RU2384867C1 - Electrical cased well logging method - Google Patents

Abstract

FIELD: physics; geophysics.

SUBSTANCE: invention relates to well logging and can be used in determining electrical resistance of rock formations surrounding a well cased by a metal column. The method involves using a probe consisting of three equidistant measuring electrodes and two current electrodes placed outside the zone of the measuring electrodes. Inphase electrical currents are simultaneously transmitted into the column through both current electrodes in defined proportions. The proportion of the currents is changed and inphase electrical currents are simultaneously transmitted into both current electrodes in the altered proportion. During transmission, the currents themselves are measured, as well as the electric field potential at the contact point between the middle measuring electrode and the column, the first potential difference on the section of the column between contacts of the two outermost measuring electrodes and the second potential difference on the same section of the column. Resistivity is determined using a corresponding formula.

EFFECT: increased accuracy, wider range of measurements.

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Description

The invention relates to the field of geophysical research of wells and may find application in determining the electrical resistance of rock formations surrounding a cased metal column well.

A known method of electric logging cased wells (RF patent No. 2172006, publ. 08/10/2001 [1]). The method includes measuring the electric field potential and its second difference using a single-pole four-electrode probe in contact with the casing. The probe is made in the form of three equidistant measuring electrodes and three current electrodes, two current electrodes are located symmetrically with respect to the middle measuring electrode, the third electrode is located in the middle at the level of the middle measuring electrode and connected to the column at a point that is not aligned with the point of contact with the column of the middle measuring electrode . Each of the three current electrodes is alternately supplied with electric current from the same pole of the source. At each of the three current supplies, the electric field potential of the middle measuring electrode is measured, the first potential difference between the two extreme measuring electrodes, the second potential difference. Electrical resistivity is determined by the corresponding formula.

The disadvantage of this method is the difficulty of its practical implementation, namely the correct measurement of the second potential difference when applying current to the third electrode, located at the level of the middle measuring electrode.

A known method of electric logging cased wells (RF patent No. 2200967, publ. 03/20/2003 [2]). The method uses a probe consisting of three measuring electrodes and two pairs of current equidistantly spaced along the column, one of which is located above the outside of the measuring electrodes, and the other is lower. Current is supplied to the column through each of both pairs of current electrodes from two probe circuits located outside the measuring circuits and current generator meters. At each current supply, the potential of the electric field of the column is measured at the point of contact with it of the middle measuring electrode, the first and second potential differences in the section of the column between the two extreme measuring electrodes. Electrical resistivity is determined by the corresponding formula.

The disadvantage of this method is the difficulty of its practical implementation: to obtain sufficient amplitudes for measuring the first and second potential differences, current dipoles hundreds of meters long are required.

Closest to the invention in technical essence is a method of electric logging of cased wells (RF patent No. 2176802, publ. 10.12.2001 [3]). The method uses a probe consisting of three equidistant measuring electrodes and two located outside the zone of the measuring electrodes, symmetrically with respect to the middle measuring electrode, current electrodes. An electric current is alternately supplied to the column through each of the two current electrodes from the same pole of the source. At each current supply, the potential of the electric field of the column is measured at the point of contact with it of the middle measuring electrode, the first potential difference in the section of the column between the two extreme measuring electrodes and the second potential difference in the same section of the column. Electrical resistivity is determined by the corresponding formula.

The disadvantage of this method is that the probe is powered by currents, the main shares of which within the measuring electrodes flow through the column and are thousands of times higher than the fractions of currents flowing into the formation within the same measuring electrodes. As a result, the accuracy of determining the parameters of the reservoir is low, and the measurement range is limited.

The proposed method solves the problem of increasing the accuracy and expanding the range of measurement of formation parameters by simultaneously supplying currents above and below the measuring electrodes - balancing currents to significantly, hundreds of times, reduce the fractions of currents flowing within the measuring electrodes along the column without reducing the fractions of currents into the reservoir within the same measuring electrodes.

The problem is solved in that in the method of electric logging of cased wells, including the supply of electric currents, measuring the potential of the electric field and its second difference using a multi-electrode probe of the second difference, made in the form of three equidistant measuring electrodes and two current, upper and lower, which are located outside the measuring electrodes, according to the invention, a probe supply method is used when simultaneously coinciding phase electric currents are supplied to both current electrodes of a certain proportion, the proportion of currents is changed and simultaneously the phase currents of electric currents are fed into both current electrodes in a changed proportion, and at each of the current supply currents themselves are measured, the electric field potential at the contact point of the middle measuring electrode with the column, the first potential difference in the section of the column between the contacts of the two extreme measuring electrodes and the second potential difference in the same section of the column, and use as the parameter of the electric logging of cased wells n electrical resistance of the surrounding rock strata, which is determined by the formula:

where K is the coefficient of proportionality, depending on the geometric dimensions of the probe;

I1 - the sum of the currents supplied to the current electrodes A1 and A2 at the first time:

I1 _A1 is the fraction of current I1 supplied to the current electrode A1;

I1 _A2 is the fraction of current I1 supplied to the current electrode A2;

I2 - the sum of the currents supplied to the current electrodes A1 and A2 at the second time moment:

I2 _A1 is the fraction of current I2 supplied to the current electrode A1;

I2 _A2 is the fraction of current I2 supplied to the current electrode A2;

U _N (I1), U _N (I2) are the potentials of the electric field at the point of contact with the column of the middle measuring electrode, respectively, when currents I1 and I2 are supplied to the first and second current electrodes of the probe;

ΔU _M2M1 (I1), ΔU _M2M1 (I2) are the first potential differences of the electric field in the column section between the contacts of the two extreme measuring electrodes of the probe, respectively, when currents I1 and I2 are supplied to the first and second current electrodes of the probe;

Δ ² U (I1), Δ ² U (I2) are the second potential differences of the electric field, respectively, when currents I1 and I2 are supplied to the first and second current electrodes of the probe.

SUMMARY OF THE INVENTION

Figure 1 shows a block diagram of a device implemented by the proposed method, where 1 is a well; 2 - casing metal string; 3 - rocks surrounding the well; 4 - downhole tool; 5 - average measuring electrode N; 6 and 7 - measuring electrodes M1 and M2 symmetrically located relative to the middle; 8 and 9 - current electrodes, respectively, A1 and A2; 10 is a block of meters: potential U _N between the middle measuring electrode 5 and the remote electrode 15, the first potential difference ΔU between the measuring electrodes 6 and 7, the second potential difference A ² U between the measuring electrodes 6, 7 and 5; 11 - current generator; 12 - reverse current electrode B connected to the second poles of the generator 11; 13 and 14 - current meters I _A1 and I _A2 , fed from the first poles of the generator 11 to the current electrodes 8 and 9, respectively. The computer that records and processes the signals U _N , ΔU, Δ ² U, I _A1 and I _A2 according to the formula (1), and the resistance curve recorder ρ _{p are} not shown in Fig. 1.

In the downhole tool 4 there is a probe consisting of an average measuring electrode 5, two additional measuring electrodes 6 and 7, and two current electrodes 8 and 9. All five electrodes are pressed against the wall of the column and have electrical contact with it.

The current generator 11 is a device that allows you to generate two currents that coincide in phase and are controlled in amplitude. It can be two synchronously operated controlled current generators or one current generator with a controlled current divider; in the latter case, the controllable current divider can be located both on the surface and in the downhole tool.

Current meters 13 and 14 can also be located both on the surface and in the downhole tool.

To carry out the measurement, perform the following steps:

- currents 8 and 9 are supplied with currents of such strength and in such a proportion that the signal of the 10 ΔU _M2M1 meter block is negative and does not exceed the amplitude of units μV, and the total current is significant, and signals U _N (I1), ΔU are recorded _M2M1 (I1), Δ ² U (I1), I1 _A1 , I1 _A2 ;

- currents of such a force and in such a proportion that the signal of the 10 ΔU _M2M1 meter block is positive and does not exceed the units of μV in amplitude and the total current is significant, and signals U _N (I2), ΔU are recorded in the current electrodes 8 and 9 _M2M1 (I2), Δ ² U (I2), I2 _A1 , I2 _A2 ;

- calculate ρ _p according to the formula (1).

It should be noted that currents spread over the column for hundreds of meters, therefore, the proportions of currents selected during the first measurement with a depth change will change slowly and maintaining the necessary proportions of currents will not be difficult.

We also note that the calculations ρ _p according to formula (1) are valid for the operation mode at the bottom of the column, when there is no need to balance the currents to decrease ΔU _M2M1 (I1), and ΔU _M2M1 (I2) must be reduced significantly, hundreds of times (I1 _A1 > 0 I1 _A2 = 0, I2 _A1 > 0 I2 _A2 > 0), and for the mode when I1 _A2 = 0, I2 _A1 = 0, then formula (1) corresponds to the prototype formula [3].

Compared with the prototype [3], the proposed method allows to increase the accuracy and expand the range of measurement of resistivity of rock formations.

The implementation of the proposed method in the practice of geophysical research of wells will give a significant economic effect, as it will improve the method and apparatus for electric logging of cased wells.

BIBLIOGRAPHY

1. Pat. 2172006 Russian Federation. Method for electric logging of cased wells. / A.S. Kashik, N.I. Rykhlinsky, G.N. Gogonenkov, R.I. Krivonosov, V.Z.Garipov (RF). - No.2000127404 / 28; declared 11/01/2000; publ. 08/10/2001.

2. Pat. 2200967 Russian Federation. The method of electric logging of cased wells. / A.S. Kashik, N.I. Rykhlinsky, R.I. Krivonosov (RF). - No. 2002114518/28; declared 06/04/2002; publ. 03/20/2003.

3. Pat. 2176802 Russian Federation. Method for electric logging of cased wells. / A.S. Kashik, N.I. Rykhlinsky, G.N. Gogonenkov, R.I. Krivonosov, V.Z.Garipov (RF). - No. 2001104501/28; declared 02/20/2001; publ. 12/10/2001.

Claims (1)

A method for electric logging of cased wells, including the supply of electric currents, measuring the potential of the electric field and its second difference using a multi-electrode probe of the second difference, made in the form of three equidistant measuring electrodes and two current, upper and lower electrodes, which are located outside the measuring electrodes, characterized in that both current electrodes simultaneously supply phase-matching electric currents in a certain proportion, change the proportion of currents and in and the current electrodes simultaneously supply phase-matching electric currents in an altered proportion, and at each current supply, the currents themselves are measured, the electric field potential at the point of contact of the middle measuring electrode with the column, the first potential difference in the column section between the contacts of the two extreme measuring electrodes and the second the potential difference in the same section of the column, and as a parameter of the electric logging of cased wells, the electrical resistivity surrounding the column is used strata of rocks, which is determined by the formula

where K is the coefficient of proportionality, depending on the geometric dimensions of the probe;
I1 - the sum of the currents supplied to the current electrodes A1 and A2 at the first time:

I1 _A1 is the fraction of current I1 supplied to the current electrode A1;
I1 _A2 is the fraction of current I2 supplied to the current electrode A2;
I2 - the sum of the currents supplied to the current electrodes A1 and A2 at the second time moment:

I2 _A1 is the fraction of current 12 supplied to the current electrode A1;
I2 _A2 is the fraction of current 12 supplied to the current electrode A2;
U _N (I1), U _N (I2) are the potentials of the electric field at the point of contact with the column of the middle measuring electrode, respectively, when currents II and 12 are applied to the first and second current electrodes of the probe;
ΔU _M2M1 (I1), ΔU _M2M1 (I2) are the first potential differences of the electric field in the column section between the contacts of the two extreme measuring electrodes of the probe, respectively, when currents I1 and I2 are supplied to the first and second current electrodes of the probe;
Δ ² U (I1), Δ ² U (I2) are the second potential differences of the electric field, respectively, when currents I1 and I2 are supplied to the first and second current electrodes of the probe.