switch SW1 is connected to the cable 24 and therefore is electrically connected to the electrode F, the switch SW2 is open and the switch SW3 is closed.

The purpose of the subsequent analysis is to determine the method of work when evaluating the experimental value ΔI in figure 1, which would be relatively accurate, which would be relatively insensitive even to significant variations in the thickness of the casing and which would not substantially depend on the inaccuracy of the location of the electrodes. Therefore, we propose that the amplifiers 26, 28 and 30 in FIG. 1 create gains, respectively, a ₁ , a ₂ and a ₃ . Further, suppose that the resistance of the string between the electrodes C and D is equal to R ₁ Ohm and that the resistance of the casing between the electrodes and E is equal to R ₂ Ohm. In the following analysis, solely for the purpose of simplification, it is assumed that: (a) there is no phase shift between the amplifiers, (b) there is no phase shift between the voltages applied to the casing and the current passing through the earth (caused by polarization effects, surface effects, or other electrochemical processes) (c) therefore, values representing the amplitudes of low-frequency alternating current quantities can be used in the subsequent simplified analysis.

The average resistance of the casing between the electrodes D and E, denoted as the value of R _A , gives the following expression:
R _A (R ₁ + R ₂ ) / 2 (1).

Thus, there is a deviation from the average resistance of the first section of the casing between the electrodes C and D, denoted by ΔR ₁ , satisfying the equation:
R ₁ = R _A + ΔR ₁ (2)
Next, there is a deviation from the average resistance of the second section of the casing between the electrodes D and E, denoted by ΔR ₂ that satisfies the equation:
R ₂ = R ₁ + ΔR ₂ (3) (3)
When the device is in the preferred zero state, the so-called zero current (denoted by IN) passes through the casing between electrodes C and E. Since it is expected that a relatively small current will flow through the formation in this state, then substantially the same current IN flows between electrodes C and D and between the electrodes D and E. Therefore, the output voltage created in this situation by the amplifier 30 and denoted by V _N gives the following expression:
V _N = a ₃ {a ₂ I _N (R _A + R ₂ ) -a ₁ I _N (R _A + R ₁ )} (4)
Regrouping the terms of equation 4, we have:
V _N = a ₃ • γ • I _N (5)
The value of γ in equation 5 is determined by the following algebraic formula:
g = a ₂ {(R _A + ΔR ₂ ) -a ₁ (R _A + ΔR ₁ )} (6)
In the preferred measuring state of the device, the full measuring current, denoted I _T , passes between the electrodes A and the electrode B at a distance of the earth. Only part of the total current flows down the casing between electrodes C and D, and this part is simply called the "measuring current" and is denoted by _IM . If I _M passes between the electrodes C and D, then a certain current flows from the casing between the electrodes D and E, which is defined as the value of δ _i2 . Therefore, the current passing down to the electrode E, denoted by I _E , gives the following expression:
I _E = I _M -δ _i2 (7)
Therefore, the voltage at the output of amplifier 30, denoted by W _m , in such a situation, when approximated by a model with lumped components, it gives the following expression:
V _m = a ₃ {a ₂ (I _M -δ _i2 ) R _A + ΔR ₂ (-a ₁ I _M / R _A + ΔR ₁ )} (8)
Equation 8 can be reduced to a simplified form:
V _m = a ₃ • γ • I _M -a ₂ a ₃ δ _i2 (R _A + ΔR ₂ ) (9)
In the device in the preferred calibration state, the calibration current I _CA passes through the casing between the electrodes C and E. Since it is expected that a relatively small current will flow through the formation in this state, substantially the same currents I _CA flow between the electrodes C and D and between the electrodes D and E. In the preferred calibration state, the switch SW2 is open, therefore, in this state, the output signal V _{CA of the} amplifier 30 gives the following expression:
V _CA = -a ₁ a ₃ I _CA (R _A + ΔR ₁ ) D (10)
Here, the parameter D is a division factor or a gain reduction parameter introduced to avoid saturation of the amplifier 30 with the switch SW2 open, although the device implementing it is not explicitly shown in FIG. 1. Basically, when SW2 is open, the output of amplifier 30 is divided by parameter D, which was recognized as a useful input for field testing. In this embodiment, gain controller 38 in FIG. 1 has at least the ability to open and close switches SW2 and SW3 and divide the output signal of amplifier 30 by a factor D.

Three main different independent measurements are carried out, the meaning of each of which is given by one of the following equations: equation 5, equation 9 and equation 10. The coefficients a ₁ , a ₂ and a _{3 are} assumed to be known with an accuracy of 0.1%. The currents listed below are measured for three different conditions: I _N , I _M and I _CA. Thus, equation 5,9,10 contains three basic unknowns: δ _i2 , R ₁ and R ₂ . Three equations with three unknowns always have a unique solution. Such a solution, which is used in practice to minimize the required measurement accuracy, is described below.

The convenient parameter ΔV, which must be calculated for the analysis presented here, is determined by the following expression:
ΔV = V _M -V _N (11)
Representing V _M from equation 9 and V _N from equation 5, has the following expression for ΔV:
ΔV = a ₃ • γ • (I _M -I _N ) -a ₂ a ₃ δ _i2 (R _A + ΔR ₂ ) (12)
Using equation 10 and resolving with respect to R _A , we have:
R _A = {-D _CA (a ₁ a ₃ I _CA )} - ΔR ₁ (13)
We substitute R _A from equation 13 into equation 12, we have two quantities ΔV expression:
ΔV = (a ₂ / a ₁ ) (σ _i2 / I _CA ) D • V _CA (member A)
+ a ₃ • γ • (I _M -I _N ) (term B) (14)
-a ₂ a ₃ σ _i2 (ΔR ₂ -ΔR ₁ ) (member C).
Equation 14 is the main result of this analysis.

Equation 14 has three members, which are indicated by adjacent letters and in square brackets. Member A provides useful information regarding the measurement of leakage current flowing into the formation. Member B is a first-order error that occurs if the measuring current I _{M is} not equal to the zero current I _N and if different parts of the casing have different resistances. Member C is a second-order error, which is equal to the product of the difference of the resistances of different parts of the casing and the leakage current going into the formation. Member C is called a second-order term because it is equal to the product of two quantities, each of which is extremely small under ideal conditions.

To give a presentation on the comparative numerical values of the terms A, B and C and their comparative significance, these terms will be calculated for the following values:
a ₁ a ₂ A (15)
a ₃ 10
D 10
ΔR ₁ = 0.1R _A
ΔR ₂ = + 0.3R _A
I _N I _CA I _o .

Further, to emphasize the importance of the term B, which is a first-order error, suppose that the currents I _M and I _{N are} almost equal, which determines the equality:
I _M I _N -0.01 • I _o (16)
First, equation 10 is used to calculate the value of V _CA in member A of equation 14, then equations 15 and 16 are substituted into equation 14, equation 6 is used to calculate γ, as a result of which for equation 14 we get:
term A = -0.80 A • a ₃ δ _i2 R _A (17)
term B = -0.004A • a ₃ • I _o • R _A (18)
term C = -0.40A • a ₃ • σ _i2 • R _A (19)
In this example, the C term that defines the second kind of error is significant. Member A carries information regarding leakage current from the casing:
Member C / Member A 0.44 (20)
The term “second-order error” causes a 44% error in calculating the leakage current, unless member A of equation 14 is used to calculate the leakage current to the formation. Therefore, we can conclude that in order to increase the accuracy of measurements, it is necessary to improve methods for compensating second-order errors.

When the leakage current is about 30 milliamps and the current I _o is 5 amperes, the ratio of members B and A is:
Member B / Member A 0.74 (21)
Therefore, if the currents I _M and I _N are equal with an accuracy of only 1% as in equation 16, 74% measurement errors can be expected in this example, if only the term A in equation 14 is used to calculate the leakage current to the formation.

The result of equation 21 also emphasizes an important aspect of this analysis. If the currents I _M and I _N would differ by 50%, which is typical for the case when the total currents supplied to the casing through the ammeter 42 remain the same for different states of the switch SW1, then in the given example, errors in measuring this value could at least 35 times more. To compensate also for large errors, the resistances R ₁ and T ₂ should be measured with an accuracy of 5 decimal places to ensure the required accuracy of the measurement of the leakage current. This 5-bit accuracy is very difficult to achieve due to cross-distortion in communication lines, magnetic pickups and other factors that should be considered in another related application. In contrast to this method of operation, the one described above requires the measurement of various quantities only with constant accuracy, usually characterized by 3-decimal places, which is much easier to put into practice.

Therefore, when using one embodiment of the invention, the currents I _M and I _N should be equated with an accuracy of one thousandth (1.0%) in order to obtain acceptable measuring errors of the leakage current with an accuracy of better than 10% if term B in equation 14 is ignored when interpreting the data.

Therefore, in order to avoid first-order measurement errors in practice, if term B is ignored (and a ₁ a ₂ A), the absolute value (I _M I _N ) ≅ 0.001 • I _o (22).

Первая строка уравнения 26 имеет соответствующие члены А и С из уравнения 14. Вторая строка уравнения 26 соответствует члену В в уравнении 14. Обратите внимание, что этот второй член представляет собой математическое произведение разности сопротивлений между различными участками обсадной колонны и разности измерительного и нулевого токов, являясь членом "ошибки первого порядка" и который очень велик, если не сделать измерительный и нулевой токи равными по причинам, обсужденным выше.Equation 14 is the main result and is used to interpret the data. Since the comparative values of the various members are established, the logic below is used to convert equation 14 to a form that is used in one embodiment of the present invention. In particular, if the amplification factors of the amplifier 26 and 28 are equal, then for equation 14 it is assumed:
a ₁ a ₂ A (23)
Substituting equation 23 into equation 6, for the quantity γ we obtain the following expression, which can be used to simplify the term B in equation 14:
g = A (ΔR ₂ -ΔR ₁ ) (24)
The term C in Equation 14 can be transformed to another form that is more suitable for analysis if Equation 23 is substituted into Equations 5 and 6, resulting in:
(ΔR ₂ -ΔR ₁ ) = V _N / (a ₃ • A • I _N ) (25)
Equation 24 is then used to simplify term B in equation 14 and equation 25 is used to simplify term C in equation 14, resulting in:

The first line of equation 26 has the corresponding members A and C from equation 14. The second line of equation 26 corresponds to term B in equation 14. Note that this second term is a mathematical product of the difference in resistance between different sections of the casing and the difference between the measuring and zero currents, being a member of the "first-order error" and which is very large, if not to make the measuring and zero currents equal for the reasons discussed above.

In a preferred mode of operation, the experimental parameters can be adjusted as follows:
I _M I _N I _{o o} I cm (27)
In practice, these currents are made equal, requiring that the voltage at the output of amplifier 30 with switch SW2 in the open state and switch SW3 in the closed state be the same for both states of switch SW1. For each position of the switch SW1, the necessary adjustments are made to the output voltage of the signal generator 14 shown in FIG. 1 so that the current flowing between electrodes C and D remains the same in a preferred embodiment of the invention.

If equation 27 is exactly satisfied, then term B in equation 26 is zero. Using equation 11 to exclude ΔV in equation 26 can lead to the following form:
δ _i2 = I _o [V _M -V _N ] / [DV _CA -V _N ] (28)
This is a result of paramount importance, which determines the value of δ _i2 , which is not affected by second-order measurement errors due to various resistances in neighboring sections of the casing. The value of δ _{i2 is} also not affected by the first order of measurement error.

Therefore, at least three measurements of various quantities must be performed in order to eliminate essentially measuring errors and obtain accurate results. In the above case, three independent measurements are performed to measure three experimentally independent quantities: δ _i2 , R ₁ and R ₂ . In other words, the first and second compensation measurements are performed to correct the third measurement of leakage current into the formation from the casing to obtain accurate results.

 Further, the device was used in three different configurations, which ensure its use as a first compensating device (preferred zero state) and then as a second compensating device (preferred calibration state) to obtain accurate measurements of leakage current (in a preferred measuring state).

 It should be briefly noted that equation 6 in the known method (1) is used to exclude various types of measurement errors. This equation is analyzed in the light of the above concepts. In other words, equation 6 effectively eliminates first-order measurement errors similar to term B in equation 14. However, it does not exclude second-order measurement errors similar to term C in equation 14. The above example shows that second-order measurement errors are significant and that a way is needed, consisting of two independent compensation steps and one measuring step, to eliminate these second order measurement errors.

The prototype (4) describes an analysis method for determining the resistivity of adjacent formations. See, in particular, Equations 6.8 and Equation 9 as directed in the Certificate of Change. However, this patent deals with ΔI, which is the total current loss between the electrodes C-D and between the electrodes D E. However, here this method of operation gives a difference between the current loss δ _i1 between the electrodes C and D and the current loss δ _i2 between electrodes D and E. Therefore, using approximation by a model with lumped components, we have:
δ _i2 = ΔI-δ _i1 (29)
To determine the resistivity of an adjacent formation, the following analysis should be performed. The contact resistance R _{c of a} cylindrical electrode of length t and diameter d in contact with the formation of resistivity ρ is calculated by the following formula:
R _c = ρ {L _n (4t / d)] / (2πt) = ρc. (thirty)
The geometric constant C is determined by the theoretical justification of equation 30. The experimentally measured value of the contact resistance R _c gives the following formula:
R _c = V _c / δ _i2
The value of V _o in equation 31 is equal to the measured potential of the casing at a current I _o included in equation 28. Therefore, the resistivity ρ gives the expression:
r = V _o / (C • δ _i2 ) (32)
It is very convenient to determine the new parameter K as follows: KI / C (33)
Then equation 32 can be rewritten:
ρ = KV _o / δ _i2 (34) (34)
Using the definition of R _c from equation 31, you can convert equation 34 to the final form:
ρ = KR _c (35)
Equation 35 is the end result used to interpret the data, the preferred mode of operation. The parameter K has a dimension in meters, which is customary because R _o is measured in ohms and resistivity is measured in units of ohm meters. Any values of the parameters K calculated according to equations 30 and 33 for a given diameter of the casing string and the distance between the voltage measuring electrodes are justified by the reliability of the argument that the section of the casing string of length t can be approximated by an ellipsoid of revolution. Therefore, it should be expected that any value of K calculated in accordance with this analysis will not be very accurate due to such an approximation. There is reason, however, to expect that the parameter K can be determined empirically with sufficient accuracy. Calibrating a device using a formation with a known resistivity that is adjacent to the casing and whose resistance was measured before installing the casing is sufficient to empirically determine parameter K. Considering equations 30 and 33, it can be expected that parameter K will be a function of pipe diameter . Based on these two equations, it can be expected that the parameter K will be a function of the distance between the voltage-measuring electrodes.

 The second main part of this specification with reference to Fig. 6 describes a device adapted to collect data during movement, which allows the implementation of the previous fundamental analysis. A preferred device, optimally adapted to collect data while moving and implementing the fundamental analysis methods described above, needs a machine that simultaneously collects data similar to those collected in a preferred measurement state, preferred zero state, and preferred calibration state. The device shown in FIG. 6, it is adapted to collect data in a similar preferred measuring state at a first operating frequency denoted by “F (I)” possibly 1 Hz, it is adapted to simultaneously collect data in a similar preferred zero state at a second operating frequency denoted “F (2)” possibly , 2 Hz, it is capable of simultaneously collecting data in a similar preferred calibration state, which is used to maintain the current flowing between electrodes C and D at two operating frequencies, such as that required by the drive Equation 27 above.

In order to ensure consistency, the elements of FIG. 6 are first briefly compared with the elements in figures 1 to 5. This introduction also serves to decipher the various inscriptions in FIG. 6. Elements N 2, 4, 6, 8 and 10 are already described. The electrodes A, B, C, D, E, F and distances L ₁ , L ₂ , L ₃ , L ₄ have already been described. The values of δ _i1 and δ _{i2 are} already described above. Amplifiers with the designations A1, A2 and A3 are similar, respectively, to the amplifiers 26, 28 and 30 shown in Figures 1, 3, 4, 5. Additionally, the device shown in Fig.6 has the following: (a) two signal generators marked " SGIat F (I) "and" GI atF (2) "and operating respectively at the first and second operating frequencies,
(b) two power amplifiers, marked with the words "RA 1" and "RA 2".

 (c) five phase-sensitive detectors labeled "PSD 1", "PSD 2", "PSD 3", "PSD 4" and "PSD 5", which respectively each have a measurement input labeled SIG, each input for the reference signal indicated by "REF", each on the output indicated by the arrow directed from the corresponding device and each of which does not perceive any signal voltage at frequencies not equal to that of the corresponding reference signal.

(d) "differential error amplifier", indicated by "Error Difference amp",
(e) a device that voltage controls the gain, referred to as a "voltage controlled gain device", designated "VCG".

(e) an additional conductive electrode labeled “H” (which is located at a distance L ₅ above electrode A, not shown),
(g) an additional voltage measuring electrode marked “I” (located at a distance L ₆ below the electrode F, not shown).

(h) current-measuring devices or ammeters indicated by "11" and "12",
(i) and a differential voltage amplifier indicated by “A4” in FIG. 6.

 The device operates as follows.

As with the similar "preferred measurement state", the SGI signal generator generates a basic signal voltage to the SCG, which determines the output level of RA I, which passes current at a frequency F (1) through meter 11, through cable 44 to electrode A and then between the electrodes A and B through the geological formation, and from the electrode B through the cable 46 back to RA 1. Thus, a voltage drop is created at a frequency F (1) along the casing between the electrodes CD and DE. Amplifier A1 senses the voltage difference of the electrodes CD, amplifier A2 senses the difference in voltage of the electrodes DE, and amplifier A3 senses the voltage difference at the outputs of amplifiers A2 and A1, respectively, which is transmitted upward through the cable to PSD 1 and PSD 3. The voltage measured PAD 1, as defined above V _M.

As with the similar “preferred zero state”, signal generator SG 2 generates a basic signal voltage that determines the output level of PA 2, which passes current at frequency F (2) through meter 12, through cable 50 to electrode H through a section of casing to the electrode F and vice versa through cable 52 to RA 2. Thus, a voltage drop is created at a frequency F (2) along the casing between the electrodes CD and DE. Amplifier A1 senses the voltage difference of the electrodes CD, amplifier A2 senses the voltage difference of the electrodes DE, and amplifier A3 senses the voltage difference at the outputs of the respective amplifiers A2 and A1, which is then transmitted upward through cable 48 to PO 1 and PO 3. Voltage, measured by PO 3, similar to that defined above V _N.

 As with the similar “preferred calibration state”, the voltage drop between the CD electrodes, amplified by amplifier A1, is fed through cable 54 to PSD 2 and PSD 4. PO 4 measures a value characterizing the current at a frequency F (1) through the casing section between the electrodes CD, PSD 2 measures a value characterizing the current at a frequency F (2) through the section of the casing between the electrodes CD. The output signals PSD 2 and PSD 4 are supplied via cables 56 and 58, respectively, to a differential error amplifier, which supplies an error signal to VCG via cable 60. The voltage applied by SG1 through cable 62 to VCG characterizes the basic level of current passing through meter 11. However, , the error voltage transmitted by cable 60 to the VCG causes adjustment of the current passing through 11, so that the currents at frequencies F (1) and F (2) passing through the casing section between the electrodes C and D are equal with an accuracy determined by equation 22.

To determine the resistivity, you need to have a potential voltage V _o . The electrode I provides voltage at one of the two inputs of amplifier A4, and the other input voltage creates electrode C. The differential voltage signal from the output of amplifier A4 is fed to PO 5 via cable 64. PSD 5 receives the reference voltage at frequency F (1) from SG1. V _o measure PSFD 5 at a frequency of F (1). Thus, the recording and calculated system is provided with information sufficient to calculate the resistivity in a form similar to equation 32.

 For the sake of clarity, note that SG1 generates an output voltage at frequency F (1), its output is connected to the node “voltage” 66, which, in turn, is connected to the reference inputs PSD 1, PSD 4 and PSD 5. Next , note SG 2 generates an output voltage at frequency F (2), its output is connected to the voltage node 68, which, in turn, is connected to the reference inputs PSD 2 and PSD 3.

The digital recording system 70 is labeled “DIG REC SYSTEM”. It is able to collect data, perform calculations on the oncoming computer and output the data processing results in the form of logging schemes, as is customary for well logging. The output of PSD 5 is connected to the input of the digital recording system by wire 72. Similarly, the output of PSD 1 is connected to the input of the digital recording system by wire 74. However, to avoid unnecessary complexity in Fig. 6, only a piece of wire (indicated by 74) is shown in the form of an arrow PSD 1 and only the end of this wire, directly connected to the input of the digital recording system, is shown as an incoming arrow (also labeled 74). In general, the outgoing digitized arrow on a phase-sensitive detector represents a wire connected to its input. The arrow entering the digital recording system is a wire connected to its input. An arrow emanating from a phase-sensitive detector, having a specific number, must be connected by a wire to the corresponding arrow included in the digital recording system. Thus, the output of the PSD 3 is connected to the input of the digital recording system by a wire 76. The output of the PSD 2 is connected by a wire 78 to the input of the digital recording system. The output of the PSD 4 is connected by a wire 80 to the input of a digital recording system. Additionally, wire 82 connects the output of amplifier A2 to the input of amplifier A3. Many coupled phase-sensitive detectors have digital outputs. Such digital outputs can be considered ideal for supplying digital data to a computer built into a digital recording system. On the other hand, many older models of phase-sensitive detectors have only similar inputs. In this case, the corresponding inputs of the digital recording system should obviously be analog-to-digital converters that communicate with the computer built into the digital recording system. This embedded computer is capable of performing standard mathematical calculations. The data thus obtained provide enough information to calculate the leakage current, similar to that defined by equation 28. The exact algebraic expression acceptable for the device depends on the coefficients selected for amplifiers A1, A2, A3 and A4, however, it is very simple to determine the equation, similar to equation 28 for a mobile system. After that, the computer will be able to calculate the leakage current to the formation. The information supplied to the digital recording system is also sufficient to determine the potential voltage V _o . The computer is able to calculate the resistivity of the adjacent geological formation using the formula 34. The dependence of the resistivity on the depth of the borehole is given in the form of a typical logging pattern typical of borehole logging.

Summarizing, we note: PSD 1 measures a value similar to V _m , PSD 3 measures a value similar to V _n , PSD 2 and PSD 4 are used to maintain a constant current between the electrodes C and D at two frequencies, and PSD 5 measures V _o . The recording and calculation system calculates the leakage current into the formation, the local potential casing stress and the resistivity of the adjacent geological formation while the system is moving up the well.

 Other components commonly used in logging operations should be equipped with a device, but they are not shown, however, among them are tools for measuring depth, logging machines and cables, housings for instruments and devices lowered into the well, etc. It is assumed that to create a detailed design and manufacture of the device should be used the values of the achieved level of technology. Typical device debugging methods should be used, including empirical determination of the maximum vertical lift speed of the device by gradually increasing the speed until a critical speed is reached at which the results are not consistent with the results obtained from stationary measurements or at lower speeds.

 Although the above description has many specifics, they should not be construed as limitations on the scope of the invention, but merely as an explanation of an example of preferred options. Further, the described device and method can be used at least for the following methods: localization of unopened oil and gas, evaluation of porous rocks, observation of water flows, measurement of quantitative saturations, permeability measurements and measurements through a drill string with a drill bit attached to it. Accordingly, the scope of the invention is not determined by the illustrated options, but is determined by the attached formula or its equivalent. YYY2 YYY4

Claims (1)

 A method for measuring the electrochemical properties of formations adjacent to a well surrounded by a casing string, comprising supplying a first sinusoidal alternating current to the casing string at a first input point near a specific part of the formation to be tested, measuring this current at a first input point where a portion of the first current is supplied to formation along the length of the casing of the well, measurement of stresses between a plurality of discrete pairs consisting of at least three measurement points spaced vertically o along the well casing, determining the part of the well casing adjacent to the specific part of the formation to be tested, the first resistance being determined between the first and second measuring points from these three measuring points, and the second resistance is determined between the second and third measuring points from these three measuring points and measuring the electrochemical parameter of the formation according to the values of differential voltages, characterized in that at the same time as the first sinusoidal variable they supply a second sinusoidal alternating current at the second input point to the casing of the well through this part of the casing of the well, determined by spaced points for measuring stress, and to the first output point of the casing of the well, the second input point and the first output point located on opposite sides of the part well casing defined by spaced stress measuring points, and differential stress values between a plurality of discrete pairs of three measuring points are measured I, spaced along the casing of the well, and also determine the magnitude of the differential stresses between the set of discrete pairs of these measurement points spaced along the casing of the well, and using this determination of the values of the differential voltage, the first compensation determination is made from the first and second resistances and from the second current, measured at the second input point of the casing, and the second compensation determination is carried out from the first and second resistances and from part of the first current, going to the formation in which the electrochemical parameter is determined by the time-varying differential stresses, including the first and second compensation definitions along the well casing.

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