RU2478991C2 - Methods and apparatus for measuring streaming potentials and determining underground formation characteristics - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: underground formation characteristics are determined using a group of electrodes which can be used to measure streaming potentials in the formation. The electrodes are mounted on a wireline cable, a logging while drilling tool or at fixed points near the completed wellbore. The measured streaming potentials are generated by drilling with overbearing pressure, cutting the mud cake in the wellbore, injecting acid or using any other method which causes fluid movement. The obtained data are interpreted in order to locate fractures. The transient process of variation of voltage which corresponds to the streaming potential having a profile with a double peak indicates fractures in the formation.

EFFECT: high accuracy of locating fractures.

18 cl, 23 dwg

Description

BACKGROUND OF THE INVENTION

1. The technical field to which the invention relates.

In general, the present invention relates to the oil and gas industry. More specifically, the present invention relates to devices and methods for measuring filtration potentials resulting from transients of pressure changes in an underground formation intersected by a wellbore. The present invention also relates to methods for carrying out studies relating to the characterization of an underground formation from measurements of a filtration potential. One such characteristic is the permeability of the formation at various depths, but the invention is not limited to this.

2. The level of technology

The background to the possibility of measuring the filtration potentials in the reservoirs intersected by the wellbore is long. In US Pat. No. 2,433,746,1947, Doll suggested that strong vibration of the downhole device in the wellbore can cause pressure oscillations and fluid movement relative to the formation, which in turn can lead to an increase in the filtration potentials available for measurement due to the effect of electrokinetic potentials. In US patent No. 2814017, 1957, Doll proposed methods for studying the permeability of underground formations by measuring phase differences between periodic pressure waves passing through the formations and potentials created by the oscillatory movement of the formation fluid caused by these pressure waves. On the other hand, it was proposed to use a periodically changing electric current to create oscillatory movement of the formation fluid, which, in turn, leads to the formation of periodic pressure waves in the formation. As a result of the measurement of the phase shift between the creating and created physical quantities, a direct indicator of the relative permeability of the formation will be obtained.

US Pat. No. 3,599,085 (Semmelink A.), entitled “Apparatus for well logging by measuring and comparing potentials caused by sonic excitation”, 1971, proposed applying low frequency acoustic energy to the formation surface in order to generate powerful electrokinetic or flow pulses in the vicinity of the acoustic generator. In accordance with the disclosure of this patent, electrokinetic pulses resulting from compression (i.e. viscosity and inertia) of the formation and filtration potential pulses cause periodic displacement of the formation fluid relative to the formation rock. The movement of the fluid creates detectable electrokinetic potentials at the same frequency as the applied acoustic energy, and having values at any given location that are directly proportional to the velocity of the fluid and inversely proportional to the square of the distance from the geometric location of the pulse points of the filtration potential. Since it was found that the fluid velocity decreases with respect to the initial value with increasing distance traveled through the formation, with a rate partially dependent on the permeability of the formation rock, it was suggested that the value of the electrokinetic potential at any given distance from the pulse gives a relative indicator of the permeability of the formation . Having obtained the ratio of the magnitudes (amplitudes of sinusoids) of the electrokinetic potential in places that are separated from the acoustic generator by which it is possible to find the effective penetration depth of the electrokinetic potential, in turn, it is possible to determine the actual permeability.

In US Pat. No. 4,427,944, 1984, Chandler proposed a stationary type downhole tool and method for determining formation permeability. The downhole tool includes a downhole logging probe, which is engaged with the surface of the formation in the right place and which includes means for pumping fluid into the reservoir and electrodes for measuring transients of changes in electrokinetic filtration potentials and response time for pumping fluid. Fluid injection is effective for generating pressure pulses in the formation that cause a transient flow in the formation. Chandler proposed measuring the characteristic response time of transient filtering potentials generated in the formation in such a way as to obtain accurate information regarding the permeability of the formation.

In US patent No. 5503001, 1996, Wong proposed a method and apparatus for measuring the filtering potential of a finite frequency and electroosmotic voltage induced due to the application of pressure oscillations with a finite frequency and alternating current. The proposed device includes an electromechanical converter, which at the final frequency creates oscillations of the pressure drop between two points, and many electrodes, with which a pressure drop and a filtering potential signal are detected between the same two points near the source of applied pressure and at the same frequency by using a synchronous amplifier or digital frequency response analyzer. Wong believes that since the pressure differential at the final frequencies between two points of the porous medium near the source of applied pressure (or current) is measured using a measuring device, this significantly reduces the effect of background noise created by hydrostatic pressure due to the depth of the measured formation.

Despite the long background and numerous ideas from the prior art, the applicant believes that, in fact, before field measurements performed in support of the present invention, downhole measurements of transient changes in the filtration potential in real oil fields were never carried out. There are several reasons for the lack of actual implementation of the proposed prior art embodiments. Wong believes that neither measuring the filtration potential nor measuring the electroosmotic potential alone provides a reliable indicator of the permeability of the formation, especially a formation with low permeability. Wong claims that attempts to measure the signal of the filtration potential using electrodes located at distances greater than one wavelength from each other are erroneous, since pressure oscillations propagate as an acoustic wave, and the pressure drop will depend on the magnitude and phase of the wave, and potential will be very weak, since a significant part of the energy will be lost to viscous scattering throughout this distance. In addition, Wong claims that applying a direct current to the formation and measuring the transient voltage in the time domain will not have an effect in formations with low permeability, since with a longer response time and a very low signal of the filtration potential at the electrode interface, the dynamics will prevail over time voltage drifts. Therefore, despite the theoretical possibilities formulated in the prior art, the generally accepted point of view of specialists in this field of technology (which Wong focuses on) is that practical measurements of the filtration potential cannot be carried out due to low signal levels, high noise levels, poor spatial resolution and insufficient long-term stability. Undoubtedly, it is difficult to obtain pressure transient data with a high spatial resolution, since the wellbore is essentially an isobaric region. Using pressure sensors placed inside the wellbore, it is impossible to obtain detailed information about transient changes in pressure inside the formation if the formation is heterogeneous. To do this, it is necessary to divide the wellbore into hydraulically isolated zones, which is a difficult and costly task. In addition, it should be clear that some of the proposed devices from the prior art, even if they function as intended, find extremely narrow application. For example, the instrument proposed by Chandler will only work in drilled wellbores until casing, and it is necessary to stop the instrument for a while at each place where measurements are to be taken. Therefore, the instrument proposed by Chandler cannot be used as a device for measuring during drilling or logging during drilling, is not applicable in completed wells for measurements during production, and cannot even be used on a moving garland of logging devices.

SUMMARY OF THE INVENTION

Therefore, the objective of the invention is to provide methods and devices for measuring the filtration potential in an underground formation.

Another objective of the invention is to provide methods and devices for measuring the filtration potentials in the reservoir during the drilling of the wellbore.

An additional objective of the invention is to provide methods and devices for measuring the filtration potentials in the reservoir near the device, constantly installed in the wellbore.

The objective of the invention is also to create methods and devices for measuring the filtration potentials in the reservoir using a movable downhole tool.

An additional objective of the invention is to create methods for determining the characteristics of the reservoir by using the results of measurements of filtration potentials.

Another objective of the invention is to create methods for determining the characteristics of cracks in the reservoir by using the results of measurements of filtration potentials.

An additional objective of the invention is to provide methods for determining one or more of the permeability of the formation, the permeability of the near-barrel part of the formation with impaired permeability, effective permeability of the fracture, and horizontal and vertical permeability of the formation by using the results of measurements of filtration potentials.

In accordance with these tasks, which will be discussed in detail below, various methods and devices for measuring the filtration potential in an underground formation are proposed. The first embodiment of the invention relates to measuring the filtration potential during the drilling of a wellbore. In this application, measurement during drilling and logging during drilling will be considered equivalent. A second embodiment of the invention relates to measuring the filtration potential using a downhole tool, which is configured to take measurements while moving along the wellbore. A third embodiment of the invention relates to measuring the filtration potential using a device that is permanently installed (eg, cemented) around the wellbore. All embodiments of the invention can be used to detect formation characteristics. In particular, since the measurement of the filtration potential is directly related to the movement of the fluids, the results of measurements of the filtration potentials can be used to track the movement of fluids in the reservoir. In turn, this information can be used to find the permeability of the formation in various layers of the rock around the wellbore and / or to detect cracks in the formation and determine their characteristics.

Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description in conjunction with the accompanying drawings.

Brief Description of the Drawings

In the drawings:

Figure 1 is a schematic view of a completed horizontal well having electrodes located near it, for measuring filtration potentials;

Figure 2 is a schematic view of the electrodes mounted on insulated connecting sections, when completing the well of Figure 1 using sand filters;

Figure 3 is a graph of transients of pressure changes measured for two zones from the zones shown in Figure 1;

Figure 4 is a graph illustrating, depending on time, transients of pressure change and filtration potentials for the well of Figure 1;

FIG. 5 is a graph illustrating filtration potentials measured with electrodes in zone 2 of FIG. 1;

FIG. 6 is a graph illustrating filtration potentials measured with electrodes in zone 3 of FIG. 1;

FIG. 7 is a graph illustrating voltage drifts at the electrodes in zone 1 of FIG. 1;

Fig. 8 is a graph illustrating filtration potentials measured with electrodes in zone 1 of Fig. 1;

Fig.9 is a schematic view of the well of Fig.1, illustrating the qualitative definitions of the characteristics, carried out according to the information obtained using electrodes located near the well;

Fig. 9a is a schematic representation of a direct model of a heterogeneous formation;

Fig. 9b is a graph of filtration potentials obtained using the direct model from Fig. 9a;

Fig. 9c is a schematic representation of a straight fracture formation model;

Fig. 9d is a graph of filtration potentials obtained using the direct model of Fig. 9c;

Figure 10 is a schematic view of a completed vertical well having electrodes located near it, for measuring filtration potentials;

11 is a schematic illustration of the method in which the electrodes were installed in the completed well of FIG. 10;

Fig. 12 is a graph of the pressure at the wellhead applied to the well of Fig. 10 over several days;

Fig.13 is a graph illustrating the pressure at the wellhead of Fig.12 and the filtration potentials measured by a group of electrodes at the location of the collector shown in Fig.10;

Fig - an enlarged version of the plot of Fig;

Fig is a graph illustrating the pressure at the wellhead of Fig and filtration potentials measured by a group of electrodes above the location of the collector;

Fig.16 is an enlarged version of the plot of Fig.15;

Fig is a graph illustrating the pressure at the wellhead of Fig and filtration potentials, measured by a group of electrodes below the location of the collector;

Fig. 18 is a schematic view of the well of Fig. 10, illustrating qualitative characterization performed by information obtained using electrodes located near the well;

Fig. 18a is a schematic representation of a straight fracture model for a vertical production well;

Fig. 18b is a graph of filtration potentials obtained using the direct model from Fig. 18a;

Fig - enlarged version of the plot of Fig, which is used to show the stability of the electrodes;

Fig. 20 is a schematic view of a completed well with an open hole and with electrodes located near the insulating zone surrounding the elevator string;

Fig is a schematic view of a completed well with a cased hole and with electrodes included in the casing;

Fig is a schematic view of a tool for logging while drilling with electrodes for measuring filtration potentials located on it;

23 is a schematic view of a cable-launched device having electrodes for measuring filtering potentials located on it;

Figa is a schematic representation of a direct model of a tool launched on a cable, which is configured to cut a clay crust of a wellbore;

Fig.23b is a graph of filtration potentials obtained using the direct model of Fig.23a, for not affected by the penetration of the zone;

Fig.23c is a graph of filtration potentials obtained using the direct model of Fig.23a, for the zone of penetration of the filtrate;

Fig.23d is a graph obtained using the direct model of Fig.23a, the sensitivity of the filtration potential to the penetration depth; and

Fig.23e is a graph illustrating the inversion of synthetic data for permeability and the best fit for the five parameters of the model.

Detailed Description of Preferred Embodiments

Before referring to the drawings, it is advisable to consider some theoretical considerations related to the physics of the invention. In reservoir rocks there is a thin charged double layer at the interface between the rock skeleton and the water in the pores. The surface of the skeleton is negatively charged, and the water is positively charged. When water moves under the action of a pressure gradient ∇ p , an electric current i _e is created along with the flow of water. Electric current is proportional to the flow of water, which is proportional to the pressure gradient:

i _e = L∇p, (one)

where L is the coupling constant, which is a property of the breed.

Transient pressure changes occur in the formation during many different operations that are carried out during the life of the well, such as drilling, injecting drilling fluid with its penetration into the formation, cementing, injecting water and acid into the formation, hydraulic fracturing and oil and gas production . Pressure transient testing is a generally accepted way of determining reservoir properties such as permeability, reservoir size, and hydrodynamic coupling between different zones and between different wells. In addition, as described below, to determine these properties can be used transient changes in the filtration potential associated with transient changes in pressure.

Modeling of reservoir pressure p can be carried out using multiphase flow models. It is advisable to begin modeling the filtration potential with the diffusion equation for a single-phase flow:

(2)

where k is the permeability, μ is the viscosity, ϕ is the porosity, and c is the compressibility of the fluid.

The filtration potential V can be calculated on the basis of the pressure field p obtained by modeling by solving the Poisson equation:

−∇ · σ∇V = ∇ · L∇p, (3)

where σ is the electrical conductivity.

From equation (2) it follows that the propagation time Δ t at a distance Δ x in the reservoir of the transient process of changing the pressure and the associated transient process of changing the filtration potential that occurs on the surface of the wellbore has the form

(four)

At an early point in time, transients of pressure and filtration potential changes are mainly sensitive to the properties of the reservoir near the wellbore, and at a late time, transients are sensitive to the properties of the reservoir both near the wellbore and at a distance from the wellbore. The properties of the reservoir at various distances from the wellbore can be determined by time-ordered interpretation of measured transients. The interpretation of pressure transients in such a time-ordered manner is generally accepted in the art. For example, transients of pressure change at an early point in time are used to determine permeability or “skin effect” disturbances, and transients of pressure change at a late point in time are used to determine reservoir boundaries.

These applications are very limited if there are measurements of only steady-state filtration potentials. For a steady state, equation (2) becomes

(5)

In this case, the pressure difference Δ p in the depth interval Δ x is proportional

(6)

The drop Δ V of the filtration potential depending on Δ p has the form

(7)

while it is proportional

(8)

Based on the established filtration potential, it is possible to obtain information only on the average value of the reservoir property, and at the same time, it is significant on intervals with large values ( Lµ ) / ( σk ). It is believed that in the presence of a clay cake, the steady-state filtration potential is dominated by a clay cake and is insensitive to reservoir properties. The permeability of the clay cake is very low, and a steady differential pressure exists mainly on the clay cake.

Although in principle it is possible to determine the properties of the reservoir at all distances from the wellbore (i.e., in the radial directions from the wellbore) by interpreting transients in an orderly manner over time, in practice the decisive question is the possibility of making measurements with sufficient quality: accuracy, spatial resolution and stability over an extended period of time. It is difficult to obtain pressure transient data with high spatial resolution because the wellbore is essentially an isobaric region. If the formation is heterogeneous, then using a pressure sensor placed inside the wellbore, it is impossible to obtain detailed information about transient pressure changes inside the formation. To accomplish this, it is necessary to segment the wellbore into hydraulically isolated zones, which is a difficult task requiring high costs in its implementation. On the other hand, the wellbore is not an equipotential surface for electric current to flow. Therefore, transient changes in the filtration potential can be measured using a group of electrodes placed inside the wellbore and electrically isolated (i.e. isolated) from each other, and information equivalent to the information during the test during the transient pressure change in hydraulically can be obtained isolated zone, since the filtration potential is determined by the pressure gradient. In fact, by using a group of isolated electrodes to measure the filtration potential, it is possible to measure the filtration potential with a higher spatial resolution than in the case of a transient test of a pressure change in a hydraulically isolated zone.

In view of the theoretical interpretation given above, according to one aspect of the invention, for measuring transients of a change in the filtration potential, insulated electrodes are placed in or near the wellbore or in the well. As will be discussed in more detail below, according to various embodiments of the invention, the electrodes can be placed on insulated sections of the drill pipe during measurements while drilling or logging while drilling or on the tool body, which is moved along the wellbore with cable logging. In applications after completion of the well, the electrodes can be placed on an insulated probe placed in an open hole when completing a well with an open hole, or on centralizers (or as parts of them) when completing the well using sand filters, or in an isolated casing environment when well completion by cementing. When completing an open hole well and with electrically isolated casing sections, metal casing can be used as electrodes. Regardless of how the electrodes are placed, the DC voltage differences, indicating the filtering potentials, are measured between the reference electrode and other electrodes of the group. The initial values of the voltage differences between the reference electrode and other electrodes, which are usually due to differences in the chemistry of the surface of the electrodes, are subtracted from all subsequent data to obtain transients of pressure change.

According to another aspect of the invention, transients of changing filtration potential are created in any of many ways. According to one embodiment of the invention related to drilling a wellbore, the pressure differential between the formation and the wellbore leads to the penetration of the drilling fluid into the formation, to transient pressure changes and transient changes in the filtration potential. According to another embodiment of the invention associated with cable logging of the wellbore, transient changes in the filtration potential are created by supplying the logging tool with one or more blades that are mounted on one or more by-pass levers, through which slots are cut in the clay crust during logging. Due to the large pressure drop between the formation and the wellbore, when cutting the clay cake, the fluid will flow through the slot, and the resulting transient pressure change can be measured. According to another embodiment of the invention, used at the completion and after completion of the well, transient processes of changing the filtration potential are created by pumping a solution to complete the well, cement, gravel, acids, proppant, by conducting a water injection test, an inflow test, etc. d. In fact, any change in production flow rate will also create transient changes in the filtration potential. Provided that there is a flow of electrically conductive fluids associated with transients of pressure change, a transient process of changing the filtration potential will occur, and it can be measured by using placed electrodes.

According to another aspect of the invention, data related to transient changes in the filtration potential obtained by the electrodes are interpreted to extract useful information. Those of skill in the art would understand that interpreting transient pressure change data (as opposed to transient change in filtration potential) in order to obtain reservoir properties, such as permeability, is a common method. In reservoirs with high permeability, transient pressure changes occur rapidly in time, while in reservoirs with low permeability transient pressure changes occur slowly. Transient changes in the filtration potential created by transient changes in pressure depend on the permeability of the formation in the same way as transients of pressure change.

As should be clear to specialists in this field of technology, there are tools for analytical and numerical simulation of transient pressure changes. The reservoir parameters of interest can be determined by varying the parameters in the model until the calculated pressure is consistent with the measured data. Formally, let R denote the set of reservoir parameters to be determined, and let f _p ( R ) denote the pressure transient obtained by the simulation. The discrepancy between the transients of the pressure change obtained by modeling and by measurement has the form:

E _p (R) = || f _p (R) -p || (9)

The discrepancy is minimized at R = R ₀ in order to obtain the inverted values of the reservoir parameters.

Quantitative interpretation of the data on the filtration potential in order to determine reservoir parameters, such as the permeability of the formation, can be carried out in the same way as the interpretation of the data on the transient process of pressure change. Let s be the set of measured transients. A set may consist only of transients of a change in the filtration potential V or it may consist of both transients of a change in the filtration potential and transients of a change in pressure. Let f _s ( R , L ) denote transients obtained by simulation, which depend on an additional set of parameters: on the coupling constants L in equation (3). (Electrical conductivity σ in equation (3) is usually known from electrical resistivity logging data). The discrepancy between the obtained simulations and the measured transients of the pressure change is found in accordance with:

E _s ( R , L ) = || f _s ( R , L ) - s || (10)

The discrepancy is minimized at R = R ₀ and L = L ₀ in order to obtain the inverted values of the reservoir parameters.

Specialists in the art should understand that Poisson's equation (3) is linear with respect to the coupling constant L , since the feedback of the filtration potential with the basic equations for pressure during electroosmosis is negligible. Therefore, the inversion of coupling constants is a direct linear inversion. In fact, the minimization of equation (10) is carried out in two stages. The first stage consists in fixing R and changing L and finding the suboptimal minimum of the discrepancy by solving the linear problem for R. The solution gives L as a function of R. In this case, the suboptimal minimum is a function of only R :

E _s1 ( R ) ≡ E _s ( R , L ( R )) (eleven)

The second stage consists in a nonlinear search for the minimum of equation (11) containing the same number of unknowns as equation (9). Therefore, the additional task of estimating coupling constants does not introduce additional computational complexity or mathematical uncertainty into the inversion problem.

According to another aspect of the invention, the measured transient changes in the filtration potential associated with the movement of fluid in the formation can be used, inter alia, to: track the movement of a trace of cement during cementing and thereby detect possible problems associated with cementing; tracking suspensions carrying gravel, and thereby controlling gravel packing; tracking the movement of acid during injection of acid into the formation, since when injecting acid, transient processes of changing the filtration potential are created; real-time monitoring of hydraulic fracturing; quantitative assessment of hydraulic fracturing; tracking the movement of water resulting from water injection; increase the efficiency of the test during the transient process of pressure change; and monitoring changes in reservoir parameters over long periods of time, including water saturation, relative permeability, and water cut.

Using the various aspects of the invention described previously, field trials were conducted on a horizontal production well, part of which is shown schematically in FIG. The horizontal production well 100 of FIG. 1 was completed in formation 105 using sand filters 114 (see FIG. 2) and segmented into three zones by annular packers 111a, 111b, 111c. The zone closest to the mouth of the horizontal well is designated as zone 1, the middle zone as zone 2 and the zone closest to the bottom as zone 3. Each zone is equipped with a valve assembly 113a, 113b, 113c, respectively, integrated throughout the filter 114, with two pressure sensors 115-1 and 115-2 associated with each valve device 113 (see Figure 2). Electrodes 118 were placed as discussed below.

Now turn to Figure 2, which shows the placement of the electrodes 118 according to the invention. As can be seen from FIG. 2, well 100 is completed using sand filter sections 114, which are connected to each other by means of insulated connecting sections 116 to form a completion column. It should be understood that the screen sections cannot be electrically isolated from formation 105 or from fluid in the annulus (not shown). The connecting sections 116 are electrically insulated. A centralizer 118 is installed in the middle of each connecting section. Due to the mass of the column 114, 116 for completion, the centralizers 118 are always in good contact with the formation 105. Therefore, in accordance with the invention, it is preferable that the centralizers 118 used as electrodes have high impedance measurement circuits. voltages and are connected to ground electronics by cable (not shown). A suitable design of centralizers is described in International Application WO 02/053871.

Those skilled in the art will appreciate that a completion column made of metal creates a short circuit for electric currents. The sections 114 of the screens used to complete the well 100 were fifteen feet long, and the connecting sections 116 were five feet long. Since the insulated connecting sections 116 of the completion column have a protective coating only in a small area near the electrodes 118, most of the electric currents can flow through the unprotected filter sections 114, resulting in a decrease in signal level. However, as shown below, there are still significant levels of signals that can be measured. It should be noted that for a quantitative interpretation, it is sufficient to take into account the leakage current in direct modeling.

As shown in FIG. 1, seven electrodes were provided for each zone with a total number of electrodes of twenty-one (designated 118-1, 118-2, ..., 118-21). With a filter section of fifteen feet long and a connecting section five feet long, the distance between adjacent electrodes in each zone was approximately twenty feet. The distance between the nearest two electrodes in different zones was a little over one hundred feet.

From the leak test data collected by pressure gauges on both sides of the completion column, it follows that zone 1 is hydraulically isolated from zone 2 and zone 3. This can be seen from FIG. 3, since the pressure 125a in zone 1 is much higher than the pressures 125b, 125c in zone 2 and zone 3, which indicates isolation. Therefore, for the voltage of the electrodes in zone 1 (from 118-1 to 118-7), the reference electrode was selected in zone 2 or zone 3, and for the voltage of the electrodes in zone 2 and zone 3, the reference electrode was selected in zone 1.

To create a transient change in the filtration potential, three electrical valves 113a, 113b, 113c and a sucker rod pump (not shown) on the formation surface were used to control fluid flow. Fluid from the annulus of each zone flowed into the pipe through the valve opening. A manometer 115-1 on the pipe side of the hole measured the pipe pressure, and a manometer 115-2 on the side of the annulus measured the pressure in the annular region between the reservoir and the filter. Pressure transients were created in formation 105 by turning the pump on and off and opening and closing valves 113, and they were measured by pressure gauges 115-2 on the annulus side.

For each of the three zones, the pressure in the annulus was equal to the reservoir pressure. As can be seen from Figure 3, the annular pressures 125b, 125c in the zone 2 and zone 3 are approximately equal, which indicates that these two zones are in hydraulic communication. Pressure 125a in the annulus in the area 1 above indicates that hydraulic isolation zone 1. By pumping fluid out of the wellbore tubular and the pressure was about 150 pounds / inch ² above the pressure in the annulus in the zone 2 and zone 3. Zone 2 and valve the zone 3 valve was opened for three hours and then closed. The pressure in the annulus in the zone 2, 125a shown in Figure 3 curve dropped by 150 pounds / inch ² (about 840 lb / ⁱⁿ² and about 690 lb / ⁱⁿ²⁾ to the level of tubing pressure immediately after opening the valve, and then it began to increase back to reservoir pressure. 3, the annular pressure in zone 3 is shown by curve 125b. The pressure growth curve in zone 3 rises faster than the pressure growth curve in zone 2, which indicates a higher permeability of zone 3 compared to the permeability of zone 2.

Specialists in the art should understand that immediately after opening the valves, a pressure gradient exists mainly in the damaged area near the well. It is known that the permeability in the damaged area or in the near-barrel part of the formation with impaired permeability is lower than the permeability of the intact formation. If the coupling constant between the pressure gradient and electric current is also less in the near-barrel part of the formation with impaired permeability than in the formation, then the filtration potential should increase with time at the initial stage, when the pressure gradient propagates from the near-barrel part of the formation with impaired permeability to the undamaged formation. At later times, the pressure increases back to the reservoir pressure, while the pressure gradient decreases and propagates deep into the reservoir, further from the electrodes, and the filtration potential decreases. The rates of the initial increase and subsequent fall of the filtration potential are determined by the permeability of the near-barrel part of the formation with impaired permeability, the thickness of the near-barrel part of the formation with impaired permeability and permeability of the undamaged formation. Figure 4 shows the transient process of changing the filtration potential, registered with the electrode 118-8 in zone 2, which first increases and then falls, and the transient process of pressure change. As expected, the time scale of the decrease in the filtration potential is comparable with the time scale of the increase in pressure.

The data on the filtration potential in zone 2, recorded using all seven electrodes from 118-8 to 118-14 in zone 2, are shown together with the pressure data in FIG. 5. Data on the filtration potential in zone 3 are shown in Fig.6. It is clear that within each zone, the reservoir is heterogeneous; all individual filtration potential curves in FIG. 5 and FIG. 6 have very different rise and fall rates, which indicates large variations in permeability. Thus, it can be seen that measuring the filtration potential using a group of electrodes provides significantly improved information compared to information that can be obtained on the basis of one pressure rise curve for each zone, which will give only the average permeability for this zone.

A close examination of the curve from electrode 118-12 in FIG. 5 reveals a double peak. The double peak is in accordance with the superposition of a rapidly increasing and rapidly falling component and a slowly increasing and slowly falling component. The fast component occurs due to the flow in the fracture having high permeability, and the slow component occurs due to the flow in the skeleton of the rock of the formation with low permeability. This interpretation is consistent with the images of the wellbore that were obtained from the downhole imaging device, and with the simulation results discussed below.

According to the invention, the value of the filtration potential is an indicator of the water fraction of the stream, and it varies from electrode to electrode. As can be seen from Figure 5, there is a small flow of water near the electrode 118-13 (that is, the filter potential remains at about 0 mV), and in Figure 6 there is no or small flow of water near the electrodes 118-16 and 118-17.

The voltage at the electrodes in zone 1 is shown in Fig.7. Since zone 1 is hydraulically isolated from zone 2 and zone 3, and the valve in zone 3 is kept closed, the voltages recorded at the electrodes drift. The drift order is less than one millivolt per day. Since the electrodes are steel centralizers, not protected from the annulus fluid, such a magnitude of drifts is expected.

In a subsequent inflow test, the valves in zone 2 and zone 3 were closed and the valve in zone 1 was open and remained open. The pressure transient and filtration potentials obtained from this test are shown in FIG. High values of the measured filtration potential and a double peak associated with electrode 118-1 indicate a crack. In addition to the fracture, significant variations in the growth times of the filtration potential (for example, when comparing electrode 118-6 with electrode 118-5) indicate large variations in the permeability of the formation over zone 1.

Now we turn to Figure 9, which summarizes the results of a qualitative interpretation of data on transient changes in the filtration potential from figures 5, 6 and 8. As can be seen from Figure 9, using the electrode 118-1, a fracture in the formation with high permeability is detected, then as data from electrodes 118-2 through 118-5 and from electrode 118-7 indicate the locations of formations having medium permeability, and data from electrode 118-6 indicate the location of formations with high permeability. In zone 2, data from electrodes 118-8 through 118-10 and 118-14 indicate the locations of formations with medium permeability, while data from electrodes 118-11 and 118-12 indicate the location of formations or minibands with low permeability. In addition, a fracture in the formation is detected using electrode 118-12. Data from electrode 118-13 indicates the location of the formation without water inflow. In zone 3, data from electrodes 118-15 and from 118-18 to 118-20 indicate the locations of the high permeability formations, while data from electrode 118-21 indicate the location of the formation with medium permeability, and data from the electrodes 118-16 and 118-17 indicate reservoir locations or minibands without water inflow. Specialists in the art should understand that the data on transients of changes in the filtration potential from figures 5, 6 and 8, summarized in Fig. 9, provide significantly more detailed information compared to the information that was previously available based on transients pressure changes.

The results of the qualitative interpretation summarized in FIG. 9 are supported by direct modeling. In particular, by using equation (2) for a single-phase flow based on a direct model of a heterogeneous formation, graphically shown in Fig. 9a, transient processes of changing the filtration potential were calculated. The characteristics obtained by modeling are shown in Fig. 9b. As can be seen from Fig. 9b, the filtration potential detected by the electrode placed in the high permeability region increases faster and decreases faster than the filter potential detected by the electrode placed in the low permeability region. Qualitatively, this characteristic obtained by modeling is in accordance with the data presented in figures 5 and 6. Similarly, transient changes in the filtration potential, calculated to support the analysis related to fractures, based on a direct model of another heterogeneous reservoir, are shown in Fig. 9c . As can be seen from Fig. 9d, the transient of the change in the filtration potential calculated on the basis of the direct model shown graphically in Fig. 9c confirms the interpretation of the filtration potential registered with the electrode 118-12 (figure 5); that is, a crack causes a double peak in the transient response of the filtration potential.

In light of the foregoing, it is clear that, if there is information about the filtration potential, the corresponding direct modeling and inversion can be carried out using the program for solving the Laplace equation and the two-phase flow model (i.e., oil and water), considered above with reference to equations (9) - ( 11), or using a multiphase flow model. As a result, the obtained data on transient changes in the filtration potential can be used to quantify the permeability of the formation, permeability of the near-stem part of the formation, effective permeability of the formation, horizontal and vertical permeabilities, hydrodynamic coupling between zones and wells and reservoir boundaries, much more more than is possible only as a result of a transient pressure test. As a result of this, a better understanding of the well and reservoir characteristics can be achieved, contributing to better control of the well and reservoir.

Now turn to figures 10-19, which reflects the possibility of using information on transient changes in the filtration potential related to the vertical injection well 200 located in the formation 205. As can be seen from Figure 10, the formation 205 includes a hydrocarbon reservoir, the location of which indicated between 1026 feet and 1047 feet. In addition, there is a thin layer of sand at a depth of 1,020.55 feet, which is hydraulically isolated from the hydrocarbon reservoir.

As can be seen from figures 10 and 11, the well 200 includes a casing 209 around which an insulating material 211 is provided. A group of electrodes 218 including electrodes 218-1 through 218-16 together with associated voltage measurement circuits that preferably are high impedance, located in the insulating material or on the outside. The casing, insulating material and group are cemented in place by the cement layer 217. Therefore, the electrodes 218-1 through 218-16 are in contact with cement 217, but not with the metal casing 209. For hydrocarbon production, the casing must be perforated to form oriented holes 219, schematically shown in FIG. 11, but without damage to the electrodes and connecting cables (not shown). In this case, no holes were made above the upper part of the collector (i.e., above 1026 feet). After perforation, electric current can flow through the perforations 219 to the metal casing 209. As the results of field tests have shown, the electrical insulation of the casing is imperfect, but functional. The lower electrode 218-16 in FIG. 10 was used as a reference electrode.

In the case of well 200, transient changes in the filtration potential were created by pumping water into the well. Water injection was controlled by a surface pump 221 and a surface valve 223 (both shown schematically in FIG. 10) and controlled by a pressure sensor 225 placed between the valve 223 and the wellhead. From the very beginning, the injectivity of the well was very low, therefore, acid treatment of the well and hydraulic fracturing were carried out. When determining the quality of well cementing, the probability of poor adhesion of cement to the casing was revealed. Therefore, the formation outside the reservoir interval of interest could be subjected to hydraulic fracturing, and the injected water could flow into such cracks.

The injection pressure at the wellhead is shown in Fig. 12. Prior to receiving the data shown in FIG. 12, the valve was closed for a long time. When the valve was opened, the discharge pressure increased sharply, and then periodically dropped and restored, since the pump was turned off for short periods of time.

Transient changes in the filtration potential detected by the main electrodes in the interval of the collector of interest are shown in Fig. 13, and in an extended time scale in Fig. 14 (it should be noted that the electrode 218-12 is out of order and therefore there is no data for it shown). It can be seen that the transient changes in the filtration potential have two components: one component changes very quickly in response to pressure changes, and the other component changes slowly over several days. The fast component is associated with water flowing into cracks with high permeability. Changes in the fast component in figures 13 and 14 are such that the filtration potentials decrease with increasing discharge pressure and increase with a decrease in discharge pressure. This was expected because the injected water, carrying positive charges, travels a distance from the wellbore and from the electrodes. The signs of the filtration potential of the well 200 are opposite to the signs of the transient changes in the filtration potential shown for the well 100, since data for the well 100 were collected during production when the water carrying the positive charges was transferred into the wellbore to the electrodes. The slow component of the transient response is due to the injection of water from the wellbore directly into the rock skeleton with low permeability or inter-reservoir flow from cracks to the rock skeleton. The direct flow of injected water into the rock skeleton is always directed from the electrodes. The inter-layer flow from cracks to the rock skeleton also occurs from electrodes if the electrodes are directly in the crack. The filtration potential recorded using such electrodes slowly decreases as water moves into the rock skeleton. If the electrode is at some distance from the crack, the interstratal flow passes by the electrode. As a result, depending on the exact location of the electrode relative to the crack, the filtration potential will either slowly decrease or slowly increase as water moves into the rock skeleton. The data in figures 13 and 14 can be interpreted as illustrating that the electrode 218-5 is located directly in a powerful crack, while the electrode 218-9 is located at a small distance from the crack.

Transient changes in the filtration potential detected by the electrodes above the interval of the collector of interest are shown in Fig. 15, and in an extended time scale in Fig. 16. The electrode 218-2 is located very close to a thin layer of permeable sand near the non-perforated section of the casing. But nevertheless, the filtration potential on the electrode 218-2 reaches a value of 150 mV, which is five times higher than the value of the filtration potential recorded with any electrode on the collector interval. This can be explained by understanding that the interval above the perforated interval has cracks, presumably coming from the cemented annulus (which was confirmed when determining the quality of cementing works). Therefore, the forms of transient changes in the filtration potentials in this interval differ from the forms in the reservoir interval. It was found that in this interval, the filtration potential contains three components: fast, medium, and slow. This is more clearly seen with respect to the electrode 218-2 in FIG. 16. The formation of cracks in the clay layer, located between the collector and a thin layer of sand, is likely. The flow through cracks in the clay layer occurs in an intermediate time scale relative to the time scales of flows through sand and rock skeleton.

Now turn to Fig, which shows the filtering potentials detected using electrodes 218-13 to 218-15 below the collector. Voltages detected by these electrodes are less than 1 mV. Therefore, it can be concluded that very little injected water flows below the reservoir interval.

Taking into account the measurements made using the electrodes, the results of which are shown in figures 13-17, a qualitative interpretation of the data on transients of the change in the filtration potential can be made and summarized is shown in Fig. 18. In particular, as can be seen in FIG. 18, a fracture with interstratal flow is present in the clay layer above the reservoir; a crack with interstratal flow is at around 1028.55 feet (electrode No. 4); a crack with interstratal flow is near the mark of 1037.55 feet (electrode No. 9); and a fissure with inter-reservoir flow is at around 1042.05 feet (electrode No. 11) (see FIG. 13).

The qualitative interpretation of Fig. 18 is supported by the direct model graphically shown in Fig. 18a and the simulation obtained by the characteristic shown in Fig. 18b, from which it is seen that the filtration potential due to inter-layer flows can have the same sign as the filtration potential due to the flow in the crack, or may have the opposite sign. Therefore, the obtained simulation data qualitatively reproduces the experimental data from the electrode 218-9 of Fig. 13 and the electrode 218-2 of Fig. 15.

Data on filtration potentials from electrodes 218-13 to 218-15 shown in Fig. 17 are presented in Fig. 19 on stretched time and voltage scales. Before the ground valve 223 was opened at time 116.43, the voltages at the electrodes 218-14 and 218-15 were stable up to one sampling level (i.e., up to 10 μV). The voltage at the electrode 218-13 was with several noise emissions up to 100 μV. Noise emissions occurring over a very short period of time are not related to the stability of the surface of the electrodes, but are probably due to interference from the wire 235 connecting the electrode and ground electronics 233 (Figure 10). These noise emissions can be reduced or eliminated by improving the wiring of electrical wires and electronics or downhole electronics.

As can be seen from Fig. 19, the voltages at the electrodes 218-13 to 218-15 are very well consistent with the pressure at the wellhead. When valve 223 is opened at time 116.43, all three voltages decrease, and when at time about 116.8 pump 221 is stopped, a small but noticeable peak is characteristic of all three voltages. The matching is very similar to that observed for much higher voltages, measured with electrodes located in the collector and in the interval above the collector. Based on this information, it can be concluded that the stability of the electrodes when they are cemented electrodes is of the order of 10 μV, and signals with levels of 100 μV are suitable for determining the properties of the collector of interest. The stability of the cemented electrode group 218 is at least one hundred times better than the stability of the open electrodes 118 in the form of centralizers shown in relation to the well 100.

According to another aspect of the invention, it is preferable that the electrodes from the group of electrodes used to detect and measure transient changes in the filtration potentials are covered or coated with a semi-porous coating material (such as cement), regardless of their use as centralizers shown in a well completed using sand filters, or in other stationary installations, or use in measuring instruments during drilling or in logging logs appliances discussed below. The semi-porous coating material must have a significant electrical conductivity, but a very low permeability, so that the ions can reach the electrode to allow stress measurements, and the new fluid cannot reach the electrode surface during the measurement period. The surfaces of the electrodes are in a stable chemical environment, which provides increased measurement stability. The currently preferred semi-porous material is cement, although semi-porous ceramics, clay, or other material may be used. Alternatively, electrodes with a liquid boundary can be used, since a semi-porous pin of an electrode with a liquid boundary stops the movement of the fluid, but allows ion diffusion. A stable electrode makes it possible to measure the transient over a longer period of time, thereby allowing for analysis of a longer formation and also allowing measurements of signals with lower levels.

With reference to measurements of filtration potentials described with reference to FIG. 10-19, it should be clear that using equations (9) through (11) discussed above and assuming that the cracks are thin with a given permeability, it is possible to determine the permeability of the formation (rock skeleton) and the effective permeability of the cracks along the length of the well. In addition, the measurement results of the filtration potentials can be used for real-time monitoring of hydraulic fracturing. For example, when performing hydraulic fracturing in well 200, the target was the average reservoir interval of interest, and hydraulic fracturing of the upper interval was undesirable. However, the injected water did not go wherever wanted. With the presence of data on the filtration potential recorded during the hydraulic fracturing procedure, at a very early time it became clear that the hydraulic fracturing fluid moves mainly to the upper interval (above the reservoir). After that, the work on hydraulic fracturing was halted, repair and insulation work was carried out, and the hydraulic fracturing work plan was brought to an end properly.

Turning now to FIG. 20, a formation 305 is shown intersected by an open hole 300 with an open hole having an elevator string 306 extending therethrough. Around the elevator column is shown an insulated probe 311 with electrodes 318-1, 318-2, ... located on the insulated probe 311. Therefore, the elevator column 306 is essentially only a means of transporting the probe 311 to its desired locations. Other means of transportation may be used, preferably relatively strong, but somewhat flexible. Specialists in the art should understand that in, on or as part of the probe 311 and electrodes 318, wires are provided connecting electrodes, measuring electronics and telemetry equipment or a storage device for storing data, but they are not shown in FIG.

The cased-hole completed well is shown in FIG. 21, with the formation 405 being intersected by the well 400. The well includes an insulated lift string 406, a casing having electrically conductive electrode portions 418-1, 418-2, 418-3, ... divided electrical insulating sections 416, which are cemented in the well with cement 417. Therefore, the metal casing is used as a group of electrodes, while the individual sections of the casing are electrically isolated from each other. Casing sections may be regular casing sections connected by means of insulating flanges, or specially designed casing sections made of two or more electrically isolated subsections. As can be seen in FIG. 21, electrodes 418 are in contact with cement 417 and with fluid within the casing. If the elevator string 406 inside the well is metal, it is preferred that the elevator string is electrically insulated or partially insulated.

According to another embodiment of the invention, there is provided an apparatus and method for measuring filtration potentials while drilling a wellbore. In particular, during the drilling process, the pressure differential between the formation and the wellbore causes the penetration of the drilling fluid into the formation and transient pressure changes and, consequently, transient changes in the filtration potential. In wells drilled using oil-based drilling mud, a filtration potential will exist if the drilling mud contains a water fraction.

Now turn to Fig, which schematically shows the design of the tool 510, suitable for measuring in the process of drilling the filtration potential in the well 500 of the well surrounded by the formation 505. The drill tool 510 includes a drill bit 507 and electrodes 518-1, 518-2 , 518-3, ..., 518-R (it is preferable that a semi-porous coating, such as cement, be applied to everything) mounted on the electrically insulated sections 511-1, 511-2, 511-3 of the drill pipe 515. The electrodes 518 are moved together with tool 510. Therefore, various electrodes in Rupp at different times will be perceived transient changes filtration capacity in fixed spatial points. The distance between the electrodes 518 in the group and the drilling speed is determined by the quantization frequency of the transient changes in the filtration potential. In other words, the point in time at which the electrode 518-2 is located in a specific place where measurements were previously performed using the electrode 518-1, depends on the drilling speed and the distance between the electrodes. In the embodiment of FIG. 22, the upper electrode 518-R is used as the reference voltage electrode, since it is farthest from the drill bit and will often reach places in the formation where the transient change in the filtration potential has already reached steady-state values. It will be understood by those skilled in the art that wires connecting electrodes, measuring electronics, and telemetry equipment are provided in, on, or in the measurement tool 510 during drilling while they are standard in the art, but are not shown in FIG. 22. FIG. 22 shows a processor 550 and its associated memory 560 for storing data that are used to obtain pilot data. It should be understood that processor 550 and storage device 560 for storing data can also be used in other embodiments, although various forward and reverse models may be used in processing on the processor.

In the case of a drilling measurement tool 510, the measurements of the filtration potential are passive stress measurements that can be made in a high-resistance wellbore by using high-impedance electronics. In wells drilled using oil-based drilling mud, it is necessary that the electrodes are as large as possible, and should be placed as close to the formation as possible to reduce electrode impedance.

Those of skill in the art should understand that, in order to properly analyze the data obtained by the logging tool 510 while drilling, a clay cake model built up during drilling should be included in the direct model. Exact models may be used, for example, disclosed in an article: Fordham E.J. and Ladva H.K. J., “Crossflow filtration of bentonite suspensions”, Physico-Chemical Hydrodynamics ”, 11 (4), 411-439 (1989).

With a tool 510 for measuring during drilling and a corresponding model, information about the filtration potential obtained and processed using the tool can provide various control data. Since the transient changes in the filtration potential created during drilling proceed rapidly over time in the case of a formation with high permeability and slowly in the case of a formation with low permeability, using the inverse model, which contains a model of the build-up of clay cake, the permeability of the formation in the zone of penetration of the filtrate and in the zone not affected by penetration.

Based on the logging tool 510 during drilling and the corresponding model, a system can be implemented for early detection of flushing fluid loss. In particular, there may be unexpected fluid loss due to natural and induced cracks during drilling. In this case, the filtration potential will increase instantly as the fluids move into the reservoir. Changes in pressure in the wellbore will be slightly slower, since the wellbore has an accumulating ability. Fluid losses visible on the surface will occur much later. For fractures induced by drilling, large changes in the filtration potential will be detected long before the fractures become significant. Therefore, monitoring the results of measurements of the filtration potential can be used for early detection of washing fluid losses.

Similarly, information about the filtration potential can be used for early detection of abnormal reservoir pressure. For example, when the reservoir pressure becomes higher than the borehole pressure, the sign of the filtration potential changes. This sign change can be observed before the fluid flows into the wellbore in an amount sufficient to observe the pressure surge. The development of flow reversal during drilling of the abnormal pressure zone can occur within a short but finite period of time. Any reversal of the flow will be immediately detected by the measurement results of the filtration potential. Therefore, measurements of the filtration potential are important in the early detection of abnormal reservoir pressure.

Turning now to FIG. 23, another embodiment of the invention is shown. On Fig presents the descent on the cable device 610 for measuring filtration potential. A cable-lowered device 610 is shown suspended on a cable 611 in a well bore 600 (having a clay crust 607) crossing the formation 605. A cable-lowered device 610 is equipped with an insulated probe 616, on which a group of electrodes 618-1, 618-2, 618- are located 3, ..., including a reference electrode 618-R and corresponding voltage measurement circuits, preferably high impedance. Preferably, the electrodes are coated with a semi-porous material, such as cement. In addition, the device 610 includes one or more levers 631, preferably outriggers, on which one or more blades 635 are fixed. The blades 635 are designed to cut slots in the clay cake 607 when the device launched on the cable is moved along the wellbore. Blades can be made of polycrystalline diamond. Since there is a large pressure drop between the reservoir and the wellbore (the largest pressure drop is on the clay cake), after the slot 635 cuts a gap in the clay cake 607, a new clay cake will quickly build up in the gap, which stops the flow of fluid. In the meantime, a pressure transient is being formed in the formation 605. In wells drilled using oil-based drilling fluid, transient changes in the filtration potential will occur if the drilling fluid has a water fraction.

As should be understood by specialists in this field of technology, in the continuous logging mode, the electrodes 618 are moved together with the device 610. Transient changes in the filtration potential are perceived at fixed spatial points by different electrodes in the group. The distance between the electrodes in the group and the logging speed is determined by the quantization frequency of the transition process of the filtration potential. The upper electrode 618-R is used as a reference voltage electrode, since it is farthest from the blades and there is still no transient process of changing the filtration potential. There are wires connecting the electrodes, measuring electronics and telemetry equipment, but they are not shown in Fig.23.

As mentioned previously, it is preferred that the levers 631 be retractable. As a result, the blades 635 can be retracted when there is no need for information on the filtration potential, and the device, if necessary, can be used for repeated flights in order to record data on the filtration potentials over a long period of time. To facilitate the matching of data from repeated flights, a gamma-ray detector 640 is provided.

As discussed previously when referring to a logging tool 510 while drilling, filtration potential measurements are passive stress measurements that can be performed in a high-resistance wellbore using high-impedance electronics. In wells drilled using oil-based drilling mud (without water fraction), it is preferable that the electrodes are relatively large (for example, but not limited to these dimensions, twelve inches by two inches), and to ensure tight contact with the formation, it is preferable that they were placed on articulated sites (not shown) or on a pressure logging probe.

In the case of using the device 610 launched on the cable, instantaneous water loss from the clay crust section is most likely to occur within a shorter period of time compared to the time necessary for the pressure transient to extend outside the filtrate penetration zone. In this case, the source of the transition process of the change in the filtration potential that occurs when cutting clay cake can be interpreted as a delta function of time. Data inversion for a short period of time can be carried out without any input from the clay peel model. After instant water loss, the clay crust will grow again due to the static process. The thickness of the clay crust will increase in proportion to the square root of time. Despite the fact that the clay crust grows in proportion to the square root of time, the inversion of data on filtration potentials over a longer period of time is still quite robust.

The penetration of the drilling fluid into the reservoir is an almost continuous process, even with a good system of circulation of the drilling fluid. Transient changes in the filtration potential that occur when the drilling fluid penetrates the formation can probably be measured if not too much time has passed from the time of drilling the well or expanding the wellbore to the time of logging. Therefore, by means of the device shown in FIG. 23, with the retracting blades (or without levers and blades), the filtration potential generated during the previous drilling and / or expansion of the wellbore and the continuous penetration of the drilling fluid into the formation can be recorded. In such a situation, a model for a long period of measurement and build-up of clay cake should be used to interpret the collected data on filtration potentials. Therefore, it should be clear that a cable-launched device for measuring the filtration potential can be used with appropriate modeling and inversion to perform formation permeability measurements in the filtrate penetration zone, outside the filtrate penetration zone and in the far zone, continuously along the length of the wellbore. Transients recorded over long periods of time without the use of a cutting knife will help determine the permeability in the far zone.

The ability to use the cable-lowered device from Fig. 23 to detect transient changes in the filtration potential and to obtain qualitative characterization is supported by the direct model from Fig. 23a and the simulation results shown in figures 23b-23e. In the model of FIG. 23a, it is assumed that instantaneous water loss due to cutting of the clay cake occurs over a shorter period of time compared with the period of time necessary for the transient process of pressure change to extend beyond the penetration zone of the filtrate. The source of the transition process of changes in the filtration potential that occurs when cutting clay cake can be interpreted as a delta function of time. It is assumed that after instant water loss by a new clay crust, any further course is suspended.

23b shows that the early transients are independent of the intact zone. The time interval defined by equation (4) is required in order for the transient process of pressure change to spread to the zone not affected by penetration. Fig. 23c shows that both the filtration potential in the early time and the filtration potential in the late time are sensitive to the permeability of the filtrate penetration zone. The fact that early data and late data are sensitive to permeabilities of different zones makes the inversion algorithm completely robust.

23d shows the dependence of the filtration potential on the thickness of the penetration zone of the filtrate. From equation (4) it can be seen that the time required for the transition of the pressure change through the filtrate penetration zone depends on the thickness Δ of the filtrate penetration zone and the permeability k of the filtrate penetration zone through the formula Δ ² / k . From equation (8) it can be seen that when approaching the steady state, the filtration potential due to the penetration zone of the filtrate depends on Δ and k through the formula combination Δ / k . The difference between the two formula combinations suggests that the thickness and permeability of the filtrate penetration zone can be individually determined by inversion.

The results of the inversion for the case of synthetic data determined from a direct model with the addition of noise in the amount of 5% are shown in Fig.23e. All inverted values of the permeability of the filtrate penetration zone, the permeability of the zone not affected by the penetration, and the thickness of the filtrate penetration zone are in very good agreement with the input values used in the direct model.

In this application, several embodiments of devices and methods for measuring filtration potentials and determining the characteristics of underground formations based on them have been described and explained. Although specific embodiments of the invention have been described, it is not intended that the invention be limited thereto, but rather that the scope of the invention is as wide as is permitted in the technical field to which the invention relates, and that the description should be construed in a similar manner. Therefore, it must be understood that although specific devices and electrode arrangements have been disclosed, modifications can be made provided that the device or arrangement includes a group of electrodes with which filtration potentials can be measured. Therefore, for example, the invention can be modified so that as a result, a two-dimensional group of electrodes can be used in some circumstances to obtain information about azimuthal filtering potentials. Specialists in the art should understand that other modifications of the invention may also be made without departing from its claimed essence and scope.

Claims (19)

1. A method of researching an underground formation traversed by a well, comprising the steps of:
a) use at least one sensor in or near the well to sense DC voltages corresponding to transient filtering potentials over a period of time; and
b) identify the fracture in the subterranean formation by identifying a double peak profile in said DC voltages corresponding to transient filtering potentials. 2. The method according to claim 1, in which additionally:
c) initiate the formation of said DC voltages corresponding to transient filtering potentials. 3. The method according to claim 2, in which:
with said initiation, a change in relative pressure is generated between the well and the formation. 4. The method according to claim 2, in which:
with said initiation, a clay crust is cut in said well. 5. The method according to claim 2, in which:
with said initiation, fluid is pumped into the well. 6. The method according to claim 5, in which: said liquid contains water. 7. The method according to claim 5, in which:
said fluid contains water contained in the drilling fluid. 8. The method according to claim 2, in which:
with said initiation, at least one component is pumped into the well for completion from the flushing fluid, cement, gravel, acids, fracturing fluid and propellant. 9. The method according to claim 2, in which:
with the mentioned initiation, the rate of formation fluid selection through the well is changed. 10. The method according to claim 1, in which:
with said use of at least one sensor, a plurality of sensors are used. 11. The method according to claim 10, in which:
the sensors of said plurality are at least partially isolated from each other. 12. The method according to claim 10, in which:
with said use of a plurality of sensors, a cable-launched device with said sensors is moved along said well. 13. The method according to item 12, in which:
said cable-lowered device includes means for cutting clay cake around a well. 14. The method according to claim 10, in which:
with said use of a plurality of sensors, an apparatus having a drill bit and said sensors is rotated. 15. The method according to claim 10, in which:
said well being an open hole, and with said use of a plurality of sensors, a plurality of sensors are used located on an insulating probe placed in said open-hole well. 16. The method according to claim 10, in which:
with said use of a plurality of sensors, said sensors are located on centralizers in wells completed using sand filters. 17. The method according to claim 10, in which:
with said use of a plurality of sensors, said sensors are placed on electrically isolated sections of the casing in a finished cased well. 18. The method of claim 10, in which:
with said use of a plurality of sensors, said sensors are located in an insulating environment of the casing in a cemented well. 19. The method of research of the underground formation, intersected by the well, which consists in the fact that
a) have multiple sensors in or near the well;
b) initiate formation of direct current voltages in the formation corresponding to transient filtering potentials;
c) use sensors to sense, over a period of time, said DC voltages corresponding to transient filtering potentials; and
d) identify the fracture in the subterranean formation by identifying a double-peak profile in said DC voltages corresponding to transient filtering potentials.

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