RU2229735C1 - Process of electric logging of cased well - Google Patents

Abstract

FIELD: geophysical investigation of wells, measurement of electric resistance of formation of rocks surrounding well cased by metal column. SUBSTANCE: according to invention there is used sonde comprising three equidistant measurement electrodes and two measurement electrodes located outside limits of zone and current electrodes positioned in symmetry relative to center measurement electrode. Electric current in the form of alternating rectangular pulses is fed into column through first (upper) current electrode. Current from output of self-balancing potentiometer which automatically keeps potential difference equal to zero between extreme measurement electrodes into second (lower) current electrode. Potential of electric field of column in point of contact of center measurement electrode with it and second potential difference of section of column between extreme measurement electrodes are measured in time intervals of galvanic excitation in each period of current on termination of induction transit process. Specific electric resistance is established by corresponding formula. EFFECT: exclusion of inductions distorting measurement results. 2 cl, 4 dwg

Description

The invention relates to the field of geophysical research of wells and is intended to determine the electrical resistance of the rock formations surrounding a cased metal column well.

A known method of electric logging cased wells (patent of the Russian Federation No. 2176802, class G 01 V 3/20, published. 10.12.2001, BI No. 34) [1].

To perform the method, a probe was used in the form of three measuring electrodes equidistantly located along the column and two current electrodes located symmetrically relative to the middle measuring electrode outside the zone of the measuring 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 of the current supplies, the electric field potential of the column is measured element-wise at the point of contact with the middle measuring electrode, the first and second potential differences in the column section between the two extreme measuring electrodes. Electrical resistivity is determined by the corresponding formula. This method has two drawbacks: the first - because of the discrete element-by-element registration of signals, the measurement speed decreases sharply and, in principle, the implementation of continuous recording of the resistance curve along the wellbore is excluded; the second - the current circuit to the lower current electrode passes by the measuring circuits of the first and second potential differences of the electric field and creates induction pickups in these circuits, the levels of which can exceed the levels of useful signals.

A known method of electric logging cased wells (patent of the Russian Federation No. 2200967, class G 01 V 3/20, publ. 03/20/2003, BI No. 8) [2]. To perform the method, a probe was used in the form of three measuring electrodes equidistantly located along the column and two current dipoles, each of which is located outside the zone of the measuring electrodes: one above and the other below. An electric current is alternately supplied to the column through each of the two current dipoles. At each of the current supplies, the electric field potential of the column is measured element-wise at the point of contact with the middle measuring electrode, the first and second potential differences in the column section between the two extreme measuring electrodes. Electrical resistivity is determined by the corresponding formula.

The indicated method also has two drawbacks: the first (the same as in [1] - due to discrete element-by-element registration of signals, the measurement speed decreases sharply and, in principle, the implementation of continuous recording of the resistance curve along the wellbore is excluded; the second - although due to the use of the lower a current dipole instead of a single-pole current electrode, the laying of a current line in the area of the measuring circuits of the first and second potential differences of the electric field is excluded, and thereby induction induction on and measuring chains, but due to the presence of a lower current dipole in the probe, which significantly increases its size, it is not possible to carry out measurements in the near-well part of the well, where, as a rule, there are oil and gas reservoirs, for the study of which, mainly, a cased hole logging method is intended .

A known method of divergent well logging (NI Rykhlinsky. Aut. St. No. 333514, class G 01 V 3/04 according to the application No. 932133 / 26-25, publ. 21.03.1972. BI No. 11) (prototype). [3].

To perform the method, a probe was used in the form of three measuring electrodes equidistantly located along the axis of the well, and two current electrodes located outside the zone of the measuring electrodes symmetrically with respect to the middle measuring electrode. To maintain the potential extreme in the zone of the measuring electrodes, which is necessary to eliminate the axial component of the current distorting the measurement results, an autocompensator is used, which regulates the current in the lower current electrode so that the first potential difference of the electric field between the extreme measuring electrodes is always zero. Electrical resistivity is determined by the corresponding formula.

The advantage of this method with auto-compensator over methods [1] and [2] with sequential element-wise registration is that it can significantly increase the speed of measurements and, in principle, allows continuous recording of the resistance curve along the axis of the well.

The disadvantage of this method, as well as the method [1], is that the current circuit to the lower current electrode passes by the measuring circuits and creates induction pickup in them, the levels of which in cased wells may exceed the levels of useful signals.

The proposed method solves the problem of eliminating the distortion of the measurement results in cased wells induction induction.

The problem is solved in that in the cased hole electric logging method, in which electric current is supplied to the casing from the generator through the first current electrode, an electric field potential is created and maintained at an extremum below the first current electrode towards the bottom of the well with the help of a compensator that feeds the second a current electrode located on the other side from the extremum point of the potential relative to the first electrode, and measure the electric potential and the second difference of the elec tric potentials in the field extreme region using three measuring electrodes located at the same distance along the column, according to the invention, the current electrodes are supplied with alternating rectangular current pulses; measuring the electric potential, the second electric potential difference and the first electric potential difference controlling the auto-compensator, is carried out after completion of the induction transient associated with the current reversal; electrical resistivity ρ _{p of the} surrounding rock strata is determined by the formula

where Ω _Z is the electrical resistance of the column in the area between the extreme measuring electrodes M ₁ M ₂ ;

- соответственно электрический потенциал поля среднего измерительного электрода N и вторая разность электрических потенциалов на участке колонны между крайними измерительными электродами M₁M₂ при равенстве нулю первой разности потенциалов между этими электродами;

- respectively, the electric potential of the field of the middle measuring electrode N and the second electric potential difference in the column section between the extreme measuring electrodes M ₁ M ₂ when the first potential difference between these electrodes is equal to zero;

k is the geometric coefficient of the probe.

In addition, according to the invention, the reverse current electrode of the self-compensation is earthed at an arbitrary point on the earth's surface or columns in its area in the interval from the wellhead to the first current electrode.

SUMMARY OF THE INVENTION

между измерительными электродами 6, 7 и 5; 16 - усилитель потенциала U_N между средним измерительным электродом 5 и удаленным электродом N∞ -17.Figure 1 is a block diagram of a device implemented by the proposed method. Here 1 is a well; 2 - casing metal string; 3 - a rock formation surrounding a well; 4 - downhole tool; 5 - average measuring electrode N; 6 and 7 - symmetrically located relative to the middle measuring electrodes M ₁ and M ₂ ; 8 - the first current electrode A ₁ ; 9 - current generator; 10 - communication line of the first pole of the generator 9 with the first current electrode 8; 11 - reverse current electrode B ₁ connected to the second pole of the generator 9; 12 - the second current electrode And ₂ ; 13 - auto-compensator And _to ; 14 - reverse current electrode B ₂ auto-compensator 13; 15 - amplifier of the second potential difference

between measuring electrodes 6, 7 and 5; 16 - potential amplifier U _N between the middle measuring electrode 5 and the remote electrode N∞ -17.

Figure 2 illustrates the distribution of electric potential along the axis of the well from a single-pole current electrode.

Figure 3 illustrates the distribution of electric potential along the axis of the well between two single-pole current electrodes at

Figure 4 illustrates the plot of the signals:

a - in the current circuits of the probe; b - in the measuring circuits of the probe. Here Δt _ind. - the time interval during which induction excitation associated with the reversal of the current in the current circuits occurs. And Δt _galv is the time interval during which galvanic excitation occurs. During the galvanic excitation of the current, there are no induction pickups on any electrical circuits of the device, including the measuring ones, due to the constancy of the magnetic field in the current circuits during this period.

Consider the principle of cased hole electrical logging, the electrical resistance of the casing of which is directly based on the direct measurement of the second potential differences of the electric field.

We place in the well (Fig. 2), at point A, a source from which a direct electric current I is supplied to the test medium (in practice, instead of direct current, low-frequency alternating current is supplied), and determine the distribution of electric potential along its axis.

It is known [3] that

and only when ωr / ωz >> 1 (a necessary condition that is always satisfied in cased wells)

where U (z) is the electric potential in the well at the observation point with z coordinate; I _z (z) is the electric current through the cross section of a cased well with the same coordinate; J _r (z) is the current flowing from the borehole wall into the surrounding rock per unit of the depth interval (linear current density with dimension [A / m]); ω _r is the electrical resistance [Ohm · m] provided by the medium to the current J _r (z); ω _z - linear electrical resistance of the well section to the axial direction current, functionally dependent on the z coordinate due to the inconstancy of the geometric and other parameters of the casing [Ohm · m].

взятое в интегральной форме, т.е.We select a segment of a well column at a point z with a high Δz and with a center at the observation point and to the closed surface of this cylindrical segment, we apply the equation of continuity of the current density vector

taken in integral form, i.e.

The surface S consists of the bases of the cylinder S _p and S _q and its side surface S _b . Therefore, the left side of equation (3) represents the sum of the three flows

thus, according to (3), we have

I _z (z + Az / 2) -I _z (z-Δz / 2) + J _r (z) (Δz = 0 (Δz), (4)

whence ΔI _z (z) / Δz = -J _r (z) +0 (1) and in the limit as Δz → 0:

We differentiate expression (1) with respect to z, taking into account that ω _z is a function of the electrical resistance of the string, which varies in a real well along its bore with a change in the z coordinate, i.e. ω _z = ω _z (z) ≠ const:

Substituting equalities (2) and (5) into equation (6), we obtain the equation of the distribution of the source potential along the axis of the well with the electrical resistance of the column being constant along the axis

It is seen from equation (7) that the measurement of the electric potential and its second derivative does not determine the desired ratio ω _z / ω _r due to the presence of the term dω _z / dz in this equation, which strongly depends on the variability of the casing electrical resistance.

The proposed method for electric logging of cased wells, the measurement results of which the inhomogeneities of the cased hole are practically unaffected, is characterized in that the electric field along the column is set so that the potential distribution curve along this column has an extremum in the region of the measuring electrodes (in the coordinate area z = z _n ), i.e. dU (z _N ) / dz = 0. Therefore, a term containing an indefinite quantity dω _z / dz is excluded from equation (7), and this equation at the point z = z _n takes the following form:

or

Based on equation (9), by measuring the potential and its derivative at a point with the coordinate z _N in the presence of an extremum and the column resistance ω _z , one can determine the desired radial resistance ω _r .

Achieving the potential extremum at the location of the measuring electrodes is carried out using two current electrodes A ₁ and A ₂ (Fig. 3) located on both sides at the same distance from the middle electrode N (measuring point), and the current is continuously maintained using the auto-compensator in the lower the current electrode A ₂ such that the potential difference between the two electrodes M ₁ and M ₂ symmetric with respect to N is zero, i.e.

The achievement of an extremum at the measurement point z = z _N means the exclusion of the axial component of the current I _z (z _N ), which in a cased hole when the medium under study is excited by one unipolar source is many times (millions of times) the radial component J _r (z _N ). In practice, to measure ω _r instead of the second derivative of the potential (equation (9)), a second finite potential difference proportional to it is used

~

~

The implementation of the proposed method for cased hole electric logging consists in eliminating from the measured parameter the distorting effects of the electrical resistance of the column, its changes and external random electrical interference through the following formula (9):

where Ω _z is the electrical resistance of the column in the area between the electrodes M ₁ M ₂ , Ohm;

ρ _p - electrical resistivity of the surrounding rock strata, Ohm · m;

- соответственно электрический потенциал поля электрода N и вторая разность электрических потенциалов на участке колонны между электродами M₁M₂ при равенстве нулю первой разности потенциалов между этими электродами, В;

- respectively, the electric potential of the field of the electrode N and the second electric potential difference in the column section between the electrodes M ₁ M ₂ when the first potential difference between these electrodes is equal to zero, V;

K is the geometric coefficient of the probe, m

Concrete example

Figure 1 presents a block diagram of the equipment made by the proposed method. The block diagram shows a well 1 in cross section with a metal casing 2, which surrounds the formation 3. The downhole tool 4 is located in the well and adjacent to the area of the formation 3, the resistivity of which is measured. In the downhole tool 4, there is a probe consisting of the middle measuring electrode N, indicated in figure 1 by number 5, two additional measuring electrodes M ₁ - 6 and M ₂ - 7 and two current electrodes of the first (upper) A ₁ - 8 and the second ( lower) A ₂ - 12. All five electrodes are pressed against the wall of the column and have electrical contact with it.

- 15. Усилитель потенциала U_N - 16 может находиться в скважинном приборе или на поверхности. Потенциал U_N центрального измерительного электрода 5 измеряется относительно удаленного электрода N∞ - 17, который расположен на дневной поверхности. Компьютер, обрабатывающий сигналы U, и Δ²U по формуле (11), и регистратор кривой сопротивления ρ_п на фиг.1 не показаны. Удельное электрическое сопротивление ρ_П получают из формулы (11). Как уже отмечалось выше, эта формула выведена из предпосылки, что результирующая осевая составляющая тока, текущего вдоль колонны между измерительными электродами 6 и 7, равна нулю. Благодаря этому, в частности, искажающее влияние электрического сопротивления колонны на результаты измерения отсутствуют и регистратор после обработки сигналов по формуле (11) регистрирует истинное сопротивление пласта ρ_п. Отметим, что автокомпенсатор 13 выполняет одинаково свою роль (поддержание равенства нулю между измерительными электродами M₁ M₂) независимо от того, в какую точку пространства заземлен его обратный токовый электрод В₂ - 14. По конструктивным соображениям его желательно заземлять в произвольной точке земной поверхности или колонны на ее участке в интервале от устья скважины до первого токового электрода.The first current electrode 8 is connected to the first pole of the alternating pulsed current generator 9 with an infralow frequency located on the day surface by a communication line 10. The second pole of the generator 11 is grounded on the day surface through the return current electrode B ₁ , indicated by the number 11. In the downhole tool 5 there are also auto-compensator 13 to maintain a zero potential difference between the electrodes 6 and 7 and the amplifier of the second potential difference

- 15. The potential amplifier U _N - 16 may be located in the downhole tool or on the surface. The potential U _{N of the} central measuring electrode 5 is measured relative to the remote electrode N∞ - 17, which is located on the day surface. The computer processing the signals U and Δ ² U according to the formula (11) and the recorder of the resistance curve ρ _{p are} not shown in Fig. 1. The electrical resistivity ρ _{P is} obtained from formula (11). As already noted above, this formula is derived from the assumption that the resulting axial component of the current flowing along the column between the measuring electrodes 6 and 7 is zero. Due to this, in particular, there is no distorting effect of the electrical resistance of the column on the measurement results, and the registrar, after processing the signals by formula (11), registers the true formation resistance ρ _p Note that the auto-compensator 13 performs equally its role (maintaining the equality of zero between the measuring electrodes M ₁ M ₂ ) regardless of what point in space its return current electrode B ₂ - 14 is grounded. For structural reasons, it is desirable to ground it at an arbitrary point on the earth's surface or columns in its area in the interval from the wellhead to the first current electrode.

In order to exclude the influence of induction interference on the measuring circuits of the downhole tool, the measurement of the electrode potential N and the second electric potential difference in the column section between the electrodes M ₁ M ₂ , as well as the extraction of the control signal of the self-compensation between the electrodes M ₁ M _{2, are} carried out in galvanic excitation time intervals Δt _{galv .} (Fig. 4) of each rectangular current pulse when the change in the magnetic field in the current circuit is zero. In this period of time there are no induction interference on the measuring circuit of the device.

Present in the formula (11), the electrical resistance of the column Ω _z in its section between the electrodes M ₁ M ₂ can be determined in different ways. In particular, one of them allows us to determine this resistance by the formula

первая разность потенциалов от действия поля первого источника A₁ при заблокированном автокомпенсаторе;Where

the first potential difference from the action of the field of the first source A ₁ with the locked auto-compensator;

- сила тока в цепи автокомпенсатора, когда результирующая разность потенциалов между электродами M₁M₂ равна нулю [

]=0, а токовые электроды A₁ и В₂ совмещены,

- current strength in the circuit of the auto-compensator, when the resulting potential difference between the electrodes M ₁ M ₂ is equal to zero [

] = 0, and the current electrodes A ₁ and B _{2 are} combined,

The proposed method is implemented in the form of a hardware model and tested in the well. The test results of the model in a cased hole confirmed the coincidence with the results of standard resistance logs obtained before casing the well.

The implementation of the proposed method in the practice of geophysical exploration of the well will give a significant economic effect, as it will allow more efficient and more accurate in comparison with other methods to control the level of oil-water contact in operating oil wells.

Claims (2)

1. The method of electric cased hole logging, in which an electric current is supplied to the casing from the generator through the first current electrode, creates and maintains an extreme potential of the electric field below the first current electrode towards the bottom of the well with the help of an compensator that feeds the current to the second current electrode located on the other side of the point of extremum of the potential relative to the first electrode, and measure the electric potential and the second difference of electric potentials in on the field extremum using three measuring electrodes located at the same distance along the column, characterized in that the current electrodes are supplied with alternating rectangular current pulses; measuring the electric potential, the second electric potential difference and the first electric potential difference controlling the auto-compensator, is carried out after completion of the induction transient associated with the current reversal; electrical resistivity ρ _P rock formations surrounding the column is determined by the formula

where Ω _Z is the electrical resistance of the column in the area between the extreme measuring electrodes M ₁ M ₂ ;

and

- respectively, the electric potential of the field of the middle measuring electrode N and the second electric potential difference in the column section between the extreme measuring electrodes M ₁ M ₂ when the first potential difference between these electrodes is equal to zero; K is the geometric coefficient of the probe. 2. The method according to claim 1, characterized in that the reverse current electrode of the self-compensator is grounded at an arbitrary point on the earth's surface or columns in its area in the interval from the wellhead to the first current electrode.

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