A real-time effect evaluation method for hydraulic fracturing of shale gas horizontal wells

Abstract

The present invention discloses a method for real-time evaluation of the fracturing effect of shale gas horizontal wells, which relates to the technical field of unconventional oil and gas stimulation and enhancement, and specifically includes the following steps: Step S1: Obtain various construction data of the horizontal section of the fractured well; Step S2: Calculate the real-time net pressure of each fracturing stage during fracturing without considering the induced stress; Step S3: Calculate the real-time induced stress difference of each fracturing stage during fracturing; Step S4: Conduct real-time evaluation of the fracturing effect and provide suggestions for the fracturing of subsequent fracturing stages; With the present invention, based on real-time second-point data, a method that can provide parameter optimization in real time following the fracturing construction is established. The calculation steps are simpler and faster than previous methods, and it has stronger practicality in the oilfield. It also considers aspects such as hole friction, string friction, and the mutual influence of multi-stage induced stress, combines complex construction problems such as casing deformation faced in production practice, gives solutions for different situations, and provides suggestions for subsequent fracturing.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional oil and gas stimulation, and particularly relates to a method for real-time evaluation of the fracturing effect of shale gas horizontal wells. Background Art

[0002] The fracturing technology is an important core means to achieve the stimulation of shale gas reservoirs; during the fracturing process, the construction curve is the most intuitive mechanical and engineering reflection of the artificial fracture of formation rocks, which includes the real-time construction casing pressure, fracturing fluid consumption, proppant pumping concentration, and surface construction displacement. At present, the fracturing construction curve has gradually been taken seriously by scholars and engineers and has become an important basic data for analyzing the dynamic expansion mechanical behavior of fractures.

[0003] Since 1981, foreign scholars have started to analyze the fracturing construction curve of conventional reservoirs by the double logarithmic curve method, and judge whether the fracturing construction is normal through the slope of the curve, so as to classify the formed fracture types. Then, domestic scholars have also enriched and improved this type of method, making more fracture types identified and considering aspects such as natural weak planes, in-situ stresses, and rock mechanical properties. However, the research of domestic scholars still stays in theoretical analysis and has less application in the field. The patent No. CN 106948800 B, "A Method for Diagnosing the Construction Conditions of Staged Fracturing of Horizontal Wells", fails to consider the influence of wellbore friction and perforation friction, only judges whether the fracture penetrates and slips, and only considers the influence of the previous fracturing stage on the induced stress, resulting in a large deviation in the calculation results; the patent CN 110056336 B, "An Automatic Diagnosis Method for the Construction Pressure Curve of Shale Gas Fracture Network Fracturing", analyzes the hydraulic fracture morphology by using the construction curve through numerical simulation technology, but does not consider the influence of the induced stress of the previous fracturing stage on the current fracturing stage, and has a long calculation time, which is not conducive to real-time application. In addition, all current analysis methods and technologies for construction curves fail to analyze the stimulation effect, and the information data mining of construction curves is still not sufficient, making the current similar technologies and methods have great limitations in actual application.

[0004] The induced stress field has a great influence on the original in-situ stress field of the reservoir. When the difference between the induced stress acting in the direction of the minimum horizontal principal stress and the induced stress acting in the direction of the maximum horizontal principal stress is large enough, it will change the original in-situ stress direction; previous studies have shown that the greater the induced stress difference, the higher the improvement of the hydraulic fracturing effect; however, through actual construction, it is found that when the induced stress difference is too large, casing deformation always occurs during the fracturing construction, resulting in the abandonment of tens or even hundreds of meters of fracturing sections and unable to be fractured, and there will be a large number of untransformed areas in the reservoir, seriously affecting the fracturing effect. Summary of the Invention

[0005] The object of the present invention is to: aiming at the problems in the current fracturing evaluation method, such as the failure to consider wellbore friction, perforation friction, and the influence of the induced stress of the previous fracturing stage on the current fracturing stage, resulting in large calculation errors, long calculation time, and being not conducive to real-time application, a real-time fracturing effect evaluation method for shale gas horizontal wells is provided. By obtaining various geological data, tool and fluid data of the horizontal section of the fractured well, fully excavating the data information contained in the construction curve, calculating the real-time net pressure of each fracturing stage without considering the induced stress, calculating the single-stage real-time induced stress difference during the fracturing process, realizing real-time fracturing evaluation, and giving adjustment suggestions for the subsequent fracturing stages, and finally forming a comprehensive index for post-fracturing analysis, the above problems are solved smoothly.

[0006] The technical solution of the present invention is as follows:

[0007] A real-time fracturing effect evaluation method for shale gas horizontal wells, comprising the following steps:

[0008] Step S1: Obtain various construction data of the horizontal section of the fractured well;

[0009] Step S2: Calculate the real-time net pressure of each fracturing stage during the fracturing process without considering the induced stress;

[0010] Step S3: Calculate the real-time induced stress difference of each fracturing stage during the fracturing process;

[0011] Step S4: Conduct real-time evaluation of the fracturing effect and put forward suggestions for the fracturing of the subsequent fracturing stages.

[0012] Further, the various construction data in step S1 specifically include: geological data, tool and fluid data.

[0013] Further, the geological data includes the test depth, average vertical depth, average Poisson's ratio, average minimum horizontal principal stress, and average maximum horizontal principal stress of the top and bottom boundaries of each fracturing stage in the logging interpretation data;

[0014] The tool and fluid data includes the density of the fracturing fluid, viscosity of the fracturing fluid, number of perforations, perforation diameter, inner diameter of the wellbore, absolute roughness of the inner wall of the wellbore, and perforation flow coefficient used in each fracturing stage.

[0015] Further, the detailed steps of step S2 are as follows:

[0016] Step S2a: Obtain the real-time construction second-point data of each fracturing stage during the fracturing process; the construction second-point data includes the real-time casing pressure at the wellhead, real-time sand concentration, and real-time construction displacement of each fracturing stage during the fracturing process;

[0017] Step S2b: Calculate the real-time static pressure of the sand-carrying string during the fracturing of each fracturing stage; the calculation formula for the real-time static pressure of the sand-carrying string during the fracturing of each fracturing stage is:

[0018]

[0019] In the formula,

[0020] P String — The real-time static pressure of the sand-carrying string during the fracturing of each fracturing stage;

[0021] ρ frac — The real-time density of the fracturing fluid during the fracturing of each fracturing stage;

[0022] g — Acceleration due to gravity;

[0023] h — The average vertical depth of each fracturing stage;

[0024] Step S2c: Calculate the real-time wellbore string friction acting on the fracture opening generated by the migration of the sand-carrying fracturing fluid in the string during the fracturing of each fracturing stage; the calculation formula for the real-time wellbore string friction during the fracturing of each fracturing stage is:

[0025]

[0026] In the formula,

[0027] P wf — The real-time wellbore string friction acting on the fracture opening generated by the migration of the sand-carrying fracturing fluid in the string during the fracturing of each fracturing stage;

[0028] λ — Hydraulic friction coefficient;

[0029] L — The test depth of the bottom boundary of each fracturing stage;

[0030] v s — The real-time flow velocity of the sand-carrying fracturing fluid during the fracturing;

[0031] D — Wellbore diameter;

[0032] ρ Fluid — The density of the fracturing fluid used in each fracturing stage;

[0033] Step S2d: Calculate the perforation friction at the fracture opening during the fracturing of each fracturing stage; the calculation formula for the perforation friction at the fracture opening during the fracturing of each fracturing stage is:

[0034]

[0035] In the formula,

[0036] P nf — The perforation friction at the fracture opening during the fracturing of each fracturing stage;

[0037] q—the real-time construction displacement of each fracturing stage during fracturing;

[0038] n nf —the number of perforations of each fracturing stage;

[0039] d nf —the perforation diameter of each fracturing stage;

[0040] α nf —the hole flow coefficient of each fracturing stage;

[0041] Step S2e: Calculate the real-time net fracturing pressure of each fracturing stage without considering the induced stress during fracturing; the calculation formula for the real-time net fracturing pressure of each fracturing stage without considering the induced stress is:

[0042] P netn = P head + P String - P nf - P wf - σ x

[0043] In the formula,

[0044] P netn —the real-time net pressure of the nth fracturing stage without considering the induced stress during fracturing;

[0045] P head —the real-time wellhead casing pressure of each fracturing stage during fracturing;

[0046] σ x —the average minimum horizontal principal stress of each fracturing stage in the logging interpretation data.

[0047] Furthermore, the calculation formula for the real-time fracturing fluid density ρ frac of each fracturing stage in step S2b is:

[0048]

[0049] In the formula,

[0050] w—the real-time sand concentration of each fracturing stage during fracturing;

[0051] ρ PropV —the bulk density of the proppant used in each fracturing stage;

[0052] ρ Prop —the apparent density of the proppant used in each fracturing stage;

[0053] ρ Fluid —the density of the fracturing fluid used in each fracturing stage.

[0054] Furthermore, the calculation formula for the real-time flow rate of the sand-carrying fracturing fluid during the fracturing process in step S2c is as follows:

[0055]

[0056] Furthermore, the hydraulic friction coefficient in step S2c is determined by the following steps:

[0057] Step S2c1: Define a Reynolds number based on the average Poisson's ratio in the logging interpretation data of each fracturing section, the density of the fracturing fluid used in each fracturing section, the wellbore diameter, and the viscosity of the fracturing fluid used in each fracturing section. The Reynolds number is determined by the following formula:

[0058]

[0059] In the formula,

[0060] Re—Reynolds number;

[0061] ν—Average Poisson's ratio in the logging interpretation data of each fracturing section;

[0062] μ—Viscosity of the fracturing fluid used in each fracturing section.

[0063] Step S2c2: When Re < 2000, the hydraulic friction coefficient is determined by the following formula:

[0064] λ = 64 / Re

[0065] λ = 64 / Re

[0066] Step S2c3: When 2000 < Re < 59.7 / (2R / D) 8 / 7 , the hydraulic friction coefficient is determined by the following formula:

[0067] λ = 0.3164 / Re 0.25

[0068] Step S2c4: When 59.7 / (2R / D) 8 / 7 < Re < [665 - 765lg(2R / D)] / (2R / D), the hydraulic friction coefficient is determined by the following formula:

[0069]

[0070] Step S2c5: When Re > [665 - 765lg(2R / D)] / (2R / D), the hydraulic friction coefficient is determined by the following formula:

[0071]

[0072] In steps S2c3 - S2c5, R—Absolute roughness of the inner pipe wall of the wellbore.

[0073] Furthermore, the detailed steps of step S3 are as follows:

[0074] Step S3a: Calculate the real-time effective net pressure of each fracturing stage during the fracturing process considering the induced stress. The calculation formula is as follows:

[0075]

[0076] In the formula,

[0077] P net (n) — The real-time effective net pressure of the nth fracturing stage during the fracturing process considering the induced stress;

[0078] σ xin — The real-time induced stress at the fracture mouth of the ith fracturing stage in the direction of the minimum horizontal principal stress on the fracture mouth of the nth fracturing stage;

[0079] Step S3b: Calculate the induced stress in the direction of the minimum horizontal principal stress generated by each fracturing stage on the next fracturing stage during the fracturing process. The calculation formula for the induced stress in the direction of the minimum horizontal principal stress is:

[0080]

[0081] In the formula,

[0082] c — The half-thickness of the reservoir;

[0083] L(i,n) — The difference in the mid-test depth between the ith fracturing stage and the nth fracturing stage;

[0084] Step S3c: Calculate the induced stress in the direction of the maximum horizontal principal stress generated by each fracturing stage on the next fracturing stage during the fracturing process. The calculation formula for the induced stress in the direction of the maximum horizontal principal stress is:

[0085]

[0086] In the formula,

[0087] σ yin — The real-time induced stress at the fracture mouth of the ith fracturing stage in the direction of the maximum horizontal principal stress on the fracture mouth of the nth fracturing stage;

[0088] Step S3d: Calculate the real-time induced stress difference of each fracturing stage during the fracturing process. The calculation formula is:

[0089]

[0090] In the formula,

[0091] Δσ′ — The real-time induced stress difference of each fracturing stage during the fracturing process.

[0092] Further, the detailed steps of step S4 are as follows:

[0093] Step S4a: Calculate the current in-situ stress field by the following formula;

[0094]

[0095] In the formula,

[0096] Δσ x ′—The real-time in-situ stress in the direction of the minimum horizontal principal stress of each fracturing stage during the fracturing process;

[0097] Δσ y ′—The real-time in-situ stress in the direction of the maximum horizontal principal stress of each fracturing stage during the fracturing process;

[0098] σ y —The average maximum horizontal principal stress of each fracturing stage in the logging interpretation data;

[0099] Step S4b: Calculate the current in-situ stress difference by the following formula;

[0100]

[0101] In the formula, Δσ xy ′—The real-time current in-situ stress difference of each fracturing stage during the fracturing process;

[0102] Step S4c: Qualitatively evaluate the fracturing effect according to the real-time current in-situ stress difference of each fracturing stage, and optimize the fracturing of the subsequent fracturing stages;

[0103] Step S4d: Integrate the timeliness and construction intensity, establish evaluation indicators, and evaluate the post-fracturing transformation effect of each fracturing stage of a single well.

[0104] Further, the evaluation indicators established in step S4d include a timeliness index, a transformation intensity index, and a single-stage fracturing effect evaluation indicator;

[0105] Among them, the timeliness index is determined by the following formula:

[0106]

[0107] In the formula,

[0108] Δt—The timeliness index of each fracturing stage;

[0109] t—The number of second-point data when the current in-situ stress difference Δσ xy ′ of each fracturing stage is between -5 and 5 MPa;

[0110] When t < 3600, Δt is uniformly taken as 0.2; when t > 7200, Δt is uniformly taken as 0.8;

[0111] The transformation intensity index is determined by the following formula:

[0112]

[0113] In the formula,

[0114] Δp—the transformation intensity index of each fracturing stage;

[0115] The evaluation index of the single-stage fracturing effect is determined by the following formula:

[0116]

[0117] In the formula,

[0118] S—the evaluation index of the single-stage fracturing effect.

[0119] Compared with the existing technologies, the beneficial effects of the present invention are as follows:

[0120] 1. A real-time fracturing effect evaluation method for shale gas horizontal wells fully utilizes all real-time second-point data of the fracturing construction curve, establishes a method that can provide parameter optimization in real time following the fracturing construction, and has simpler and faster calculation steps and stronger field practicability compared with previous methods. In addition, it also considers aspects such as hole friction, string friction, and the mutual influence of multi-stage induced stresses that are not fully considered in many existing technologies and methods, combines construction complex problems such as casing deformation faced in production practice, gives solutions for different situations, and provides suggestions for subsequent fracturing. Finally, the present invention also forms an evaluation index for the fracturing effect, which can be used for comparative analysis with the transformed volume monitored by microseismic and the productivity parameters obtained from production logging and distributed fiber optic logging, achieving a good positive correlation and realizing a relatively accurate evaluation of the transformation effect when the shale gas horizontal well is not yet put into production. BRIEF DESCRIPTION OF THE DRAWINGS

[0121] Figure 1 is a flowchart of a real-time fracturing effect evaluation method for shale gas horizontal wells;

[0122] Figure 2 is a relationship diagram between the fracturing effect evaluation index and the gas production contribution rate of each fracturing stage of Well A, a shale gas horizontal well, in the second embodiment; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0123] It should be noted that relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising a..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element.

[0124] The features and performance of the present invention will be further described in detail below in conjunction with embodiments.

[0125] Embodiment 1

[0126] Please refer to Figure 1 , a real-time effect evaluation method for hydraulic fracturing of shale gas horizontal wells, specifically including the following steps:

[0127] Step S1: Obtain various construction data of the horizontal section of the fracturing well;

[0128] Step S2: Calculate the real-time net pressure of each fracturing stage during the fracturing process without considering the induced stress;

[0129] Step S3: Calculate the real-time induced stress difference of each fracturing stage during the fracturing process;

[0130] Step S4: Conduct real-time evaluation of the fracturing effect and put forward suggestions for the fracturing of subsequent fracturing stages.

[0131] The specific construction data in Step S1 includes: geological data, tool and liquid data; that is, the purpose of Step S1 is to obtain various geological data, tool and liquid data of the horizontal section of the fracturing well.

[0132] Among them, the geological data includes the test depth, average vertical depth, average Poisson's ratio, average minimum horizontal principal stress, and average maximum horizontal principal stress of the top and bottom boundaries of each fracturing section in the logging interpretation data of the single-well horizontal section; the tool and liquid data includes the density of the fracturing fluid, viscosity of the fracturing fluid, number of perforations, perforation diameter, inner diameter of the wellbore, absolute roughness of the inner wall of the wellbore, and orifice flow coefficient used in each fracturing section; the logging interpretation data is the integration of test data at a data point every 0.125 m, and the test data of each data point needs to include the above-mentioned geological data; the horizontal well is divided into multiple fracturing sections, and it is necessary to obtain the length of each fracturing section, the test depth of the top and bottom boundaries of each fracturing section, the average vertical depth, average density, average Poisson's ratio, average minimum horizontal principal stress, and average maximum horizontal principal stress of each fracturing section; the above-mentioned average values are all obtained from the average values of each data point belonging to the fracturing section.

[0133] The detailed steps of step S2 are as follows:

[0134] Step S2a: Obtain the real-time construction second-point data of each fracturing section during the fracturing process; the construction second-point data includes the real-time wellhead casing pressure P Head 、real-time sand concentration w, and real-time construction displacement q during the fracturing process of each fracturing section;

[0135] Step S2b: Calculate the static pressure of the sand-carrying string of each fracturing section during the fracturing process; the calculation formula for the static pressure of the sand-carrying string of each fracturing section during the fracturing process is:

[0136]

[0137] In the formula,

[0138] P String —The static pressure of the sand-carrying string of each fracturing section during the fracturing process, unit: MPa;

[0139] ρ frac —The density of the real-time fracturing fluid of each fracturing section during the fracturing process, unit: kg / m 3 ;

[0140] g—Acceleration due to gravity, unit: m / s 2 , taking the value of 9.8;

[0141] h—The average vertical depth of each fracturing section, unit: m;

[0142] Among them, the calculation formula for the density ρ frac of the real-time fracturing fluid of each fracturing section is:

[0143]

[0144] In the formula,

[0145] w—the real-time sand concentration during the fracturing process of each fracturing stage, unit: kg / m 3 ;

[0146] ρ PropV —the bulk density of the proppant used in each fracturing stage, unit: kg / m 3 ;

[0147] ρ Prop —the apparent density of the proppant used in each fracturing stage, unit: kg / m 3 ;

[0148] ρ Fluid —the density of the fracturing fluid used in each fracturing stage, unit: kg / m 3 ;

[0149] Step S2c: Calculate the real-time wellbore string friction acting on the fracture opening generated by the transport of the sand-carrying fracturing fluid in the string during the fracturing process of each fracturing stage; the calculation formula for the real-time wellbore string friction during the fracturing process of each fracturing stage is:

[0150]

[0151] In the formula,

[0152] P wf —the real-time wellbore string friction acting on the fracture opening generated by the transport of the sand-carrying fracturing fluid in the string during the fracturing process of each fracturing stage, unit: MPa;

[0153] λ—hydraulic friction coefficient, dimensionless;

[0154] L—the test depth of the bottom boundary of each fracturing stage, unit: m;

[0155] v s —the real-time flow velocity of the sand-carrying fracturing fluid during the fracturing process, unit: m / s;

[0156] D—wellbore diameter, unit: m;

[0157] ρ Fluid —the density of the fracturing fluid used in each fracturing stage, unit: kg / m 3 ;

[0158] Among them, the real-time flow velocity of the sand-carrying fracturing fluid during the fracturing process is calculated using the following formula:

[0159]

[0160] In the formula,

[0161] q—the real-time construction displacement of each fracturing stage, unit: m 3 / min;

[0162] Among them, the hydraulic friction coefficient is determined by the following steps:

[0163] Step S2c1: Define a Reynolds number according to the average Poisson's ratio in the logging interpretation data of each fracturing stage, the density of the fracturing fluid used in each fracturing stage, the wellbore diameter, and the viscosity of the fracturing fluid used in each fracturing stage. The Reynolds number is determined by the following formula:

[0164]

[0165] In the formula:

[0166] Re - Reynolds number, dimensionless;

[0167] ν - Average Poisson's ratio in the logging interpretation data of each fracturing stage;

[0168] μ - Viscosity of the fracturing fluid used in each fracturing stage;

[0169] Step S2c2: When Re < 2000, the hydraulic friction coefficient is determined by the following formula:

[0170] λ = 64 / Re (6)

[0171] Step S2c3: When 2000 < Re < 59.7 / (2R / D) 8 / 7 At this time, the hydraulic friction coefficient is determined by the following formula:

[0172] λ = 0.3164 / Re 0.25 (7)

[0173] Step S2c4: When 59.7 / (2R / D) 8 / 7 < Re < [665 - 765lg(2R / D)] / (2R / D), the hydraulic friction coefficient is determined by the following formula:

[0174]

[0175] Step S2c5: When Re > [665 - 765lg(2R / D)] / (2R / D), the hydraulic friction coefficient is determined by the following formula:

[0176]

[0177] In the above steps S2c3 - S2c5, R - Absolute roughness of the inner wall of the wellbore;

[0178] Step S2d: Calculate the hole friction at the fracture mouth during the fracturing process of each fracturing stage; the calculation formula for the hole friction at the fracture mouth during the fracturing process of each fracturing stage is:

[0179]

[0180] Wherein,

[0181] P nf — The hole friction at the fracture mouth of each fracturing stage during fracturing, unit: ;

[0182] q — The real-time construction displacement of each fracturing stage during fracturing, unit: MPa;

[0183] n nf — The number of perforations of each fracturing stage, unit: pieces;

[0184] d nf — The perforation diameter of each fracturing stage, unit: m;

[0185] α nf — The hole flow coefficient of each fracturing stage, dimensionless, taking the value of 0.85;

[0186] Step S2e: Calculate the real-time net fracturing pressure of each fracturing stage without considering the induced stress during fracturing; the calculation formula for the real-time net fracturing pressure of each fracturing stage without considering the induced stress is:

[0187] P netn =P head +P String -P nf -P wf -σ x (11)

[0188] Wherein,

[0189] P netn — The real-time net pressure of the nth fracturing stage without considering the induced stress during fracturing, unit: MPa;

[0190] P head — The real-time wellhead casing pressure of each fracturing stage during fracturing, unit: MPa;

[0191] σ x — The average minimum horizontal principal stress of each fracturing stage in the logging interpretation data, unit: MPa.

[0192] The detailed steps of Step S3 are as follows:

[0193] Step S3a: Calculate the real-time effective net fracturing pressure of each fracturing stage considering the induced stress during fracturing, and the calculation formula is as follows:

[0194]

[0195] Wherein,

[0196] P net (n) — The real-time effective net pressure of the nth fracturing stage considering the induced stress during fracturing;

[0197] σ xin — The real-time induced stress of the minimum horizontal principal stress direction generated by the fracture mouth of the i-th fracturing stage on the fracture mouth of the n-th fracturing stage;

[0198] Step S3b: Calculate the induced stress of the minimum horizontal principal stress direction generated by each fracturing stage itself and the previous stage on the next fracturing stage during the fracturing process; The calculation formula for the induced stress of the minimum horizontal principal stress direction is:

[0199]

[0200] In the formula,

[0201] σ xin — The real-time induced stress of the minimum horizontal principal stress direction generated by the fracture mouth of the i-th fracturing stage on the fracture mouth of the n-th fracturing stage, unit: MPa;

[0202] c — The semi-thickness of the reservoir, unit: m;

[0203] P net (n) — The real-time effective net pressure of the n-th fracturing stage considering the induced stress during the fracturing process;

[0204] L(i,n) — The difference between the mid-test depth of the i-th fracturing stage and the mid-test depth of the n-th fracturing stage, unit: m;

[0205] Step S3c: Calculate the induced stress of the maximum horizontal principal stress direction generated by each fracturing stage on the next fracturing stage during the fracturing process; The calculation formula for the induced stress of the maximum horizontal principal stress direction is:

[0206]

[0207] In the formula,

[0208] σ yin — The real-time induced stress of the maximum horizontal principal stress direction generated by the fracture mouth of the i-th fracturing stage on the fracture mouth of the n-th fracturing stage, unit: MPa;

[0209] Because the fracturing stages of the shale gas well are divided into the 1st, 2nd, 3rd, 4th..., so calculate the real-time effective net pressure of each stage considering the induced stress during the fracturing process in turn; First, calculate the 1st fracturing stage. Since there is no previous fracturing stage before the 1st fracturing stage, there is no real-time induced stress generated by the previous stage on the fracture mouth of the 1st fracturing stage. Therefore, according to formula (12), it is calculated that: P net (1) = P net1 ; Then substitute P netSubstitute the value of (1) into formulas (13) and (14) to calculate the real-time induced stress of the minimum horizontal principal stress direction at the fracture tip of the first fracturing stage on the fracture tip of the second fracturing stage and the real-time induced stress of the maximum horizontal principal stress direction; subsequent stages can be calculated in turn.

[0210] Step S3d: Calculate the real-time induced stress difference of each fracturing stage during the fracturing process; subtract the sum of the real-time induced stresses of the maximum horizontal principal stress direction from the sum of the real-time induced stresses of the minimum horizontal principal stress direction acting on the currently fracturing stage generated by itself and the previous stage to obtain the real-time induced stress difference; its specific calculation formula is:

[0211]

[0212] In the formula,

[0213] Δσ′—The real-time induced stress difference of each fracturing stage during the fracturing process, unit: MPa.

[0214] The detailed steps of Step S4 are as follows:

[0215] Step S4a: As described in the previous steps, calculate the net pressure and induced stress in real time during the construction process to obtain the real-time induced stress difference, and calculate the current in-situ stress field by the following formula;

[0216]

[0217] In the formula,

[0218] Δσ x ′—The real-time in-situ stress of the minimum horizontal principal stress direction of each fracturing stage during the fracturing process, unit: MPa;

[0219] Δσ y ′—The real-time in-situ stress of the maximum horizontal principal stress direction of each fracturing stage during the fracturing process, unit: MPa;

[0220] σ y —The average maximum horizontal principal stress of each fracturing stage in the logging interpretation data, unit: MPa;

[0221] Step S4b: Calculate the current in-situ stress difference by the following formula;

[0222]

[0223] In the formula, Δσ xy ′—The real-time current in-situ stress difference of each fracturing stage during the fracturing process, unit: MPa;

[0224] Step S4c: According to the real-time current in-situ stress difference of each fracturing stage, qualitatively evaluate the fracturing effect and optimize the fracturing of the subsequent fracturing stages;

[0225] Specifically, the detailed operation steps of step S4c are as follows:

[0226] Record the current in-situ stress difference Δσ xy ′ The number of second-point data with < -5 MPa. When the number is greater than 7,200, and the corresponding time is 2 hours or more, it is regarded as a fracturing stage with severely insufficient transformation duration; otherwise, the next determination is carried out;

[0227] Record the current in-situ stress difference Δσ xy ′ Evaluate the timeliness based on the number of second-point data between -5 and 5 MPa:

[0228] ① When the number of second-point data is greater than 5,400, and the corresponding time is 1.5 hours (5,400 seconds), it is recorded as a sufficient transformation duration, and the construction process of this stage can be continued to the next stage;

[0229] ② When the number of second-point data is between 3,600 and 5,400, and the corresponding time is 1 - 1.5 hours, it is recorded as an effective transformation duration. Then, for the subsequent fracturing stages, it is necessary to moderately increase the optional perforation parameters or the displacement to improve the fracturing effect. Each improvement can adjust at most one of the perforation parameters and the displacement. The perforation parameters are the number of holes and the number of clusters. The number of clusters increased each time shall not exceed 1 cluster, and the maximum number of clusters in a single stage shall not exceed 11 clusters. The number of holes increased each time shall not exceed 6 - 8 times the number of clusters, and the maximum number of holes in a single stage shall not exceed 66 holes; The displacement increased each time shall not exceed 0.25 m 3 / min, and the upper limit shall not exceed 18 m 3 / min. If the next stage meets the sufficient transformation duration, it will be continued to the next - next stage. If it still does not meet, continue to improve the perforation parameters or the displacement until the sufficient transformation duration is achieved;

[0230] ③ When the number of second-point data is less than 3,600, and the corresponding time is less than 1 hour, it is recorded as an insufficient transformation duration. For the subsequent fracturing stages, it is necessary to strengthen the increase of perforation parameters and the increase of displacement. Each improvement can adjust at most one of the perforation parameters and the displacement. The perforation parameters are the number of holes and the number of clusters. The number of clusters increased each time shall not exceed 2 clusters, and the maximum number of clusters in a single stage shall not exceed 11 clusters. The number of holes increased each time shall not exceed 6 - 8 times the number of clusters, and the maximum number of holes in a single stage shall not exceed 66 holes; The displacement increased each time shall not exceed 0.5 m 3 / min, and the upper limit shall not exceed 18 m 3 / min. If the next stage meets the sufficient transformation duration, it will be continued to the next - next stage. If it still does not meet, continue to improve the perforation parameters or the displacement until the sufficient transformation duration is achieved;

[0231] ④ Record the current in-situ stress difference Δσ xyThe number of second-point data exceeding 5 MPa. When there are more than 300 data points continuously during the fracturing process, that is, the time exceeding 5 MPa lasts for more than 5 minutes, it is considered that the fracturing reconstruction intensity is too high, the proppant pumping will be difficult, and there is a risk of casing deformation. The construction displacement should be reduced in time, and it should be reduced step by step at a rate of 0.5 m 3 / min. Observe the change of the induced stress difference for 1 minute. If it is still greater than 10 MPa, continue to reduce it until the induced stress difference is maintained within 10 MPa. And implement measures to reduce the displacement or perforation parameters for the next section. At most, only one of the perforation parameters and displacement can be adjusted each time. The perforation parameters are the number of holes and the number of clusters. The number of clusters reduced each time shall not exceed 2 clusters, and the number of clusters in a single section shall not be less than 6 clusters at most. The number of holes reduced each time shall not exceed 6-8 times the number of clusters, and the number of holes in a single section shall not be less than 36 holes at most; the displacement is reduced by no more than 0.25 m 3 / min, and the lower limit is at least 14 m 3 / min;

[0232] Record the real-time average value of the current in-situ stress difference. The initial point of the real-time average value is the time when the proppant starts to be pumped, and the end point is the time when the pump stops. Use this to evaluate the intensity of the fracturing construction; when the average value of the current in-situ stress difference is below -5 MPa, it is determined that the linear fracture is not sufficiently reconstructed; when it is between -5 and 0 MPa, it is determined that the vertical wellbore strip fracture is effectively reconstructed; when it is between 0 and 5 MPa, it is determined that the vertical wellbore fracture network is sufficiently reconstructed; when it is between 5 and 10 MPa, it is determined that the parallel wellbore strip fracture is effectively reconstructed; when it is greater than 10 MPa, it is determined that the parallel wellbore linear fracture is not sufficiently reconstructed and is a risk section;

[0233] Step S4d: Integrate the timeliness and construction intensity, establish evaluation indicators, and evaluate the post-fracture reconstruction effect of each fracturing section of a single well to facilitate subsequent indoor theoretical analysis;

[0234] The evaluation indicators established in step S4d include the timeliness index, the reconstruction intensity index, and the single-section fracturing effect evaluation index;

[0235] Among them, the timeliness index is determined by the following formula:

[0236]

[0237] In the formula,

[0238] Δt—the timeliness index of each fracturing section, dimensionless;

[0239] t—the number of second-point data of the current in-situ stress difference Δσ xy ′ between -5 and 5 MPa, unit: piece;

[0240] When t < 3600, Δt is uniformly taken as 0.2; when t > 7200, Δt is uniformly taken as 0.8;

[0241] Among them, the transformation intensity index is determined by the following formula:

[0242]

[0243] In the formula,

[0244] Δp—the transformation intensity index of each fracturing stage;

[0245] Among them, the evaluation index of the single-stage fracturing effect is determined by the following formula:

[0246]

[0247] In the formula,

[0248] S—the evaluation index of the single-stage fracturing effect, dimensionless.

[0249] Example Two

[0250] Example Two is the specific application of Example One on the shale gas horizontal well A. Please refer to Figure 1-2 .

[0251] Step S1: Obtain various geological data, tool and liquid data of the horizontal section of the fracturing well.

[0252] In this embodiment, the geological data of the shale gas horizontal well A includes the test depth, vertical depth, Poisson's ratio, minimum horizontal principal stress, maximum horizontal principal stress, and corresponding fracturing stage number in the logging interpretation data of the single well horizontal section, as shown in Table 1.

[0253] Table 1 - Geological data of each fracturing stage of the shale gas horizontal well A

[0254]

[0255]

[0256]

[0257] The tool and liquid data include the fracturing fluid density, fracturing fluid viscosity, number of perforations in each fracturing stage of the well horizontal section, hole diameter, wellbore inner diameter, absolute roughness of the wellbore inner wall, and hole flow coefficient.

[0258] In this embodiment, the fracturing fluid of the shale gas horizontal well A is all slickwater; the fracturing fluid density is 1050 kg / m 3, the viscosity of the fracturing fluid is 5 mPa·s, the number of perforations within a section is designed to be 48, the hole diameter is measured as 0.011 m according to the tool, the inner diameter of the wellbore is 0.1143 m, the absolute roughness of the inner wall of the wellbore is taken as 0.000005 m, and the hole flow coefficient is taken as 0.85.

[0259] Step S2: Calculate the real-time net fracturing pressure for each fracturing section without considering the induced stress.

[0260] Obtain the real-time construction second-point data, including the real-time wellhead casing pressure P Head 、real-time sand concentration w, and real-time construction displacement q for each fracturing section in the horizontal section of the well during fracturing.

[0261] In this embodiment, according to the real-time calculation and construction optimization for each section, the real-time wellhead casing pressure, real-time sand concentration, and real-time construction displacement of each section from section 1 to section 44 in the shale gas horizontal well A are statistically counted, as shown in Table 2; the specific calculation method should be calculated using the actual real-time construction second-point data.

[0262] Table 2 - Average values of real-time casing pressure, real-time sand concentration, and real-time construction displacement for each fracturing section of shale gas horizontal well A

[0263]

[0264]

[0265] Calculate each pressure and friction according to formulas (1) to (11), as shown in Table 3.

[0266] Table 3 - Real-time string static pressure, string friction, hole friction, and net pressure without considering induced stress for each fracturing section during construction

[0267]

[0268]

[0269]

[0270] Step S3: Calculate the real-time induced stress difference for each section during fracturing.

[0271] Calculate according to formulas (12) to (15) to obtain data such as the induced stress difference for each section, the difference between the in-situ stress difference and the induced stress difference, etc., as shown in Table 4; during fracturing, to ensure the construction quality, mainly by changing the number of clusters and holes, changing in steps of 1 - 2 clusters and 6 - 12 holes, most of the effective transformation of the entire well section is achieved.

[0272] Table 4 - Net pressure considering induced stress, induced stress difference, and difference between in-situ stress difference and induced stress difference for each fracturing section

[0273]

[0274]

[0275] Step S4: Conduct real-time evaluation of the fracturing effect, provide suggestions for the fracturing of subsequent fracturing stages, and successfully complete the fracturing.

[0276] Calculate the evaluation indexes according to Formulas (16) to (20), as shown in Table 5.

[0277] Table 5 - Evaluation Results of Each Fracturing Stage

[0278]

[0279]

[0280] The gas production profile test was carried out on Well A, a horizontal shale gas well. By correlating the gas production contribution rate with the above-mentioned fracturing effect evaluation indexes, it was found that (see the appendix) Figure 2 , the correlation degree is strong, and the post-fracture evaluation is effectively realized, proving that this method is applicable to evaluating the fracturing transformation effect of each fracturing stage before the production determination of horizontal wells.

Claims

1. A real-time effect evaluation method for shale gas horizontal well fracturing, characterized in that, It includes the following steps: Step S1: Obtain various construction data of the horizontal section of the fracturing well; Step S2: Calculate the real-time net fracturing pressure of each fracturing stage during the fracturing process without considering the induced stress; Step S3: Based on the calculation results of Step S2, calculate the real-time induced stress difference of each fracturing stage during the fracturing process; Step S4: Conduct real-time evaluation of the fracturing effect and put forward suggestions for the fracturing of subsequent fracturing stages; The detailed steps of Step S2 are as follows: Step S2a: Obtain the real-time construction second-point data of each fracturing stage during the fracturing process; the construction second-point data includes the real-time wellhead casing pressure, real-time sand concentration, and real-time construction displacement of each fracturing stage during the fracturing process; Step S2b: Calculate the real-time static pressure of the sand-carrying string for each fracturing stage during the fracturing process P String ; Step S2c: Calculate the real-time wellbore string friction acting on the fracture opening generated by the migration of the sand-carrying fracturing fluid in the string during the fracturing of each fracturing stage P wf ; Step S2d: Calculate the hole friction at the fracture mouth during the fracturing process for each fracturing stage P nf ; Step S2e: Calculate the real-time net fracturing pressure of each fracturing stage during the fracturing process without considering the induced stress, and the formula is as follows: In the formula, P netn — The n real-time net pressure of the fracturing stage without considering the induced stress during the fracturing process; P head — The real-time wellhead casing pressure during the fracturing of each fracturing stage; σ x — The average minimum horizontal principal stress of each fracturing stage in the logging interpretation data; The detailed steps of Step S4 are as follows: Step S4a: Calculate the current in-situ stress field by the following formula; In the formula, Δ σ x ′— the in-situ stress in real time of the minimum horizontal principal stress direction during the fracturing process of each fracturing stage; Δ σ y ′—The in-situ stress in real time in the direction of the maximum horizontal principal stress during the fracturing process of each fracturing stage; σ y — The average maximum horizontal principal stress of each fracturing stage in the logging interpretation data; σ xin — The i real-time induced stress at the fracture opening of the n fracture stage in the direction of the minimum horizontal principal stress generated at the fracture opening of the fracture stage; σ yin — the i real-time induced stress in the direction of the maximum horizontal principal stress generated at the fracture port of the n fracture stage; Step S4b: Calculate the current in-situ stress difference by the following formula; where Δ σ xy ′—the real-time current in-situ stress difference of each fracturing stage during the fracturing process; Step S4c: Qualitatively evaluate the fracturing effect according to the real-time current in-situ stress difference of each fracturing stage during the fracturing process, and optimize the fracturing of subsequent fracturing stages; Step S4d: Integrate timeliness and construction intensity, establish evaluation indicators, and evaluate the post-fracturing transformation effect of each fracturing stage of a single well; the evaluation indicators include timeliness index, transformation intensity index, and single-stage fracturing effect evaluation index.

2. The real-time effect evaluation method for shale gas horizontal well fracturing according to claim 1, wherein The specific construction data in Step S1 specifically includes: geological data, tool and liquid data.

3. The real-time effect evaluation method for shale gas horizontal well fracturing according to claim 2, characterized in that, The geological data includes the test depth, average vertical depth, average Poisson's ratio, average minimum horizontal principal stress, and average maximum horizontal principal stress of the top and bottom boundaries of each fracturing stage in the logging interpretation data; The tool and liquid data includes the density of the fracturing fluid, viscosity of the fracturing fluid, number of perforations, perforation diameter, inner diameter of the wellbore, absolute roughness of the inner wall of the wellbore, and hole flow coefficient used for each fracturing stage.

4. The real-time effect evaluation method for hydraulic fracturing of shale gas horizontal wells according to claim 3, wherein The calculation formula for the static pressure of the sand-carrying string of each fracturing stage during the fracturing process is: In the formula, P String — The static pressure of the sand-carrying string in real time during the fracturing of each fracturing stage; ρ frac — The real-time fracturing fluid density during the fracturing process of each fracturing stage; g — acceleration due to gravity; h — The average vertical depth of each fracturing stage; The calculation formula for the real-time wellbore string friction of each fracturing stage during the fracturing process is: In the formula, P wf — The real-time wellbore string friction acting on the fracture opening generated by the migration of the sand-carrying fracturing fluid in the string during the fracturing of each fracturing stage; λ — coefficient of hydraulic friction; L —The test depth at the bottom boundary of each fracturing stage; v s — The real-time flow rate of the sand-carrying fracturing fluid during the fracturing process; D — Wellbore diameter; ρ Fluid — The density of the fracturing fluid used in each fracturing stage; The calculation formula for the hole friction at the fracture mouth of each fracturing stage during the fracturing process is: In the formula, P nf — the hole friction at the fracture mouth during the fracturing process of each fracturing stage; q — The real-time construction displacement of each fracturing stage during the fracturing process; n nf — The number of perforations in each fracturing stage; d nf —The perforation aperture of each fracturing stage; α nf — Perforation flow coefficient of each fracturing stage.

5. A real-time effect evaluation method for shale gas horizontal well fracturing according to claim 4, characterized in that The real-time fracturing fluid density of each fracturing stage in the step S2b ρ frac is calculated by the formula: In the formula, w — The real-time sand concentration during the fracturing process of each fracturing stage; ρ PropV — The bulk density of the proppant used for each fracturing stage; ρ Prop — Apparent density of proppant used for each fracturing stage; ρ Fluid — The density of the fracturing fluid used in each fracturing stage.

6. The real-time effect evaluation method for hydraulic fracturing of shale gas horizontal wells according to claim 4, characterized in that The calculation formula for the real-time flow velocity of the sand-carrying fracturing fluid in Step S2c during the fracturing process is: 。 7. A real-time effect evaluation method for shale gas horizontal well fracturing according to claim 6, characterized in that In Step S2c, the hydraulic friction coefficient is determined by the following steps: Step S2c1: Define a Reynolds number according to the average Poisson's ratio in the logging interpretation data of each fracturing stage, the density of the fracturing fluid used for each fracturing stage, the wellbore diameter, and the viscosity of the fracturing fluid used for each fracturing stage. The Reynolds number is determined by the following formula: In the formula, Re - Reynolds number; ν — The average Poisson's ratio in the logging interpretation data of each fracturing stage; μ - the viscosity of the fracturing fluid used for each fracturing stage; Step S2c2: When Re < 2000, the hydraulic friction coefficient is determined by the following formula: Step S2c3: When 2000 < Re < 59.7 / (2 R / D) 8 / 7 , the hydraulic friction coefficient is determined by the following formula: Step S2c4: When 59.7 / (2R / D) 8 / 7 <Re < [665 - 765lg(2 R / D)] / (2 R / D), the hydrodynamic friction coefficient is determined by the following formula: Step S2c5: When Re > [665 - 765lg(2 R / D)] / (2 R / D), the hydraulic friction coefficient is determined by the following formula: In steps S2c3 - S2c5, R — Absolute roughness of the inner pipe wall in the wellbore.

8. A real-time effect evaluation method for shale gas horizontal well fracturing according to claim 1, characterized in that The detailed steps of Step S3 are as follows: Step S3a: Calculate the real-time effective net fracturing pressure of each fracturing stage during the fracturing process considering the induced stress, and the calculation formula is as follows: In the formula, P net ( n ) — the n real-time effective net pressure of the fracturing stage considering the induced stress during the fracturing process; P netn — The n real-time net pressure of the fracturing stage without considering the induced stress during the fracturing process; σ xin — The i real-time induced stress at the fracture mouth of the n frac stage in the direction of the minimum horizontal principal stress generated at the fracture mouth of the frac stage; Step S3b: Calculate the induced stress on the minimum horizontal principal stress direction generated by each fracturing stage during the fracturing process; the calculation formula for the induced stress on the minimum horizontal principal stress direction is: In the formula, c — Half thickness of the reservoir; L ( i , n ) — the i test depth at the midpoint of the n frac stage and the difference in the test depth at the midpoint of the Step S3c: Calculate the induced stress on the maximum horizontal principal stress direction generated by each fracturing stage during the fracturing process; the calculation formula for the induced stress on the maximum horizontal principal stress direction is: In the formula, σ yin — the i real-time induced stress in the direction of the maximum horizontal principal stress generated at the fracture tip of the n fracture stage; Step S3d: Calculate the real-time induced stress difference of each fracturing stage during the fracturing process; The calculation formula is: In the formula, Δ σ ′—The real-time induced stress difference of each fracturing stage during the fracturing process.

9. The real-time fracturing effect evaluation method for shale gas horizontal wells according to claim 8, characterized in that 。 10. A real-time fracturing effect evaluation method for shale gas horizontal wells according to claim 9, wherein The timeliness index is determined by the following formula: In the formula, Δ t —Timeliness index of each fracturing stage; t — The number of second-point data where the current in-situ stress difference Δ σ xy ′ between -5 and 5 MPa; When t <3600, Δ t both take the value of 0.2; when t >7200, Δ t both take the value of 0.8; The transformation intensity index is determined by the following formula: In the formula, Δ p — Modification intensity index of each fracturing stage; The single-stage fracturing effect evaluation index is determined by the following formula: In the formula, S — Evaluation index for the effect of single-stage fracturing.

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