RU215374U1 - DEVICE FOR MEASURING ELECTRIC MACROANISOTROPY OF ROCKS IN A WELL - Google Patents

Abstract

The utility model relates to the field of geophysical research in oil and gas wells, namely to devices for studying the anisotropy of the electrical properties of rocks surrounding the well. EFFECT: device for detecting and measuring electrical macroanisotropy of rocks surrounding a borehole. Essence: supply and receiving electrodes are placed on the outer surface of the non-conductive body. All electrodes lie in one secant plane passing through the body axis and form a probe, i.e. a set of generator and measuring dipoles separated by a distance L. Justification of the design and parameters of the probe is based on the construction of a mathematical model of the device, numerical modeling and analysis of the connection of the probe signal with the parameters of the geoelectric section. It is shown that a combination of features, including the dipole nature of the electric field created by the claimed device, and the choice of probe length L≥0.8 m ensures that the useful model obtains the claimed technical result: the probe signals are uniquely related to the macroanisotropy coefficient of the medium Λ and practically do not depend on the longitudinal conductivity of the cut. In the range L≥0.8 m, the inventive device serves as a direct action sensor that detects the presence of electrical anisotropy when the probe passes through the section: the probe signal is associated with the macroanisotropy parameter by a simple functional relationship and can be calibrated in the values of the parameter Λ. This property of the proposed device allows the probe to be calibrated inside the well, when passing through a known interval of isotropic formations, since in this case Λ=1.

Description

The utility model relates to the field of geophysical research in oil and gas wells, namely to devices for studying the anisotropy of the electrical properties of rocks surrounding the well. The electrical anisotropy of reservoir layers and their accompanying geological formations (we are talking about the average macroanisotropy of the medium created by the alternation of parallel layers of isotropic media with different electrical conductivity) is a stable sign of oil and gas potential in the fields of Western Siberia.

At present, a number of devices are known - analogues used to determine the electrical macroanisotropy of rocks and formations intersected by a well. These devices are structurally made in the form of induction probes with several generator and measuring coils differently oriented with respect to the tool axis [1], or they represent a hardware and software complex of direct and alternating current probes assembled in a single housing, which implements several geophysical methods in the process of logging. As an example, let us point to the instrumental and methodological autonomous complex for geophysical research of production wells SKL-A-160 [2], produced by the research and production enterprise of geophysical equipment "Luch": http://www.looch.ru.

In the first analogue [1], combined orthogonal coils serve as the source and receiver of the electromagnetic field, and measurements are performed at several distances. The combined three-component source and receiver make it possible to measure the EMF corresponding to all components of the total electromagnetic field tensor. It is known that the main disadvantages of determining the electrical macroanisotropy using three-component induction systems are the small values of the signals of the cross components, the strong influence of the shape of the well, the inconsistency of measurements due to different mechanisms for averaging the electrical resistivity for different components, as well as a strong loss of accuracy in determining the vertical resistance at large values coefficient of electrical macroanisotropy [3].

The second analogue [2] (the SKL-A-160 complex used in Western Siberia) implements 10 geophysical methods for well research, allows solving a number of geological and geophysical problems and, among other things, is focused on determining the macroanisotropy of electrical resistance from a combination of lateral logging sounding signals ( BKZ) and data of high-frequency electromagnetic logging sounding (VEMKZ).

The VEMKZ equipment includes 5 or 9 three-coil probes with a length of 0.5 to 2 m. Each probe operates at one frequency in the range from 14 to 0.875 MHz, the generator and two receiving coils are coaxial to the device, the phase difference and the ratio of emf amplitudes are measured . The BKZ module in the stand-alone SKL-A-160 equipment differs from the cable versions by placing electrodes on a rigid case with a fixed diameter of 0.102 m. BKZ to the coefficient of electrical anisotropy of the section; the need for preliminary identification of the anisotropic intervals of the section [4].

It has now been established that the BKZ method, due to physical reasons related to the joint symmetry of the field and the medium, fundamentally allows obtaining information on the transverse formation resistance [5, 6]. However, during probing, as the probe length increases, the measured signal becomes less sensitive to the transverse resistance value, and the apparent resistance tends to an asymptotic value equal to the longitudinal resistance (anisotropy paradox).

It is known that the solution of inverse problems and the interpretation of BKZ in anisotropic sections is a complex and ambiguous procedure. In [7], the equivalence of cylindrically layered anisotropic media to isotropic formations was revealed. A homogeneous anisotropic reservoir intersected by a well, when studied with BKZ probes, turns out to be equivalent to a heterogeneous isotropic reservoir with a decrease in resistance with distance from the borehole walls (KM equivalence). When interpreting, it is necessary to specify the values of the horizontal conductivity of an anisotropic medium. This circumstance makes it impossible to directly identify anisotropic intervals within the framework of the BKZ method.

Closest to the claimed device for measuring the electrical anisotropy of rocks (prototype) is a device for laboratory simulation proposed in [8]. The device includes a dipole-equatorial probe, the generator and receiving dipoles of which are placed on a plastic plate (frame) 1 mm thick and 1.2-1.8 cm wide. The moments of the dipoles are oriented perpendicular to the frame axis. The frame length during the experiment did not exceed 10 cm.

On the surface of the frame, lead spherical electrodes were installed, rigidly fixed in the frame to prevent their movement. The electrodes were connected with the help of wires to the standard electroprospecting equipment Elektrotest S. For research, a laboratory setup was designed, which, in addition to the probe, included a model of an anisotropic medium. The model consisted of a set of cardboard and plexiglass plates. As a result of model experiments, the fundamental possibility of determining the coefficient of electrical macroanisotropy from the results of measurements obtained by a dipole-equatorial probe was confirmed. The main disadvantages of the prototype include the need to pre-set the known values of the longitudinal electrical resistance of the macroanisotropic medium to estimate the anisotropy coefficient from the measurement results.

The technical problem of the claimed utility model is the elimination of the above disadvantages, and the technical result is the creation of a device for detecting and measuring the electrical macroanisotropy of rocks surrounding the well.

A scientifically based approach to obtaining a technical result was formulated in the copyright certificate of one of the authors [9]. A logging method is proposed to determine the formation anisotropy coefficient. The method is devoid of such disadvantages of BKZ as anisotropy paradox and QM equivalence. It is proved that the paradox of anisotropy and QM equivalence is based on the axial symmetry of the field, and it is concluded that both phenomena are absent in the simplest nonaxisymmetric fields (for example, of the dipole type).

The following design of the device (probe) is claimed. A non-conductive hollow cylinder of radius d serves as the body of the device, on the outer surface of which there are supply (A and B) and receiving (M and N) electrodes (Fig. 1). All electrodes lie in one secant plane passing through the axis of the cylinder and are located at opposite points of diameters AB and MN. The length of the probe is the distance L between the diameters AB and MN. The device is powered by direct current I flowing from electrodes A and B. The probe signal ΔU _MN is the measured potential difference between electrodes M and N.

The rationale for the proposed design is based on the construction of a mathematical model of the device, numerical modeling and analysis of the relationship between the signal measured by the probe and the cut parameters in a cylindrically layered medium.

обладающей сопротивлением ρ_c и пересекающей анизотропный пласт бесконечной мощности. Коэффициент анизотропии пласта, его продольное и поперечное сопротивления равны Λ,ρ_t,ρ_n соответственно (Фиг. 1).Let the probe be in a well with a radius

having resistance ρ _c and crossing an anisotropic layer of infinite thickness. The formation anisotropy coefficient, its longitudinal and transverse resistances are equal to Λ,ρ _t ,ρ _n respectively (Fig. 1).

It is convenient to start studying the relationships between the field characteristics and the parameters of geoelectric models by constructing and analyzing asymptotic expressions for the field components.

в двухслойной анизотропной среде, справедливое в случае относительно больших длин зондов

и предположении бесконечной длине непроводящего корпуса зонда:Using the method described in [6], one can obtain an approximate expression for the probe signal

in a two-layer anisotropic medium, valid in the case of relatively large probe lengths

and assuming an infinite length of the non-conductive probe body:

и из (1) следуют основные свойства асимптотического представления

The following notation is introduced here:

and from (1) the main properties of the asymptotic representation follow

1. The value of the probe signal is inversely proportional to the square of the reservoir anisotropy coefficient Λ

2. The remaining electrical and geometric properties of the medium are included in the expression for the signal through the so-called contrast ratio k _t , which contains only the values of the longitudinal electrical formation resistance and well resistance. Since the modulus of the value of this coefficient does not exceed unity, the conditions for the validity of asymptotics (1) are practically not related to the values of the contrast coefficient.

сигналы длинных зондов

равномерно по геоэлектрическим параметрам описываются простым приближенным соотношением (1).It has been established that in a practically significant range of values of the macroanisotropy coefficient

long probe signals

uniformly in geoelectric parameters are described by a simple approximate relation (1).

Let us neglect in (1) the terms containing the contrast ratio compared to unity. As a result, we obtain a simple expression that uniquely relates the signal measured by the probe to the anisotropy coefficient:

(2)

(2)

On the basis of numerical simulation and analysis of electrical signals in macroanisotropic media, an analysis of the measured signals in the considered probe system of the proposed device was performed. The minimum length of the probe is established, at which the measured signal is described by approximate expressions.

Referring to the data of Fig. 2, where for a number of values of the formation anisotropy coefficient (code of curves - Λ) the dependence of the apparent anisotropy parameter Λ _k on the length of the probe is presented.

Based on formula (1), Λ _k is introduced as follows:

(3)

(3)

For the asymptotic representation of the signal (2), the apparent parameter has the following form:

Of those shown in Fig. 2 of the mathematical modeling data, it follows that with an increase in the length of the probe, the apparent anisotropy parameter introduced by formula (3) monotonically increases with an increase in the length of the probe, and at L=0.8-1 m for all considered values of the model parameters differs from the true value of the anisotropy in the smaller side, no more than 3-5%. As L increases further, the parameter Λ _k tends to the corresponding value Λ.

The set of features, including the dipole nature of the electric field generated by the claimed device, and the choice of the length of the device L=0.8-1 m ensures that the utility model obtains the claimed technical result. The probe signals of the declared length are uniquely related to the macroanisotropy coefficient of the medium and practically do not depend on its longitudinal conductivity.

In the probe length range L>0.8 m, the proposed device can serve as a direct-acting sensor that detects the presence of electrical anisotropy when the probe passes through the section. Indeed, due to the simplicity of functional relationships (3), (4), the measured signal does not need to solve the corresponding inverse problem to estimate the anisotropy and can be calibrated in terms of the parameter Λ.

As follows from (4), the parameter Λ _k in the case of an isotropic reservoir is practically equal to 1, regardless of the values of the longitudinal resistivity of the medium. This property of the proposed device allows its calibration inside the well, when passing through the interval of isotropic formations.

The device is illustrated by drawings, which show:

Fig. 1 is a general view of the structural scheme of the technical solution (view in the direction perpendicular to the secant plane).

Fig. 2 - dependence of the apparent anisotropy parameter on the probe length for different values of the anisotropy coefficient. Receiving M, N and current A, B push-button electrodes of the device are located on an infinitely long non-conductive case with a radius of d=0.03 m, placed in a two-layer cylindrical-layered anisotropic medium with parameters ρс=2 Ohm m, a=0.108 m, ρt=3.2 Ohm m Code of curves - the true value of the formation electrical anisotropy parameter.

Fig. 3 experimental diagrams of the layout of the proposed device, obtained on the physical model "well tank with electrolyte air"

An example implementation of the utility model

The device is operated as follows. Using a signal generator (conditionally not shown), a direct current I is passed in the electrode circuit AB and, thereby, an electric field is created in the environment of the device. Using a block of measuring equipment (conditionally not shown), the potential difference ΔU _MN of the receiving electrodes M and N is measured. The values of I and ΔU _MN are fed to the input of an operational amplifier (conditionally not shown), which at the output generates a value Λ _k equal to the anisotropy coefficient of the medium.

The proposed device is practically feasible, since, as follows from (2), in the considered model, the signal value of a galvanic probe 1 m long is estimated to be acceptable, from the point of view of metrology, ~ 0.001 V at a current I=1 A.

A physical simulation of the operation of the device was performed. The physical model is located in the room and is arranged as follows (Fig. 3). In the central part of the tank with a volume of 3.75 × 3.75 × 2.53 cu. m. a through well with a diameter of 0.13 m and a depth of 4 m was drilled. The bottom and walls of the tank are covered with a layer of concrete 0.2 m thick. A waterproofing layer made of tar is applied over the concrete. A concrete funnel is laid out at the bottom of the tank at the wellhead. The well and tank are filled with electrolyte. The thickness of the electrolyte layer in the tank is 1.94 m.

Before each experiment, the entire volume of liquid was thoroughly mixed for 1 hour using a submersible pump. During the experiment, the specific resistance of the electrolyte was kept equal to 1 Ω⋅m, with an error of less than 1.5%.

When creating a prototype probe, a piece of fiberglass pipe with a diameter of 0.077 m and a length of 2 m was used as a non-conductive case. Button current electrodes with a diameter of 0.01 m were placed at a distance of 0.6 m from the edge of the pipe. Pairs of measuring electrodes. The lengths of the probes and the size of the probe body were chosen based on the conditions of applicability of the model of an infinitely long non-conductive probe base.

To measure the sensor signal, a verified set of electrical prospecting equipment ANCh-3 (operating frequency 4.88 Hz) was used. Passport measurement error is 3-5%. Before the start of measurements, the probe was installed on the bottom of the well. With the help of a winch, the structure was moved up to a distance of 0.1 m. After a pause of 10 seconds, measurements were taken. The measured signals are normalized to the theoretical signals of the probe in a homogeneous medium with a resistance of 1 Ω⋅m. The results are presented in Fig. 3 as diagrams of normalized probe signals of various lengths depending on the z-coordinate of the sensor receiver (solid lines).

In the section of the diagrams z<0.4 m, the normalized signal differs significantly from unity, which is explained by the influence of the “electrolyte-air” boundary. At z=0.9 m, the normalized signal tends to the theoretical limit equal to unity and for probes 0.1, 0.2, 0.4 m long is 1.01, 1.05, 1.07, respectively. This result experimentally confirms the conclusions obtained as a result of numerical analysis, the correctness of the choice of the model base and all design solutions. With further advancement of the probe, the diagrams reflect the presence of a concrete wall and a funnel and then reach the values corresponding to the "well - sandy loam" model.

The experimental data corresponding to the range z=3.2-3.6 m were inverted within the framework of the cylindrical-layered model "well - isotropic reservoir". The best agreement between the experimental and theoretical data for all sensor lengths was achieved at a resistivity value of the isotropic formation of ~106 Ohm⋅m. The calculated signals for this case are shown in Fig. 3 by a broken line.

The laboratory physical model described above is used to control the measured parameters of the VIKIZ equipment sets produced by the research and production enterprise of geophysical equipment "Luch": http://www.looch.ru. The phase shift recorded by the VIKIZ probes in the soil interval is ~2 degrees, which corresponds to the resistance of sandy loam - 100 Ohm⋅m.

Literature:

1. Anderson B., Barber T., Leverage R. et al. 3D induction logging: old measurements from a new angle. Oil and Gas Review, Volume 19, Number 2, Schlumberger, 2008.

2. Epov M.I., Kayurov K.N., Eltsov I.N., Sukhorukova K.V., Petrov A.N., Sobolev A.Yu., Vlasov A.A. [2010] New hardware complex for geophysical logging of SCR and software and methodological tools for interpretation EMF Pro. Drilling and oil, 2, 16 19.

3. Epov M.I., Eremin V.N., Manshtein A.K., Petrov A.N., Glinskikh V.N. Device for measuring the specific electrical conductivity and electrical macroanisotropy of rocks: Patent of the Russian Federation. 2014. No. 2528276.

4. Sukhorukova K.V. Determination of electrophysical parameters of terrigenous deposits based on joint numerical inversion of electrical and electromagnetic logging data in vertical and deviated wells. The dissertation submitted for the degree of Doctor of Technical Sciences in the specialty 25.00.10 - geophysics, geophysical methods of prospecting for minerals. Novosibirsk, 2017.

5. Verzhbitsky V.V. BKZ probes in a triaxial anisotropic medium // Geology and Geophysics. 1993. No. 4. pp.145-147.

6. Dashevsky Yu.A. The study of electrical anisotropy of rocks in wells using stationary geoelectrics: Proc. allowance / Novosibirsk state. un-t. Novosibirsk, 2008. 102 p. ISBN 978-5-94356-625-7.

7. Kunz K. S., Moran J. H. Some effects of formation anisotropy on resistivity measurements in boreholes // Geophysics. - 1958. - Vol. 23, no. 4. - P. 770-794.

8. Kaurkin M.D. Laboratory modeling of dipole resistivity probes and electromagnetic probes with toroidal antennas. The dissertation submitted for the degree of candidate of technical sciences in the specialty 25.00.10 geophysics, geophysical methods of mineral prospecting. Moscow, 2015.

9. Tabarovsky L.A., Dashevsky Yu.A. The method of electrical logging. Author's certificate of the USSR No. 693314, 1979.

Claims (2)

1. A device for measuring the electrical macroanisotropy of rocks, containing a signal generator, a block of measuring equipment, an operational amplifier, a housing and pairs of push-button electrodes, characterized in that the housing is made non-conductive, and pairs of push-button electrodes placed on the surface are used as a source and receiver of the field housings in the same sectional plane at a distance of 0.8-1 m from each other. 2. A device for measuring the electrical macroanisotropy of rocks according to claim 1, characterized in that it is configured to calibrate the signal.

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