CN112099098A - Well wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging - Google Patents
Well wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging Download PDFInfo
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Abstract
The invention discloses a well wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging, which comprises the steps of improving the structure of an electrical imaging logging device, obtaining the measured impedance Z of a button electrode according to the flowing path of current, acquiring signals and obtaining a calculation expression of the measured impedance Z of the button electrode in the actual logging process, deducing the thickness of a mud cake and a calculation formula of formation impedance according to the real part and the imaginary part of the measured impedance of the button electrode, establishing a uniform formation model, constructing a variation problem met by the electrical imaging logging, solving the variation problem by using a finite element method, calculating an electrode coefficient K, and calculating the formation impedance Z by using the electrode coefficient KfConversion to formation resistivity RtThe method can calculate the formation resistivity and the mud cake thickness, distinguish the types of cracks and holes by calculating the mud cake thickness on the basis of acquiring the formation resistivity, and can more truly reflect the geological characteristic changeAnd (4) transforming.
Description
Technical Field
The invention relates to the technical field of geophysical exploration methods, in particular to a well wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging.
Background
In a crack type oil and gas reservoir and a crack-hole type oil and gas reservoir, cracks and holes are main oil and gas storage and migration channels, and the crack and the hole can be accurately identified and judged and evaluated, so that the method has important significance for oil and gas exploration and development.
The well logging is also called as geophysical well logging, belongs to the field of applied geophysical, measures parameters such as formation resistivity, dielectric constant, density and the like and distribution thereof in an underground borehole by using measurement methods such as electromagnetism, sound waves, nuclear magnetism, radioactivity, electrochemistry and the like, and is widely applied to the field of oil gas exploration and development.
The electric imaging well logging is an electric method well logging method, also called microresistivity scanning imaging well logging, and is characterized by that it utilizes array button electrodes densely distributed on the polar plate of well logging instrument to form microresistivity scanning array, and combines the matched components of shielding electrode, return electrode, transmitting and collecting circuit, etc. to measure several tens of resistivity curves to hundreds of resistivity curves at the same time, and utilizes the image processing process to obtain clear well wall stratum resistivity distribution image. The images may be used for formation fracture, hole identification, thin layer analysis, geological structure interpretation, and the like.
When the field logging operation is carried out, the well is filled with mud, and the effects of keeping the pressure in the well, lubricating and the like are achieved. The most common type of mud is water-based mud, i.e. mud having a continuous phase of water and a dispersed phase of other constituents, which has a low resistivity. The originally used electrical imaging logging instruments were suitable for low resistivity water-based muds. However, in considerable cases, water-based muds are difficult to meet on-site operating requirements. The oil-based mud has the advantages of good lubricity, high temperature and high pressure resistance, well wall stability maintaining, operation efficiency improving and the like, and is widely applied to the environments of large inclined wells, horizontal wells, shale strata, deep sea reservoirs and the like at present. However, the oil-based mud uses the oil phase as the continuous phase, has high resistivity which is usually hundreds of times or even tens of thousands of times of the resistivity of the water-based mud, and limits the original electric imaging logging instrument suitable for the water-based mud.
For this reason, research work on an electrical imaging logging instrument suitable for high resistivity oil-based mud and a data processing method thereof needs to be performed. The existing oil-based mud electrical imaging logging instrument has a plurality of defects, such as low resolution, single measurement parameter, difficulty in quantitatively representing formation parameter change, difficulty in identifying and judging well wall cracks, holes and the like. Aiming at the problem of identifying and judging the cracks and holes of the well wall in the oil-based mud environment, a method of jointly interpreting well logging data of electricity, sound waves and density is often adopted, so that the cracks and holes are difficult to identify under the condition that one method is lacked, and the interpretation and evaluation of crack-type and crack-hole type oil and gas reservoirs are seriously disturbed.
Disclosure of Invention
The invention aims to provide a well wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging, which is suitable for the electrical imaging logging of high-resistivity oil-based mud, can calculate the formation resistivity and the mud cake thickness, distinguishes the types of cracks and holes by measuring the mud cake thickness on the basis of measuring the formation resistivity, and can reflect the geological characteristic change more truly so as to solve the technical problems mentioned in the background technology.
The purpose of the invention is realized by the following technical scheme:
a well wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging comprises the following steps:
s1, improving the structure of the electric imaging logging device, making the button electrode I and the button electrode II recess towards the inside of the polar plate, and obtaining the expressions of the distances between the surfaces of the button electrode I and the button electrode II and the stratum, wherein the expressions are respectively as follows:
wherein d is1Is the recess distance between the surface of the button electrode I and the formation, d2Is the recess distance, h, between the surface of the button electrode II and the formation1Is the distance h between the button electrode I and the surface of the polar plate2Is a button electrode II anddistance between the surfaces of the polar plates, TmIs the thickness of mud cake between the polar plate and the stratum and satisfies h1≠h2；
S2, obtaining the measured impedance Z of the button electrode I and the button electrode II according to the flowing path of the current, wherein the measured impedance Z is the impedance Z of the mud cakemAnd formation impedance ZfSumming;
s3, collecting signals and according to the real part U of the potential difference U of the button electrode I, the button electrode II and the reference electrode in the actual logging processrAnd imaginary part UiReal part I of the current I of button electrode I and button electrode IIrAnd imaginary part IiRecording the frequency f to obtain a calculation expression of the measured impedance Z of the button electrode I and the button electrode II;
s4, deducing to obtain the mud cake thickness T according to the real part and the imaginary part of the measured impedance Z of the button electrode I and the button electrode IImAnd formation impedance ZfThe formula (4) is calculated;
s5, establishing a uniform stratum model, and setting the stratum resistivity as Rt *Placing an electric imaging polar plate at the center of the stratum model, and constructing a variation problem met by electric imaging logging in the stratum model;
s6, solving the variation problem in the step S5 by using a finite element method to obtain the potential difference U and the real current part I between the button electrode I, the button electrode II and the reference electroderAnd imaginary part IiCalculating the real part and the imaginary part of the measured impedance of the button electrode I and the button electrode II according to the calculation expression of the measured impedance Z in the step S3;
s7, measuring the real part and the imaginary part of the impedance of the button electrode I and the button electrode II obtained by calculation in the step S6, and the formation resistivity R set in the step S5t *Calculating an electrode coefficient K;
s8, in the actual logging process, according to the relation between the formation resistivity and the formation impedance in the electrical logging, the electrode coefficient K obtained in the step S7 is used for comparing the formation impedance Z obtained in the step S4 with the electrode coefficient K obtained in the step S7fConversion to formation resistivity Rt；
S9, in the actual logging process, according to the stepsThe real part and the imaginary part of the impedance of the button electrode I and the button electrode II calculated in the step S3 and the thickness T of the mud cake in the step S4mCalculating the thickness T of the mud cakem；
S10, calculating the thickness T of the mud cakemAnd formation resistivity RtAnd judging the types of well wall cracks and holes of the electric imaging logging.
Further, the mudcake impedance Z described in step S2 is based on the capacitive coupling principlemExpressed as:
wherein j is an imaginary unit
RmD is the distance between the surface of the button electrode and the stratum, omega is the angular frequency of the emission current, omega is 2 pi f between the frequency f of the emission current and omega,mris the relative dielectric constant of the oil-based mud,0the dielectric constant is 8.85 multiplied by 10 in vacuum-12F/m，SbThe surface area of the button electrode;
the measured impedances of button electrode I and button electrode II are then:
further, the calculation expression of the measured impedance Z of the button electrode I and the button electrode II described in step S3 is:
measuring the real part Z of the impedance ZrAnd an imaginary part ZiRespectively expressed as:
and (4) according to the formula (6) and the formula (7), respectively calculating the real part and the imaginary part of the impedance of the button electrode I and the button electrode II by using the acquired signals.
Further, the step S4 of extracting the real part and the imaginary part of the measured impedance Z of the button electrode I and the button electrode II respectively specifically includes:
introduction of parameters
θ＝ωmr 0RmExtracting the real part and the imaginary part of the measured impedance Z of the button electrode I and the button electrode II respectively according to the equations (3) and (4), and respectively expressing the real part and the imaginary part as follows:
wherein A is1Is the real part of the button electrode I, B1Is the imaginary part of button electrode II, A2Is the real part of the button electrode II, B2Is the imaginary part of the button electrode II.
Further, finishing the formula (8) to obtain the mud cake thickness TmAnd formation impedance ZfThe calculation formula (2) is specifically as follows:
the formula (8-b) is divided by the formula (8-d) to obtain
By substituting formula (1) for formula (9)
Obtaining the mud cake thickness T by the finishing formula (10)mThe calculation formula of (A) is as follows:
substituting formula (8-b) for formula (8-a) and substituting formula (8-d) for formula (8-c) yields the following expression:
solving the formula (12) to obtain the formation impedance ZfIs calculated by the expression (i.e.)
Further, the variation problem in step S5 is:
wherein Ω is the solution region, ρ is the complex resistivity, UEIs the electrode potential, IEIs the electrode current.
Further, in the homogeneous formation model, the formation resistivity R is knownt *Solving the variation problem by using a finite element method to obtain the potential difference U, the real part and the imaginary part of the current between the button electrode I, the button electrode II and the reference electrode, and calculating the real part A of the measured impedance of the button electrode I according to the formulas (5) to (7)1 *Imaginary part B1 *And the real part A of the measured impedance of the button electrode II2 *Imaginary part B2 *The specific numerical value of (1).
Further, the calculation formula of the electrode coefficient K in step S7 is:
the measured real part A of the impedance of the button electrode I calculated in the step S61 *Imaginary part B1 *And the real part A of the measured impedance of the button electrode II2 *Imaginary part B2 *The specific value of (2) is substituted into the formula (15) to obtain the value of the electrode coefficient K.
Further, in step S8, during actual logging, the formation impedance Z is measured by using the value of the electrode coefficient K calculated in the homogeneous formation modelfConversion to formation resistivity RtThe method specifically comprises the following steps:
further, the mud cake thickness T is calculated according to the formula (11) and the formula (16)mAnd formation resistivity RtAnd drawing a mud cake thickness curve and a formation resistivity curve and judging the types of the cracks and holes of the well wall.
The invention has the beneficial effects that:
compared with the prior art, the method for identifying and judging the cracks and the holes of the well wall based on the oil-based mud electrical imaging logging only depends on one logging means, namely the electrical imaging logging is adopted to identify and judge the cracks and the holes of the well wall stratum in the oil-based mud environment, so that the problem that the cracks and the holes of the well wall stratum in the oil-based mud environment need to be identified and judged by means of a plurality of other logging means such as acoustic logging, electrical imaging logging, induction logging and the like is avoided. In addition, the method is different from an imaging method utilizing total measured impedance, the formation resistivity and the mud cake thickness can be calculated, formation resistivity imaging can be carried out, the influence of high-resistance oil-based mud is reduced, and formation resistivity change is reflected more truly. The invention can reflect the thickness change of the mud cake, which is beneficial to analyzing the stratum permeability, enriches the stratum evaluation information based on the electric imaging logging information and can reflect the geological characteristic change more truly.
Drawings
FIG. 1 is a schematic diagram of the configuration of an oil-based mud electrographic logging device of the present invention;
FIG. 2 is a schematic diagram of an electrical imaging logging pad of the present invention;
FIG. 3 is a schematic diagram of the measurement process of the improved plate of the present invention;
FIG. 4 is a schematic structural diagram of a homogeneous stratigraphic model of the present invention;
FIG. 5 is a graph of high resistance oil based mud filtrate for open fracture filling in accordance with the present invention;
FIG. 6 is a graph of closed fracture filled with high resistivity minerals according to the present invention;
FIG. 7 is a graph of the closed fracture filling low resistivity mineral of the present invention;
FIG. 8 is a graph of the mixing of the open fracture-filling oil based mud filtrate with formation water in accordance with the present invention;
FIG. 9 is a flow chart of the identification and determination method of the present invention;
in the figure, 1-button electrode I, 2-button electrode II, 3-pole plate, 4-formation, 5-wellbore, 6-mudcake, 7-logging instrument, 8-armored cable, 9-derrick, 10-logging truck, 11-winch, 12-support arm, 13-first insulating medium, 14-shield electrode, 15-second insulating medium, 16-return electrode, 17-third insulating medium, 18-first current, 19-second current, 20-crack, 21-hole.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the following embodiments, and it should be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without inventive effort based on the embodiments of the present invention, are within the scope of the present invention.
Example 1
Referring to fig. 1 to 9, the present invention provides a technical solution:
before describing the method for identifying and determining the cracks 20 and holes 21 in the borehole wall based on the oil-based mud electrical imaging logging of the present invention, firstly, the usage of the oil-based mud electrical imaging logging apparatus of the present invention will be described, referring to fig. 1, a borehole 5 penetrates through a formation 4, the borehole 5 is filled with oil-based mud, and the formation 4 contains a plurality of sub-layers of different types. Under the pressure difference between the wellbore 5 and the formation 4, the oil-based mud invades the formation 4 and adheres to the walls of the wellbore a mud cake 6 of uneven thickness. A logging instrument 7 is suspended in the wellbore 5, where the logging instrument 7 is an electrical imaging logging instrument 7. The logging instrument 7 is connected with a derrick 9 on the ground through an armored cable 8, and the other end of the armored cable 8 is connected with a winch 11 on a logging truck 10. The logging truck 10 is provided with a microcomputer control system (not shown) for controlling the motion state of the downhole logging instrument 7. The logging instrument 7 is connected with the imaging polar plate 3 by utilizing the supporting arm 12, and when the logging instrument works, the supporting arm 12 pushes against the imaging polar plate 3, so that the imaging polar plate 3 is in close contact with a well wall.
When the formation 4 contains the fracture 20, the fracture can be divided into an open fracture and a closed fracture according to the opening and closing state of the fracture 20, the open fracture is filled with flowable fluid such as formation water, oil gas and the like, and the closed fracture is filled with minerals such as feldspar, calcite and the like. Similarly, when a hole 21 exists in the formation 4, the hole 21 is filled with fluids such as formation water, oil and gas, or minerals. In the process of drilling and logging, in order to prevent accidents such as blowout and the like, the pressure of a shaft 5 is generally higher than the pressure of a stratum 4, so that mud in the shaft invades into the stratum 4, particularly the permeable stratum 4, therefore, when open cracks and connected holes exist in the shaft wall stratum 4, original fluids in the cracks 20 and the holes 21 are displaced, mud filtrate is filled, and a layer of mud cake 6 is attached to the shaft wall; on the other hand, the slurry cannot penetrate into the impermeable mineral-filled fractures 20, cavities 21, and no mud cake 6 adheres to the borehole wall.
In oil-based muds, the oil phase is the continuous phase, resulting in high resistivity values of the oil-based mud; the mineral such as feldspar, calcite, etc. also has a high resistivity value, and the mineral such as pyrite, etc. has a low resistivity value. Therefore, in an oil-based mud well, it is difficult to distinguish open fractures 20, holes 21 filled with high-resistivity mud from closed fractures 20, holes 21 filled with high-resistivity mineral using conventional microresistivity scanning imaging, subject to the effects of high-resistivity oil-based mud invasion; in addition, due to the mixing of the mud filtrate and the formation water, it is difficult to distinguish between open fractures 20 and holes 21 filled with mixed mud filtrate and formation water fluid and closed fractures 20 and holes 21 filled with low-resistance minerals by using conventional microresistivity scanning imaging.
The details of the imaging pad 3 of the oil-based mud electrographic logging device described above are described in further detail to facilitate understanding and enabling the practice of the invention by those of ordinary skill in the art:
referring to fig. 2, which is a front view and a side view of the plate 3, two rows of button electrodes, i.e., button electrode I1 and button electrode II 2, are distributed at the middle position of the plate 3, the number of the two rows of button electrodes is not limited, and the number of the button electrodes at the upper row is 1 more than that of the button electrodes at the lower row. The button electrode is provided with a first insulating medium 13, a rectangular annular shielding electrode 14 is arranged around the button electrode, and a second insulating medium 15 is arranged between the shielding electrode 14 and the polar plate 3. The two ends of the polar plate 3 are respectively provided with 1 return electrode 16, and a third insulating medium 17 is arranged between the return electrodes 16 and the polar plate 3.
When in work, the polar plate 3 is tightly attached to the stratum 4, and a thin mud cake layer 6 is arranged between the polar plate and the stratum 4. The button electrode I1, the button electrode II 2 and the shielding electrode 14 are kept at the same potential and respectively emit a first current 18 and a second current 19 with certain frequencies, and the two currents pass through the thin mud cake layer to enter the stratum 4 and then pass through the thin mud cake layer 6 to flow back to the return electrode 16. Here, the formation 4 contains fractures 20, holes 21, which are open or closed with mineral fill.
It is not feasible to distinguish open fractures, vugs invaded by high resistivity oil-based mud from closed fractures, vugs filled with high resistivity mineral, or open fractures, vugs filled with mixed mud filtrate and formation water fluid from closed fractures, vugs filled with low resistivity mineral, by relying solely on resistivity differences, and other measurement parameters are required. Under the influence of mud invasion, mud cakes 6 are attached to the well wall near the open cracks and holes, and the cracks and holes filled with minerals are not permeable, and no mud cakes 6 are attached to the well wall near the cracks and holes. Therefore, on the basis of measuring the formation resistivity, open fractures, cavities and closed fractures, cavities can be distinguished by measuring the thickness of the mud cake 6.
In order to achieve the above object, the following method can be adopted:
referring to fig. 9, a borehole wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging is characterized by comprising the following steps:
s1, improving the structure of the electric imaging logging device, and the structure and the measurement process of the improved polar plate 3 are shown in figure 3, wherein other structures are unchanged, only the button electrodes I1 and the button electrodes II 2 are modified, specifically, the surfaces of the button electrodes I1 and the button electrodes II 2 are not consistent with the surface of the polar plate 3 any more, but are sunken into the polar plate 3, and the sunken space between the surface of the button electrode and the surface of the polar plate 3 is filled with oil-based mud during logging. d1Is the recess distance, d, between the surface of the button electrode I1 and the ground layer 42Is the recess distance, h, between the surface of the button electrode II 2 and the formation 41Is the distance h between the button electrode I1 and the surface of the polar plate 32Is the distance between the button electrode II 2 and the surface of the polar plate 3 and satisfies h1≠h2(ii) a And satisfies the following conditions:
wherein, TmIs the mud cake thickness between the plate 3 and the formation 4.
S2, obtaining the measured impedance Z of the button electrode I1 and the button electrode II 2 according to the flowing path of the current, wherein the measured impedance Z is the impedance Z of the mud cakemAnd formation impedance ZfSumming;
the thickness of the mudcake 6 is generally small, where the mudcake 6 between the button electrode I1, the button electrode II 2, the current return electrode 16 and the formation 4 is equivalent to a cylinder, the height of the cylinder is equal to the thickness of the mudcake 6, and the bottom area of the cylinder is the surface area of the corresponding electrode. The measured impedance Z of the button electrode is composed of three parts, namely, the button electrode and the stratum 4 according to the flowing path of the currentImpedance of mud cake ZmEarth formation impedance ZfThe mudcake impedance Z between the formation 4 and the return electrodem'. The surface area of the return electrode is set to be much larger than that of the button electrode, and thus has ZmIs far greater than Zm' the mudcake impedance Z between the formation 4 and the return electrode is ignored in subsequent calculationsm', obtaining the measured impedance Z as the impedance Z of the mud cakemAnd formation impedance ZfConclusion of the sum.
In particular, the mudcake can be impedance Z according to the capacitive coupling principlemThe equivalent resistance of the mud cake is considered to be in parallel connection with the equivalent capacitance of the mud cake. And according to the shape of the electrode, the mud cake between the surface of the electrode and the stratum of the well wall is in a cylinder shape. According to the above analysis, the mudcake impedance ZmCan be expressed as:
wherein j is an imaginary unit
RmD is the distance between the surface of the button electrode and the stratum 4, omega is the angular frequency of the emission current, omega and the frequency f of the emission current satisfy omega-2 pi f,mris the relative dielectric constant of the oil-based mud,0the dielectric constant is 8.85 multiplied by 10 in vacuum-12F/m，SbThe surface area of the button electrode; the measured impedances of button electrode I1 and button electrode II 2 are then:
s3, collecting signals and according to the potential difference of the button electrode I1, the button electrode II 2 and the reference electrode in the actual logging processReal part of UrAnd imaginary part UiReal part I of current I of button electrode I1 and button electrode II 2rAnd imaginary part IiAnd recording the frequency f to obtain a calculation expression of the measured impedance Z of the button electrode I1 and the button electrode II 2.
Specifically, the calculation expression of the measured impedance Z of the button electrode I1 and the button electrode II 2 in step S3 is as follows:
measuring the real part Z of the impedance ZrAnd an imaginary part ZiRespectively expressed as:
according to the formulas (6) and (7), the real part and the imaginary part of the impedance of the button electrode I1 and the button electrode II 2 can be respectively calculated by utilizing the acquired signals.
S4, deducing to obtain the mud cake thickness T according to the real part and the imaginary part of the measured impedance Z of the button electrode I1 and the button electrode II 2mAnd formation impedance ZfThe formula (2) is calculated.
Specifically, the real part and the imaginary part of the measured impedance Z of the button electrode I1 and the button electrode II 2 are respectively extracted, specifically:
introduction of parameters
θ＝ωmr 0RmThe real part and the imaginary part of the measured impedance Z of the button electrode I1 and the button electrode II 2 are extracted according to equations (3) and (4), respectively, and are expressed as:
wherein A is1Is the real part of the button electrode I1, B1Is the imaginary part, A, of the button electrode I12Is the real part of the button electrode II 2, B2Is the imaginary part of the button electrode II 2.
Finishing the formula (8) to obtain the thickness T of the mud cakemAnd formation impedance ZfThe calculation formula (2) is specifically as follows:
the formula (8-b) is divided by the formula (8-d) to obtain
By substituting formula (1) for formula (9)
Obtaining the mud cake thickness T by the finishing formula (10)mThe calculation formula of (A) is as follows:
substituting formula (8-b) for formula (8-a) and substituting formula (8-d) for formula (8-c) yields the following expression:
the stratum impedance Z can be obtained by solving the above formulafIs calculated by the expression (i.e.)
S5, establishing a uniform stratum model, and setting the stratum resistivity as Rt *Placing the electric imaging plate 3 at the center of the stratum model, as shown in figure 4, solving the electric imaging well logging in the stratum model satisfiesThe problem of variation of (2);
specifically, the variation problem in S5 is:
wherein Ω is the solution region, ρ is the complex resistivity, UEIs the electrode potential, IEIs the electrode current.
S6, solving the variation problem in the step S5 by using a finite element method to obtain the potential difference U and the real current part I between the button electrode I1, the button electrode II 2 and the reference electroderAnd imaginary part IiThe real part A of the measured impedance of the button electrode I1 is calculated from the equations (5) to (7)1 *Imaginary part B1 *And the real part A of the measured impedance of the button electrode II 22 *Imaginary part B2 *The specific numerical value of (1).
S7, real part and imaginary part of the measured impedance of the button electrode I1 and the button electrode II 2 calculated in the step S6, and the formation resistivity R known in the step S5t *And calculating the electrode coefficient K.
Specifically, the calculation formula of the electrode coefficient K is:
the measured real part A of the impedance of the button electrode I1 calculated in the step S61 *Imaginary part B1 *And the real part A of the measured impedance of the button electrode II 22 *Imaginary part B2 *The specific numerical value of (2) is substituted into the formula (15), so that the numerical value of the electrode coefficient K can be obtained through solving.
S8, during actual logging, according to the relation between the formation resistivity and the formation impedance in electrical logging, the electrode coefficient K obtained in the step S7 is used for comparing the formation impedance Z obtained in the step S4fConversion to formation resistivity Rt。
Specifically, in actual measurementDuring well drilling, the stratum impedance Z is measured by the aid of the electrode coefficient K calculated in the uniform stratum modelfConversion to formation resistivity RtThe method specifically comprises the following steps:
s9, during actual well logging, according to the real part and the imaginary part of the impedance of the button electrode I1 and the button electrode II 2 calculated in the step S3 and the mud cake thickness T in the step S4mCalculating the thickness T of the mud cakem。
S10, calculating the thickness T of the mud cakemAnd formation resistivity RtAnd judging the types of well wall cracks and holes of the electric imaging logging.
Specifically, the mud cake thickness T is calculated according to the formula (11) and the formula (16)mAnd formation resistivity RtAnd drawing a mud cake thickness curve and a formation resistivity curve and judging the types of the cracks and holes of the well wall.
In the formulae (11) and (16), B1Is the imaginary part of the measured impedance of the button electrode I1, B2Is the measured impedance imaginary part, h, of the button electrode II 21Is the distance between the surface of the button electrode I1 and the surface of the polar plate 3, h2Is the distance h between the surface of the button electrode II 2 and the surface of the polar plate 31And h2Is determined during the design of the plate 3, during which T is calculatedmIs a known amount. As can be seen from the expression (11), only the imaginary impedance B of the button electrode I1 is measured1And the imaginary impedance B of the button electrode II 22Then the thickness T of the mud cake can be calculatedm(ii) a According to the expression (16), the real impedance part A of the button electrode I1 is calculated1And the real part of impedance A of the button electrode II 22The formation resistivity R can be calculatedtThe method lays a foundation for identifying and judging cracks and holes based on oil-based mud electrical imaging logging.
In order to illustrate and explain the above-mentioned contents of the present invention, and to facilitate the understanding of the contents of the present invention by researchers in the field, the following specific cases are mentioned, and the details are as follows:
simulating the figure 3, establishing a stratum model containing a horizontal fracture, wherein the fracture is located in the middle of the stratum, the width of the fracture is 2cm, the radial extension length is 50cm, and the diameter of a well hole is 20 cm; the mud resistivity in the well is 10000 omega ∙ m. The filling material and resistivity characteristic distribution in the crack are divided into four cases: opening a crack, and filling high-resistance oil-based mud; closing the crack and filling high-resistance minerals; thirdly, closing the crack and filling low-resistance minerals; and fourthly, opening the crack, mixing the high-resistance oil-based mud with the formation water and presenting the characteristic of low resistance.
Respectively establishing numerical simulation models under the four conditions by using a finite element method, moving an imaging polar plate from the lower side of a crack to the upper side of the crack in a borehole, respectively measuring the potential difference U and the current between the button electrode I1, the button electrode II 2 and a reference electrode, and calculating the real impedance part A of the button electrode I1Imaginary part B1And the real part of impedance A of the button electrode II 22Imaginary part B2. The thickness value of the mud cake is calculated by the formula (11), and the resistivity R of the earth formation is calculated by the formula (16)tThe results are shown in FIGS. 5 to 8. In fig. 5 to 8, the left side graph is a mud cake thickness curve, and the right side graph is a formation resistivity curve.
Referring to fig. 5, at the middle fracture position, two curves are protruded to the right, the mud cake thickness and the formation resistivity are displayed as relatively high values, which shows that the mud cake has permeability, the mud cake is attached to the well wall, the fluid in the fracture is displaced by the high-resistance oil-based mud filtrate, the fracture can be judged to be opened, and the fracture is filled with the high-resistance oil-based mud filtrate.
Referring to fig. 6, at the middle crack position, the left curve protrudes to the left, and the mud cake thickness shows a low value; the right side curve protrudes rightwards, and the formation resistivity presents a high value; it is shown here that there is no or very low permeability, essentially no mud invasion occurs, no mud cake on the borehole wall or very small mud cake thickness. At this time, the cracks exhibited high resistivity values, which indicate that the cracks are closed cracks filled with high-resistivity minerals such as quartz, mica, and the like.
Referring to FIG. 7, at the intermediate fracture site, where the two curves are convex to the left, the mudcake thickness and formation resistivity are shown to be relatively low, indicating that there is no or very low permeability, substantially no mud invasion, no mudcake on the borehole wall, or very small mudcake thickness. At this time, the crack exhibits a low resistivity state, thus indicating that the crack is a closed crack filled with low resistivity substances such as pyrite, siderite, and dispersed slime.
Referring to fig. 8, at the middle fracture position, the left curve protrudes to the right, the right curve protrudes to the left, the mud cake thickness shows a high value, which shows that the mud cake adheres to the well wall, and mud invades into the fracture; however, the low resistivity in the fracture is low, which indicates that the fluid in the fracture is not completely displaced, and the high-resistivity slurry filtrate is mixed with the low-resistivity fluid such as high-salinity formation water in the fracture, so that the whole fracture is in a low-resistivity state.
The above curve morphology and analysis also apply when holes are found in the electrographic image. In addition, when the crack is in a medium-high angle state, the crack needs to be preliminarily identified by means of a sinusoidal form in an electrical imaging image, and then the crack is analyzed and judged by the method. Compared with the prior art, the invention provides a logging means, namely, the well wall stratum cracks and holes are identified and judged in the oil-based mud environment by adopting the electrical imaging logging, so that the well wall stratum cracks and holes in the oil-based mud environment are not identified and judged in a combined manner by means of other logging means such as acoustic logging, electrical imaging logging, induction logging and the like. In addition, the method is different from an imaging method utilizing total measured impedance, the formation resistivity is calculated, formation resistivity imaging can be carried out, the influence of high-resistance oil-based mud is reduced, and the change of the formation resistivity is reflected more truly. The invention can reflect the thickness change of the mud cake, which is beneficial to analyzing the stratum permeability, enriches the stratum evaluation information based on the electric imaging logging information and can reflect the geological characteristic change more truly.
The foregoing is illustrative of the preferred embodiments of this invention, and it is to be understood that the invention is not limited to the precise form disclosed herein and that various other combinations, modifications, and environments may be resorted to, falling within the scope of the concept as disclosed herein, either as described above or as apparent to those skilled in the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. A well wall crack and hole identification and judgment method based on oil-based mud electrical imaging logging is characterized by comprising the following steps:
s1, improving the structure of the electric imaging logging device, enabling the button electrode I (1) and the button electrode II (2) to be sunken towards the inside of the polar plate (3), and obtaining expressions of distances between the surfaces of the button electrode I (1) and the button electrode II (2) and the stratum (4), wherein the expressions are respectively as follows:
wherein d is1Is the recess distance between the surface of the button electrode I (1) and the ground layer (4), d2Is the recess distance h between the surface of the button electrode II (2) and the formation (4)1Is the distance h between the button electrode I (1) and the surface of the polar plate (3)2Is the distance T between the button electrode II (2) and the surface of the polar plate (3)mIs the thickness of the mud cake (6) between the polar plate (3) and the stratum (4) and satisfies h1≠h2；
S2, obtaining the measured impedance Z of the button electrode I (1) and the button electrode II (2) according to the flowing path of the current, wherein the measured impedance Z is the mud cake impedance ZmAnd formation impedance ZfSumming;
s3, during the actual logging process,collecting signals and according to the real part U of the potential difference U of the button electrode I (1), the button electrode II (2) and the reference electroderAnd imaginary part UiReal part I of the current I of button electrode I (1) and button electrode II (2)rAnd imaginary part IiRecording the frequency f to obtain a calculation expression of the measured impedance Z of the button electrode I (1) and the button electrode II (2);
s4, deducing to obtain the mud cake thickness T according to the real part and the imaginary part of the measured impedance Z of the button electrode I (1) and the button electrode II (2)mAnd formation impedance ZfThe formula (4) is calculated;
s5, establishing a uniform stratum model, and setting the stratum resistivity as Rt *An electric imaging polar plate (3) is placed at the center of the stratum model, and the variation problem met by the electric imaging logging is built in the stratum model;
s6, solving the variation problem in the step S5 by using a finite element method to obtain the potential difference U and the real current part I between the button electrode I (1), the button electrode II (2) and the reference electroderAnd imaginary part IiCalculating the real part and the imaginary part of the measured impedance of the button electrode I (1) and the button electrode II (2) according to the calculation expression of the measured impedance Z in the step S3;
s7, according to the measured impedance real part and imaginary part of the button electrode I (1) and the button electrode II (2) calculated in the step S6 and the formation resistivity R set in the step S5t *Calculating an electrode coefficient K;
s8, in the actual logging process, according to the relation between the formation resistivity and the formation impedance in the electrical logging, the electrode coefficient K obtained in the step S7 is used for comparing the formation impedance Z obtained in the step S4 with the electrode coefficient K obtained in the step S7fConversion to formation resistivity Rt；
S9, in the actual logging process, according to the real part and the imaginary part of the impedance of the button electrode I (1) and the button electrode II (2) calculated in the step S3 and the mud cake thickness T in the step S4mCalculating the thickness T of the mud cakem；
S10, calculating the thickness T of the mud cakemAnd formation resistivity RtAnd judging the types of well wall cracks and holes of the electric imaging logging.
2. The method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 1, wherein: the mudcake impedance Z in step S2 is determined according to the capacitive coupling principlemExpressed as:
wherein j is an imaginary unit
RmD is the distance between the surface of the button electrode and the stratum (4), omega is the angular frequency of the emission current, omega and the frequency f of the emission current satisfy the condition that omega is 2 pi f,mris the relative dielectric constant of the oil-based mud,0the dielectric constant is 8.85 multiplied by 10 in vacuum-12F/m，SbThe surface area of the button electrode;
the measured impedances of button electrode I (1) and button electrode II (2) are then:
3. the method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 2, wherein: the calculation expression of the measured impedance Z of the button electrode I (1) and the button electrode II (2) described in step S3 is:
measuring the real part Z of the impedance ZrAnd an imaginary part ZiRespectively expressed as:
and (4) according to the formula (6) and the formula (7), respectively calculating the real part and the imaginary part of the impedance of the button electrode I (1) and the button electrode II (2) by using the acquired signals.
4. The method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 3, wherein: the step S4 of extracting the real part and the imaginary part of the measured impedance Z of the button electrode I (1) and the button electrode II (2), respectively, specifically includes:
introduction of parameters
θ＝ωmr 0RmThe real and imaginary parts of the measured impedance Z of the button electrode I (1) and of the button electrode II (2) are extracted according to equations (3) and (4), respectively, and are expressed as:
wherein A is1Is the real part of the button electrode I (1), B1Is the imaginary part, A, of button electrode II (1)2Is the real part of the button electrode II (2), B2Is the imaginary part of the button electrode II (2).
5. The method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 4, wherein: finishing the formula (8) to obtain the thickness of the mud cakeTmAnd formation impedance ZfThe calculation formula (2) is specifically as follows:
the formula (8-b) is divided by the formula (8-d) to obtain
By substituting formula (1) for formula (9)
Obtaining the mud cake thickness T by the finishing formula (10)mThe calculation formula of (A) is as follows:
substituting formula (8-b) for formula (8-a) and substituting formula (8-d) for formula (8-c) yields the following expression:
solving the formula (12) to obtain the formation impedance ZfIs calculated by the expression (i.e.)
6. The method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 5, wherein: the variation problem described in step S5 is:
wherein omegaTo solve for the region, ρ is the complex resistivity, UEIs the electrode potential, IEIs the electrode current.
7. The method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 6, wherein: in the homogeneous formation model, the formation resistivity R is knownt *Solving the variation problem by using a finite element method to obtain the potential difference U, the real part and the imaginary part of the current between the button electrode I (1), the button electrode II (2) and the reference electrode, and calculating the real part A of the measured impedance of the button electrode I (1) according to the formulas (5) to (7)1 *Imaginary part B1 *And the real part A of the measured impedance of the button electrode II (2)2 *Imaginary part B2 *The specific numerical value of (1).
8. The method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 7, wherein: the calculation formula of the electrode coefficient K in step S7 is:
the measured real impedance part A of the button electrode I (1) calculated in the step S61 *Imaginary part B1 *And the real part A of the measured impedance of the button electrode II (2)2 *Imaginary part B2 *The specific value of (2) is substituted into the formula (15) to obtain the value of the electrode coefficient K.
9. The method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging as claimed in claim 8, wherein: in step S8, during actual logging, the formation impedance Z is measured by using the value of the electrode coefficient K calculated in the homogeneous formation modelfConversion to formation resistivity RtThe method specifically comprises the following steps:
10. the method for identifying and judging the cracks and holes on the well wall based on the oil-based mud electrical imaging logging of claim 9, wherein the method comprises the following steps: calculating the thickness T of the mud cake according to the formula (11) and the formula (16)mAnd formation resistivity Rt，And drawing a mud cake thickness curve and a formation resistivity curve and judging the types of the cracks and holes of the well wall.
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| CN109782359A (en) * | 2019-02-20 | 2019-05-21 | 电子科技大学 | Multi-frequency bearing calibration based on oil-base mud environment micro resistor |
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