RU2200967C1 - Method of electric logging of cased wells - Google Patents

Abstract

FIELD: geophysical examination of wells. SUBSTANCE: method of electric logging can find use in determination of electric resistance of formations of rocks surrounding well cased by metal string. Method employs sonde comprising three measurement electrodes equidistantly arranged along string and two pairs of current electrodes, one pair being placed above and beyond limits of measurement electrodes and the other pair being positioned beneath them. Two current generators located outside of measurement circuits of sonde and meters are used to feed current into string through both pairs of current electrodes. Potential of electric field of string in point of its contact with center measurement electrode is measured during each current feed as well as first and second differences of potentials across section of string between extreme measurement electrodes. Specific electric resistance is established by corresponding formula. EFFECT: increased accuracy of measurement of specific electric resistance of rocks. 5 dwg

Description

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

 A known method of electric logging cased wells (patent of the Russian Federation 2172006, CL G 01 V 3/20, published. 08/10/2001). To perform the method, a probe was used in the form of three measuring electrodes equidistantly located along the column 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. Each of the three current electrodes is alternately supplied with electric current from the same source pole. At each of the three current supplies, the electric field potential of the middle measuring electrode is measured, the first and second potential difference of the electric field in the column section between the two extreme measuring electrodes. Electrical resistivity is determined by the corresponding formula.

 The disadvantage of this method is that the current circuits to the lower and middle current electrodes pass by the measuring circuits of the first and second potential differences of the electric field and create capacitive and induction pickups in the circuits of these circuits, the levels of which can exceed useful signals. Therefore, when creating devices by this method, it is necessary to apply complex circuits that compensate for these interference, which generally complicates the creation of the device, its setup and operation.

 Closest to the proposed invention by technical essence is a method of electric logging of cased wells (patent of the Russian Federation 2176802, CL G 01 V 3/20, published. 10.12.2001, the prototype). 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 outside the zone symmetrically with respect to the middle measuring electrode. 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 and second potential differences in the section of the column between the two extreme measuring electrodes. Electrical resistivity is determined by the corresponding formula.

 Due to the presence of only one current circuit, supplying current to the lower current electrode past the measuring circuits, the electrical interference to these circuits is less. Nevertheless, they have to be fought through the use of complex compensating schemes.

 The proposed method solves the problem of completely eliminating the interference of the current circuits to the measuring ones, which creates favorable conditions for the manufacture of devices by this method, their configuration and operation, and ultimately increases the accuracy of measuring the electrical resistivity of the rock formations surrounding the column.

где к - коэффициент, полученный из уравнения

вытекающего из необходимости условия наличия экстремума потенциала электрического поля вдоль колонны в пределах зоны измерительных электродов зонда;
К - коэффициент пропорциональности, имеющий размерность метры и зависящий от геометрических размеров зонда;
U_N(J_A1 B1), U_N(J_A2 B2) - потенциалы электрического поля в точке контакта с колонной среднего измерительного электрода соответственно при подаче токов в верхнюю и нижнюю пары токовых электродов зонда;

- первые разности потенциалов электрического поля на участке колонны между контактами с ней двух крайних измерительных электродов зонда соответственно при подаче токов в верхнюю и нижнюю пары токовых электродов зонда;

- вторые разности потенциалов электрического поля на том же участке колонны соответственно при подаче токов в верхнюю и нижнюю пары токовых электродов зонда;
J_A1 B1, J_A2 B2 - токи, подаваемые к колонне в точках контакта с ней верхней и нижней пары токовых электродов зонда.The problem is solved in that in the method of electric logging of cased wells, including the supply of electric current to the current electrodes of the second difference probe, made in the form of three measuring electrodes equidistantly located along the column, and two direct current, upper and lower electrodes located outside the measuring electrodes symmetrically with respect to the average measuring electrode, measurement at each current supply to the current electrodes of the electric field potential at the contact point of the average of the electrode with the column and the first and second potential difference on the column section between the contacts of the two extreme measuring electrodes, and the calculation of the electrical resistivity of the rock formations surrounding the column, according to the invention, an upper current generator with an upper current generator is located outside the measuring electrodes and their electric circuits a reverse current electrode above the upper forward current electrode and a lower current generator with a lower reverse current electrode below the lower direct the current electrode, through the upper current generator feed the upper forward and reverse current electrodes, through the lower current generator feed the lower forward and reverse current electrodes, and the electrical resistivity of the surrounding rock strata is determined by the formula

where k is the coefficient obtained from the equation

the necessary condition for the presence of an extremum of the electric field potential along the column within the zone of the measuring electrodes of the probe;
K is a proportionality coefficient having a dimension of meters and depending on the geometric dimensions of the probe;
U _N (J _{A1 B1} ), U _N (J _{A2 B2} ) are the potentials of the electric field at the point of contact with the column of the middle measuring electrode, respectively, when currents are supplied to the upper and lower pairs of current probe electrodes;

- the first potential differences of the electric field in the column section between the contacts with it of the two extreme measuring electrodes of the probe, respectively, when currents are supplied to the upper and lower pairs of current probe electrodes;

- the second potential difference of the electric field in the same section of the column, respectively, when applying currents to the upper and lower pairs of current electrodes of the probe;
J _{A1 B1} , J _{A2 B2} - currents supplied to the column at the points of contact with it of the upper and lower pairs of current electrodes of the probe.

SUMMARY OF THE INVENTION
With electric logging of cased wells, capacitive and induction pickups occur in the circuits of electrical circuits, the levels of which can exceed useful signals. Therefore, when creating devices by this method, it is necessary to apply complex circuits that compensate for these interference, which generally complicates the creation of the device, its commissioning and operation.

 In the proposed method, the task of completely eliminating the interference of current circuits to the measuring ones is solved. The problem is solved through the use of a probe made in the form of three measuring electrodes equidistantly located along the column, and two direct current, upper and lower electrodes located symmetrically relative to the middle measuring electrode outside the measuring electrodes. Outside the measuring electrodes and their electrical circuits, the probe has an upper current generator with an upper reverse current electrode above the upper forward current electrode and a lower current generator with a lower reverse current electrode below the lower direct current electrode. The upper forward and reverse current electrodes are fed through the upper current generator, and the lower forward and reverse current electrodes are fed through the lower current generator. When cased hole logging is carried out, an electric current is supplied to the probe current electrodes, at each current supply to the current electrodes, the electric field potential is measured at the contact point of the middle measuring electrode with the column and the first and second potential differences in the column section between the contacts of the two extreme measuring electrodes, and calculating the electrical resistivity of the rock formations surrounding the column.

 When supplied with alternating current, direct current electrodes are fed by common-mode currents from current generators, while the currents in the forward and reverse current electrodes are in antiphase.

 The excitation and determination of the current value in the lower current generator is carried out using the first potential difference of the electric field arising in the section of the column between the extreme measuring electrodes from the action of the electric field of the upper current generator, with the current value in the current electrode circuit of the lower current generator, ensuring the resultant zero result the first difference in electric potentials and creating the conditions for the appearance of an extremum of the electric field potential there.

Figure 1 is a block diagram of one embodiment of the device. Here 1 is a well, 2 is a casing metal string; 3 - a rock formation surrounding a well; 4 - probe; 5 - average measuring electrode; 6 and 7 - measuring electrodes M ₁ and M ₂ symmetrically located relative to the middle measuring electrode, 8 - upper current generator; 9 - direct current electrode A _{1 of the} upper current generator; 10 - remote to the wellhead reverse current electrode B _{1 of the} upper current generator; 11 - lower current generator; 12 - direct current electrode A _{2 of the} lower current generator; 13 - reverse current electrode B _{2 of the} lower current generator; 14 - switch current electrodes A ₁ and A _{2 of the} upper and lower current generators; 15 - amplifier of the first potential difference ΔUM ₂ M ₁ between the measuring electrodes 6 and 7; 16 - amplifier of the second potential difference Δ ² UM ₂ NM ₁ between the measuring electrodes 6, 7 and 5; 17 - amplifier potential UN between the middle measuring electrode 5 and the remote measuring electrode N ^∞ -18.
Figure 2 shows a block diagram of a second embodiment of a device implemented by the proposed method, where 19 is a reverse current electrode B _{1 of the} upper current generator, remote from the direct current electrode A ₁ up the axis of the well at a predetermined distance.

In FIG. 3 is a block diagram of a third embodiment of a device implemented by the proposed method, where 20 is an error signal amplifier in the column section between the measuring electrodes M ₁ and M _{2 of the} lower current generator (auto-compensator) - 21, which provides current through the lower current dipole A ₂ In _{2, the} resultant potential difference (potential extremum) is equal to zero on the column section between the measuring electrodes M ₁ and M ₂ .

 Figure 4 illustrates the distribution of electric potential along the axis of the well from a unipolar side electrode or dipole.

 Figure 5 illustrates the distribution of electric potential along the axis of the well between two pairs of current electrodes (dipoles).

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

и только при Ω_r/Ω_z≫1 (необходимо условие, которое в обсаженных скважинах всегда выполняется)
J_r(z) = U(z)/Ω_r, (2)
где U(z) - электрический потенциал в скважине в точке наблюдения с координатой z; J_z(z) - электрический ток через поперечное сечение обсаженной скважины с этой же координатой; J_r(z)- ток, стекающий со стенки скважины в окружающую породу на единицу интервала глубин (линейная плотность тока с размерностью [А/м] ); Ω_r - электрическое сопротивление [Ом•м], оказываемое средой току J(z); Ω_z - электрическое сопротивление отрезка скважины току осевого направления, функционально зависящее от координаты z вследствие непостоянства геометрических и др. параметров обсадной колонны.We place it in the well (Fig. 4), at point A, the source from which direct electric current J 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 [2] that

and only for Ω _r / Ω _z ≫1 (a condition is necessary that is always satisfied in cased wells)
J _r (z) = U (z) / Ω _r , (2)
where U (z) is the electric potential in the well at the observation point with z coordinate; J _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 (z); Ω _z is the electrical resistance of the borehole segment to the axial direction current, which functionally depends on the z coordinate due to the inconstancy of the geometric and other parameters of the casing string.

взятое в интегральной форме, т.е.We select a segment of a well column at a point z with a height Δz and centered 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.

Поверхность S состоит из оснований цилиндра Sp и Sq и его боковой поверхности S_b. Следовательно, левая часть уравнения (3) представляет сумму трех потоков

Таким образом, согласно (3), имеем
J_Z(z+Δz/2)-J_Z(z-Δz/2)+J_r(z)•Δz = 0(Δz), (4)
откуда ΔJ_Z(z)/Δz = J_r(z)+0(1) и в пределе при Δz-->0:

Продифференцируем выражение (1) по z, учитывая, что Ω_z/ есть функция электрического сопротивления колонны, изменяющегося в реальной скважине вдоль ее ствола с изменением координаты z, т.е. Ω_z(z)≠const

Подставив в уравнение (6) равенства (2) и (5), получим уравнение распределения потенциала источника вдоль оси скважины с непостоянным вдоль оси электрическим сопротивлением колонны

Анализ уравнения (7) показывает, что измерение электрического потенциала и его второй производной при возбуждении исследуемой среды одним однополюсным источником или одним дипольным источником не определяет искомое отношение Ω_z/Ω_r ввиду присутствия в этом уравнении члена dΩ_z/dz, сильно зависящего от изменчивости параметров обсадной колонны.

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

Thus, according to (3), we have
J _Z (z + Δz / 2) -J _Z (z-Δz / 2) + J _r (z) • Δz = 0 (Δz), (4)
whence ΔJ _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 (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

An analysis of equation (7) shows that the measurement of the electric potential and its second derivative when the medium under investigation is excited by one single-pole source or one dipole source does not determine the desired ratio Ω _z / Ω _r due to the presence of the term dΩ _z / dz in this equation, which is highly dependent on the variability casing string parameters.

или

На основании уравнения (9), измерив потенциал и его вторую производную в точке с координатой z_N, при наличии там экстремума, можно определить искомое отношение

Достижение экстремума потенциала в месте нахождения измерительных электродов осуществляют при помощи двух источников A₁B₁ и А₂В₂ (фиг.5), расположенных с обеих сторон от среднего электрода N (точка измерения), и подбора в них токов таких величин, чтобы разность потенциалов между двумя симметричными относительно N электродами M₁ и М₂ равнялась нулю, т.е.The proposed method for electric logging of cased wells, the measurement result of which the heterogeneity of the cased hole is practically unaffected, differs 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, the 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 second derivative at a point with coordinate z _N , if there is an extremum, we can determine the desired ratio

Achieving the potential extreme at the location of the measuring electrodes is carried out using two sources A ₁ B ₁ and A ₂ B ₂ (figure 5) located on both sides of the middle electrode N (measuring point), and selection of currents of such values in them so that the potential difference between two electrodes M ₁ and M ₂ symmetric with respect to N was zero, i.e.

Достижение экстремума в точке измерения z=z_Nозначает исключение осевой составляющей тока J_z(z_N), которая в обсаженной скважине при возбуждении исследуемой среды одним однополюсным источником многократно больше (в миллионы раз) радиальной составляющей J_r(z_N). На практике для измерения Ω_r/Ω_z вместо второй производной потенциала (уравнение (9)) используют пропорциональную ей вторую конечную разность потенциалов

Реализация предлагаемого способа электрического каротажа обсаженных скважин заключается в исключении из измеряемого параметра искажающих влияния электрического сопротивления колонны, его изменения, внешних случайных электрических помех и электрических наводок токовых цепей на измерители первой и второй разностей электрических потенциалов через формулу

где к - коэффициент, полученный из уравнения

вытекающего из необходимости условия наличия экстремума потенциала электрического поля вдоль колонны в пределах зоны измерительных электродов зонда с целью обнуления там осевого тока.

The achievement of an extremum at the measurement point z = z _N means the exclusion of the axial component of the current J _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 / Ω _z, 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 electric resistance of the column, its changes, external random electrical noise and electrical pickups of current circuits on the meters of the first and second differences of electric potentials through the formula

where k is the coefficient obtained from the equation

arising from the necessary conditions for the presence of an extremum of the electric field potential along the column within the zone of the measuring electrodes of the probe in order to zero the axial current there.

 To eliminate the distorting effect of the resistance of the column itself, the factor in front of the second bracket in formula (11) is used.

 An example of a specific implementation.

 In FIG. 1 shows a block diagram of equipment made by the proposed method. The block diagram shows a well 1 in cross section with a metal casing 2, which is surrounded by the formation 3. The probe 4 is located in the well and adjacent to the section of the formation 3, the electrical resistivity of which is measured. There are two current generators in the probe. The upper generator 8 is used to power the upper forward current electrode of the probe 9 and the upper reverse current electrode 10, which can be assigned to the wellhead (Fig. 1) or placed in the well at a predetermined distance from the upper direct current electrode, as shown in Fig. 1. 2, where it is indicated by the number 19. The lower generator 11 is used to power the lower forward electrode 12 and the lower return electrode 13.

 Measuring devices 15, 16 and 17 are placed in the probe between the upper and lower current generators for measuring the first (between the electrodes 6 and 7 of the probe) and the second (between electrodes 6, 7 and 5) potential differences, as well as for measuring the potential of the column at the point of contact with it a measuring electrode 5 probe.

The electrical resistivity ρ _n in this particular embodiment 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 specific electrical resistance obtained by the proposed method for the rock formations surrounding the column.

 There is an option when the lower generator is turned on in the auto-compensator 21 mode (Fig. 3). In this embodiment, the first potential differences from the action of the field of the upper generator and from the action of the field of the lower generator (auto-compensator) in the column section between the measuring electrodes 6 and 7 are equal and opposite in sign, so that the resulting potential difference in this section from the total field of two sources equal to zero. Moreover, the coefficient K in formula (11), due to the equality of the first potential differences from each source separately, is equal to unity, which leads to a simplification of formula (11) for determining the electrical resistivity of rock formations surrounding the column.

The proposed method in the variants of the flowcharts of FIGS. 1 and 2 is implemented as a hardware model and tested in the well. It should be noted that in this layout, the points J _A1B1 and J _A2B2 stabilized and were equal to 5 amperes. Moreover, in this layout, the potentials U _N from the excitation by each of the two sources, depending on the change in the electrical resistance of the strata surrounding the column, were from 0.5 mV to 1.0 mV, the first potential differences from 2.0 μV to 15 μV, the second potential differences - from 0 to 0.5 mkV.

 Note that, in comparison with the prototype, the proposed method allows to increase the accuracy of measuring the resistivity of rock formations by eliminating the interference of current circuits on the measuring circuit.

 The implementation of the proposed method in the practice of geophysical research of wells will give a significant economic effect, as it will reliably control the level of water-oil contact in operating oil wells.

Claims (1)

A method for electric cased hole logging, comprising applying an electric current to the current electrodes of a second difference probe made in the form of three measuring electrodes equidistantly located along the column and two direct current, upper and lower electrodes located symmetrically relative to the middle measuring electrode outside the measuring electrodes , measurement at each current supply to the current electrodes of the electric field potential at the point of contact of the middle measuring electrode with the of the first and second potential differences in the section of the column between the contacts of the two extreme measuring electrodes, and the calculation of the electrical resistivity of the rock formations surrounding the column, characterized in that an upper current generator with an upper reverse is located outside the measuring electrodes and their electric circuits the current electrode above the upper forward current electrode and the lower current generator with the lower reverse current electrode below the lower direct current electrode, through the upper current generator feed the upper forward and reverse current electrodes, the lower direct and reverse current electrodes are fed through the lower current generator, and the electrical resistivity of the rock formations surrounding the column is determined by the formula

where k is the coefficient obtained from the equation

the necessary condition for the presence of an extremum of the electric field potential along the column within the zone of the measuring electrodes of the probe;
K is a proportionality coefficient having a dimension of meters and depending on the geometric dimensions of the probe;

- potentials of the electric field at the point of contact with the column of the middle measuring electrode, respectively, when applying currents to the upper and lower pairs of current electrodes of the probe;

- the first potential difference of the electric field in the column section between the contacts with it of the two extreme measuring electrodes of the probe, respectively, when applying currents to the upper and lower pairs of current probe electrodes;

- second potential differences of the electric field in the same section of the column, respectively, when applying currents to the upper and lower pairs of current electrodes of the probe;

- currents supplied to the column at the points of contact with it of the upper and lower pairs of current electrodes of the probe.

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