A method for optimizing the soaking time based on hydration expansion stress

Abstract

The present invention discloses a method for optimizing the soaking time based on the hydration expansion stress. Considering the high content of shale clay minerals, after the rock undergoes a hydration reaction with the fracturing fluid, induced microfractures are generated due to disintegration, softening and expansion, improving the overall permeability of the reservoir, so as to maximize the soaking construction effect after fracturing. The method obtains the fracture pressure and pore pressure through a two-dimensional pore model and logging in-situ stress data, in combination with the rock tensile strength failure criterion; obtains the dynamic moisture content profile through a forced imbibition model and a moisture diffusion equation; obtains the hydration expansion stress through a three-dimensional stress-strain mathematical model of rock mechanics; combines the conditions for crack generation in the force field with the force field changes under different soaking days to obtain the optimal crack length and the corresponding shortest soaking time. The soaking time optimization method provided by the present invention can reduce the construction time and improve the construction effect.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas engineering, and particularly to a method for optimizing the shut-in time based on hydration expansion stress. Background Art

[0002] Geological exploration results show that the mineral composition of shale reservoirs is mainly composed of clay minerals, quartz and carbonate minerals; generally, it has the characteristics of high brittle mineral content, strong heterogeneity and large vertical variation; the reservoir spaces are mainly interlayer fractures and dissolution pores, and organic matter pores are not developed. Compared with conventional reservoirs, shale reservoirs have problems such as strong heterogeneity, small permeability and porosity, and sensitive reservoir lithology.

[0003] Hydraulic fracturing is one of the main means of shale development in China at present. Different from conventional sandstones, shale wells show special characteristics such as low flowback rate and high productivity after fracturing, and the water production decreases while the production increases after shut-in. And according to the statistical results of the oilfield, shut-in after fracturing transformation of shale reservoirs is beneficial to increase production.

[0004] The reason why shut-in can increase production is that the clay mineral components in shale are water-unstable. After being soaked by fracturing fluid, they undergo hydration expansion. After expansion, the strength of shale decreases and induced microfractures are generated. Under the support of the skeleton, a complex fracture network is formed to improve the seepage channel, and the single-well production is increased by increasing the permeability.

[0005] However, at present, the shut-in time mostly depends on the experience of on-site engineers to determine, and the considered factors are relatively single. Most methods only consider the oilfield development progress and engineering parameters, which are likely to cause the shut-in time to be too short to achieve the expected effect, and are also likely to cause reservoir pollution due to excessive shut-in.

[0006] At the same time, there are many simplified factors in the model calculation method for optimizing the shut-in time of shale wells at present. It cannot well explain the initiation law of rock microfractures from a microscopic perspective and cannot truly reflect the actual initiation situation; and the current research results on shut-in time are all applicable to specific wells, and a general method for determining the shut-in time cannot be obtained. Summary of the Invention

[0007] In view of this, the purpose of the embodiments of the present invention is to provide a method for optimizing the shut-in time based on hydration expansion stress.

[0008] To achieve the above technical purpose, the present invention provides the following technical solutions.

[0009] A method for optimizing the shut-in time based on hydration expansion stress, characterized by comprising the following steps:

[0010] Collect the basic parameters required for calculation;

[0011] The fracture pressure and pore pressure are obtained by means of a two-dimensional pore model and logging in-situ stress data, in combination with the tensile strength failure criterion of rocks.

[0012] The forced imbibition depth in the shale bedding direction is obtained through a forced imbibition model, and then the dynamic water content profile is obtained by combining with the water diffusion equation.

[0013] The rock mechanical parameters after rock hydration are obtained from the water content data, and the hydration swelling stress is obtained by combining with the three-dimensional stress-strain mathematical model.

[0014] The optimal fracture length and the corresponding shortest soaking time are obtained by combining the crack generation conditions in the force field with the force field changes under different soaking days.

[0015] Furthermore, the basic parameters required for calculation include formation parameters and fracturing construction parameters.

[0016] Furthermore, the formation parameters include in-situ stress parameters, rock mechanical parameters, and formation fluid parameters, and the fracturing construction parameters include fracturing fluid parameters and construction time parameters.

[0017] Furthermore, the combination of crack generation conditions in the force field includes:

[0018] If the sum of the pore pressure and the swelling stress is greater than the fracture pressure, cracks will be generated; otherwise, cracks will not be generated.

[0019] Furthermore, the obtaining of the fracture pressure by means of a two-dimensional pore model and logging in-situ stress data, in combination with the tensile strength failure criterion of rocks, further includes:

[0020] According to the superposition principle of linear elasticity theory, the stress field model of an elliptical hole under the combined action of the far-field maximum principal stress, the far-field minimum principal stress, and the uniform pressure is decomposed into the superposition of stress fields under the action of each force alone. After determining the geometric relationship of the elliptical hole, the circumferential stress at the hole edge is discriminantly analyzed to obtain the fracture pressure.

[0021] Furthermore, the obtaining of the forced imbibition depth in the shale bedding direction through a forced imbibition model and then the dynamic water content profile by combining with the water diffusion equation further includes:

[0022] The flow of fracturing fluid in a curved capillary is described by using the Hagen-Poiseuille equation and the capillary bundle model. The forced dialysis depth is calculated according to the capillary distribution frequency, and then the water content profile is obtained by combining with the water diffusion equation.

[0023] Furthermore, the obtaining of the rock mechanical parameters after rock hydration from the water content data and the hydration swelling stress by combining with the three-dimensional stress-strain mathematical model further includes:

[0024] In the triaxial stress, the circumferential stress is used as the swelling stress, and the semi-analytical method is used to process the strain boundary conditions at the hole boundary and the far field. Then, the displacement field generated by water absorption after shale hydration is obtained by using the analytical expression of the trial function, and the swelling stress distribution of different water absorption profiles is obtained by substituting it into the stress-strain mathematical model.

[0025] The present invention provides a method for optimizing the soaking time based on the hydration swelling stress. This method obtains the fracture pressure and pore pressure through a two-dimensional pore model and well logging geostress data; obtains the dynamic profile of water content through a forced imbibition model and a water diffusion equation; obtains the hydration swelling stress through a rock mechanics triaxial stress-strain mathematical model; combines the rock tensile strength failure criterion with the force field change under different soaking days to obtain the optimal fracture length and the corresponding shortest soaking time. The present invention provides a method for calculating the longest fracture length generated by the fracturing soaking construction, which can reduce the construction time while ensuring the construction effect and improve the construction effect. Brief Description of the Drawings

[0026] Figure 1 Schematic diagram of the physical model of the elliptical hole under stress for the present invention.

[0027] Figure 2 Schematic diagram of the eccentric angle and central angle of the elliptical hole model for the present invention.

[0028] Figure 3 Schematic diagram of the calculation results of the ratio of the axial stress to the far-field stress at different central angles for the present invention.

[0029] Figure 4 Schematic diagram of the calculation results of the stress at the hole edge under different far-field stresses for the present invention.

[0030] Figure 5 Schematic diagram of the calculation results of the circumferential stress distribution at the hole edge at different central angles for the present invention.

[0031] Figure 6 Schematic diagram of the calculation results of the tensile fracture initiation pressure at different central angles for the present invention.

[0032] Figure 7 Schematic diagram of the calculation results of the fracture initiation pressure at different stress azimuth angles for the present invention.

[0033] Figure 8 Schematic diagram of the calculation results of the fracture initiation pressure at different elliptical aspect ratios for the present invention.

[0034] Figure 9 Schematic diagram of the calculation results of the fracture initiation pressure under different tensile strengths for the present invention.

[0035] Figure 10 Schematic diagram of the calculation results of the water content profile under different imbibition days for the present invention.

[0036] Figure 11 Schematic diagram of the calculation results of the circumferential expansion stress at different self-priming days of the present invention.

[0037] Figure 12 Schematic diagram of the calculation results of the variation of the crack length with the self-priming days of the present invention. Detailed implementation manners

[0038] Combined with the description of the drawings and the specific implementation manners of the present invention, the details of the present invention can be more clearly understood. However, the specific implementation manners of the present invention described herein are only for the purpose of explaining the present invention and should not be construed in any way as a limitation of the present invention. Under the teaching of the present invention, those skilled in the art can conceive any possible variations based on the present invention, and these should all be regarded as falling within the scope of the present invention.

[0039] In the present invention, an optimization method for the shut-in time based on the hydration expansion stress is proposed, and the method includes the following steps:

[0040] 1. Collect the basic parameters required for calculation. The basic parameters required for calculation include formation parameters and fracturing construction parameters. The formation parameters include in-situ stress parameters, rock mechanics parameters, and formation fluid parameters. The fracturing construction parameters include fracturing fluid parameters and construction time parameters.

[0041] 2. Obtain the breakdown pressure and pore pressure through a two-dimensional pore model and well logging in-situ stress data, combined with the rock tensile strength failure criterion;

[0042] Assume that in a two-dimensional infinite formation plane, the shale medium satisfies the linear elastic theory, the pores (or fractures) of a single clay sheet are elliptical in shape, and the far field is subjected to the action of the maximum principal stress σ1 and the minimum horizontal principal stress σ3. At the same time, the elliptical hole is subjected to a uniform pressure P n acting. For the pores of the clay sheet, P n includes the original pore pressure P p , capillary force P c , and osmotic pressure P π .

[0043] The relationship between the eccentric angle and the central angle in the elliptical coordinate system satisfies the following relationship:

[0044]

[0045] In the formula:

[0046] a - The length of the major semi-axis, mm

[0047] b - The length of the minor semi-axis, mm

[0048] α - The stress azimuth angle, that is, the angle between the principal stress σ1 and the positive direction of the major axis of the ellipse, °;

[0049] θ——The central angle of a point on the periphery of the elliptical hole (the angle between the line connecting any point on the ellipse and the center of the ellipse and the positive direction of the major axis of the ellipse), °;

[0050] η——The eccentric angle of a point on the edge of the elliptical hole (the angle between the line connecting the corresponding point of any point on the ellipse on the auxiliary circle and the center of the ellipse and the positive direction of the major axis of the ellipse), °.

[0051] According to the superposition principle of linear elasticity theory, the stress field model of the elliptical hole under the combined action of σ1, σ3, and P n can be decomposed into the superposition of the stress fields under the individual actions of σ1, σ3, and P n The results of a large number of shale self - absorption experiments show that the shale water - absorption failure is mainly in the tensile failure mode. Therefore, more attention needs to be paid to the distribution of tensile stress on the edge of the elliptical hole, that is, the circumferential stress on the hole edge.

[0052] When subjected to the far - field compressive stress σ1, the circumferential stress on the edge of the elliptical hole is:

[0053]

[0054] In the formula:

[0055] σ1——The far - field maximum principal stress, MPa;

[0056] σ η ——The circumferential stress on the edge of the elliptical hole, MPa.

[0057] When σ1 is parallel to the major axis of the ellipse (α = 0°), the maximum and minimum circumferential stresses are in the directions of the minor axis and the major axis respectively, and tensile stress will be generated at the endpoints of the major axis:

[0058]

[0059] When subjected to the far - field compressive stress σ3, the angle between σ3 and the major axis of the ellipse is π / 2 + α. According to formula (3), the circumferential stress on the edge of the elliptical hole is calculated similarly as:

[0060]

[0061] In the formula:

[0062] σ3——The far - field minimum principal stress, MPa.

[0063] By superposing formula (3) and (4), the circumferential stress when the elliptical crack in the infinite formation is under bi - axial compression is obtained:

[0064]

[0065] The circumferential stress on the edge of the hole when the elliptical crack is under uniform pressure:

[0066]

[0067] When subjected to the internal pressure of the pores, a stress σ is generated on the pore boundary. η <0, which is a tensile stress.

[0068] The failure of the elliptical hole edge starts with tensile cracking, and the cracking criterion is:

[0069] σ η | η＝0 / π ≤ -σ t (7)

[0070] In the formula:

[0071] σ t —— The tensile strength of shale, MPa.

[0072] Assuming that σ1 and σ3 are parallel to the major and minor axes of the ellipse (α = 0°) respectively, combining equations (6) and (7) gives:

[0073]

[0074] Let ΔP = P n -σ3, and the discriminant model for crack generation can be obtained:

[0075]

[0076] Similarly, the discriminant model for the generation of vertical cracks at the minor axis endpoints can be obtained:

[0077]

[0078] Assume that the major axis direction of the ellipse is parallel to the horizontal stress, and assume that the horizontal stress is isotropic, that is, the difference between the maximum and minimum horizontal principal stresses is not considered.

[0079] General case:

[0080] σ η = σ η1 + σ η2 ≤ -σ t (11)

[0081] Finally, substituting equations (3) and (4) into equation (11), the crack initiation pressure P f :

[0082]

[0083] In the formula:

[0084] P f —— The crack initiation pressure when the elliptical hole edge undergoes tensile cracking, MPa.

[0085] The pore pressure is calculated using the pore pressure equivalent density:

[0086] p p = ρ p gh (13)

[0087] In the formula:

[0088] P p —— Pore pressure, MPa;

[0089] ρ p —— Equivalent density of pore pressure, kg / m 3 ;

[0090] g —— Acceleration of gravity, m / s 2 ;

[0091] h —— Formation depth, m.

[0092] 3. Obtain the forced imbibition depth in the shale bedding direction through the forced self - suction model, and then combine with the water diffusion equation to obtain the dynamic water content profile;

[0093] Assume that the flow conditions satisfy:

[0094] 1) The cross - sectional area of the cylindrical pipe along the way is constant;

[0095] 2) The flow in the cylindrical pipe is laminar;

[0096] 3) The fluid is an incompressible Newtonian fluid.

[0097] Then it can be inferred that the forced imbibition of the fracturing fluid into the horizontal capillary in the quasi - steady state follows the Hagen - Poiseuille's law:

[0098]

[0099] In the formula:

[0100] L f —— The length of the curved path of the liquid in a single curved capillary in time t, m;

[0101] t —— Swelling time, s;

[0102] D —— Capillary diameter, m;

[0103] μ —— Viscosity of the swelling liquid, Pa·s;

[0104] P c —— Capillary pressure, Pa;

[0105] P h —— Hydraulic pressure of the swelling liquid in the fracture, Pa.

[0106] The capillary pressure can be determined by Equation (15):

[0107]

[0108] In the formula:

[0109] σ —— Interfacial tension, N / m;

[0110] θ —— Contact angle, °.

[0111] Substituting Equation (15) into Equation (14) gives Equation (16):

[0112]

[0113] L f Is the length of the curved path of the liquid in a single curved capillary in time t. Then the straight-line length of the distance moved by the liquid column in a single curved capillary in time t is:

[0114]

[0115] In the formula:

[0116] L s —— The straight-line length of the distance moved by the liquid column in a single curved capillary in time t, m;

[0117] τ —— The ratio of the curved length to the straight-line length, dimensionless, and can be estimated using the following formula:

[0118] τ = 1 + 0.41ln(1 / φ) (18)

[0119] In the formula:

[0120] φ —— Porosity, dimensionless.

[0121] Substituting Equation (17) into Equation (16) and integrating gives:

[0122]

[0123] The above calculation is only the distance moved by the liquid column in a single capillary. By performing a probability summation on it, the average forced imbibition depth at time t can be obtained:

[0124]

[0125] In the formula:

[0126] f i —— Distribution frequency of capillaries with different diameters, dimensionless.

[0127] After obtaining the dialysis depth, the water content profile can be obtained according to the water diffusion equation. Let W(r, t) be the weight percentage of the adsorbed water at a distance r from the well axis at time t. In the case of cylindrical coordinates, the basic equation for water adsorption is:

[0128]

[0129] In the formula:

[0130] W — water content, dimensionless;

[0131] r — adsorption distance around the wellbore, m.

[0132] Boundary conditions:

[0133]

[0134] In the formula:

[0135] a — radius around the wellbore, m

[0136] C f — water absorption diffusion and penetration constant, related to the properties of the shale and the working fluid, as well as the temperature and pressure of the action.

[0137] W s — saturated water content, %

[0138] W0 — original water content of the formation, %

[0139] The water absorption on the wellbore surface will quickly reach the saturation value W s , which is related to the properties of the working fluid. For wellbore stability, it is required that the value of W0 generated by the interaction between the working fluid and the shale should be as small as possible. The solution of the basic equation of water adsorption can be written in the form expressed by the zeroth-order Bessel functions of the first and second kinds, and can also be solved by numerical methods. However, for the convenience of solution, the characteristic equation describing water adsorption can be written as:

[0140]

[0141] In the formula:

[0142] x — one-dimensional adsorption distance, m.

[0143] Combined with the boundary conditions, the solution of the characteristic equation is:

[0144]

[0145]

[0146] Thus, the water adsorption equation in the cylindrical coordinate system can be transformed into a one-dimensional water adsorption equation, and then combined with the forced dialysis distance to obtain the water content profile.

[0147] 4. Obtain rock mechanics parameters such as the elastic modulus and Poisson's ratio after rock hydration through the water content data, and combine with the three-dimensional stress-strain mathematical model to obtain the hydration expansion stress;

[0148] After shale hydration, there are different water contents on the spontaneous imbibition profiles of different lengths, forming a complex medium with variable modulus and variable strength. To calculate the swelling stress after shale hydration, it is necessary to study the influence of water content on its modulus, Poisson's ratio and other strength parameters.

[0149] The increase in water content after mud shale hydration has a huge impact on its Young's elastic modulus. When the water content increases, the modulus rapidly drops to the order of 10 3 MPa, and at the same time, the elastic strain will increase sharply under the action of stress.

[0150] Mathematically, the description of E = f(W) can be expressed as:

[0151] E = 4×10 4 exp[-11(W - 0.02) 1 / 2 (26)

[0152] The influence of water content on the Poisson's ratio μ of mud shale, that is, the relationship of μ = f(W), can be described by a linear equation, namely:

[0153] μ = 0.2 + 1.3W (27)

[0154] Some scholars conducted one-dimensional experiments on shale samples in contact with fresh water under zero confining pressure conditions. According to the laws summarized from the experimental results, the relationship curve between the average water absorption of mud shale and the average volume expansion strain was drawn. Therefore, according to the experimental results, the vertical strain ε v The empirical expression with water content can be obtained:

[0155] ε v = 0.0333(ΔW) + 0.832(ΔW) 2 (28)

[0156] Because the research object is the plane strain problem of axisymmetric shale hydration cracks on a two-dimensional plane, the simplified equilibrium equation is:

[0157]

[0158] Then the following geometric equations hold for the radial strain component and tangential strain component in the hydrated shale:

[0159]

[0160] In the formula:

[0161] u —— Radial displacement, cm.

[0162] The constitutive equation of mud shale after water absorption is

[0163]

[0164] In the formula:

[0165] σ r ,σ θ ,σ z —— Radial, tangential and vertical stress components after shale hydration expansion, MPa;

[0166] ε r ,ε θ ,ε v —— Radial, tangential and vertical strain components in hydrated shale, m;

[0167] m —— Ratio of the expansion strain perpendicular to the bedding direction to the expansion strain parallel to the bedding direction, dimensionless, here m = 0.71;

[0168] E —— Elastic modulus of mud shale, MPa;

[0169] μ —— Poisson's ratio of mud shale, dimensionless.

[0170] According to the stress-strain relationship expression in the equilibrium equation, the derivative relationship of the radial stress component with respect to the radial length can be obtained:

[0171]

[0172] Combined with the geometric equation, we can get:

[0173]

[0174]

[0175] Where:

[0176] Obtained from the discrete data points of the shale hydration degree variation diagram with distance, and Obtained according to the elastic modulus and Poisson's ratio expressions:

[0177]

[0178] The boundary conditions of the above equations are:

[0179]

[0180] In the formula:

[0181] S —— Assumed far-field horizontal in-situ stress, MPa, Substitute into the constitutive equation, and rewrite the boundary conditions as: at r = a (wellbore radius):

[0182]

[0183] At r = b(b >> a):

[0184]

[0185] The above boundary conditions can also be written as:

[0186]

[0187] Where:

[0188]

[0189] After calculation, the general solution of the boundary conditions is:

[0190]

[0191] In the formula:

[0192] C0 —— undetermined coefficient, dimensionless

[0193] In order to find this undetermined coefficient, a difference scheme is established, and the r-axis is divided into a set of equally spaced line segments:

[0194] r = r k = r a + Kh, K = 0, 1, …… n

[0195] Take nh to be sufficiently large to ensure: r n >> r0

[0196] Then substitute into the rewritten boundary conditions to get:

[0197]

[0198]

[0199] When n is large enough, it can be considered that u n = u n-1 , then u n can be found according to the known conditions; substitute it back into the general solution to find the undetermined coefficient C0, and then find u1.

[0200] At the same time, write the geometric equation in the following form:

[0201]

[0202] Where:

[0203]

[0204] The analytical solution of this equation is:

[0205]

[0206] In the formula:

[0207] C1, C2 —— undetermined coefficients, dimensionless;

[0208] KummerM, KummerU —— confluent hypergeometric functions, dimensionless;

[0209] Therefore, theoretically, the undetermined coefficients C1 and C2 can be obtained by substituting the deformation at the boundary conditions, and then the radial displacement u corresponding to different distances can be obtained by using the analytical method. However, due to the characteristics of the confluent hypergeometric function, the calculation results will jump due to the discontinuity of the discrete points during the calculation process. Therefore, referring to the form of the analytical solution, an approximate analytical expression of u and r is constructed:

[0210]

[0211] In the formula:

[0212] a, b —— undetermined coefficients, dimensionless;

[0213] Substituting the above approximate analytical expression into the boundary conditions, the variation of u with r can be obtained. Based on this, the numerical solution is realized by using the semi-analytical method, and the displacement field generated by water absorption after shale hydration is obtained. Then, the obtained displacement field is substituted into the stress-strain relationship formula to calculate the stress distribution generated by shale water absorption at different water absorption profiles.

[0214] 5. Calculation Examples and Analysis

[0215] According to the analysis of the elliptical hole stress field model above, the hole-edge cracking pressure calculated based on the tensile cracking criterion under the action of bi-axial in-situ stress has been obtained; through analysis, it is known that the process of forming cracks during the soaking operation is the process in which the pressure in the hole continuously exceeds the fracture pressure, which can be expressed by the following formula:

[0216]

[0217] Before the occurrence of hydration swelling, the pressure p in the hole n only includes the pore pressure p p ; with the appearance of the swelling stress, this item needs to be added to the pressure in the hole, that is:

[0218] p n = p p + σ θ ≥ p f (49)

[0219] The swelling stresses corresponding to different self-priming times and different water saturation profiles are different. Cracks occur at the profile distance where the pore pressure is greater than the fracture pressure, and no cracks occur at the profile distance where the pore pressure is less than the fracture pressure, corresponding to different lengths of hydration cracks.

[0220] The longer the length of the hydration crack, the deeper the degree of communication with the reservoir, and the better the soaking well stimulation effect. According to on-site experience, it is necessary to extend the soaking well time as much as possible to obtain a better stimulation effect; however, this time is not the longer the better. First, because the increase amplitude of the hydration crack length with the soaking well time will slow down. At this time, if the soaking well time is too short, the crack will not reach the expected extension effect. If the soaking well time is too long, it will delay the well opening time, thus affecting the production efficiency. Second, too long soaking well time will cause unforeseen reservoir damage and may even have a negative impact. Considering the above two factors, those skilled in the art can optimize the appropriate soaking well time.

[0221] Therefore, an example analysis of optimizing the soaking well time was carried out based on actual field data;

[0222] (1) Basic parameters

[0223] Table 1 Calculation basic parameters

[0224]

[0225] Table 2 Calculation parameters of capillary bundle model

[0226]

[0227] (2) Calculation results of working conditions

[0228] Calculations were carried out based on the basic parameters in Table 1 and Table 2, and the calculation results are as Figure 3 - Figure 12 shown.

[0229] As Figure 3 shown, when α = 0°, that is, when the far-field stress is parallel to the major axis of the ellipse, tensile stress appears at the end point in the major axis direction, and the stress value is the smallest; when α = 0 - 90°, that is, when the far-field stress has an angle with the major axis of the ellipse, there is an optimal angle with the lowest stress value, and the stress value is negative, which is tensile stress; when α = 90°, that is, when the far-field stress is parallel to the minor axis, the lowest value appears at the end point of the minor axis, and the lowest value is equal to the far-field stress, with the opposite sign.

[0230] As Figure 4 shown, when the stress azimuth angle is 0° and 90°, the minimum value is at the end point of the minor axis (θ = 90°). When the stress azimuth angle is 45°, the minimum value is near θ = 50°, and the maximum value is near θ = 175°.

[0231] As Figure 5As shown, when the pressure inside the hole acts alone, the circumferential stress at the hole edge is zero at the short-axis endpoints (θ = 90°); it is the lowest at the long-axis endpoints (θ = 0°), being negative, i.e., tensile stress is generated, and the numerical value is much greater than the pressure inside the hole.

[0232] As Figure 6 shown, the tensile fracture initiation pressure at the elliptical hole edge is the lowest at the long-axis endpoints (θ = 0°) of the ellipse. The fracture initiation pressure increases slowly near the long-axis endpoints, but the overall change is not significant. When the central angle exceeds 5°, the fracture initiation pressure increases sharply, i.e., tensile failure occurs at the endpoints of the ellipse during the fracture initiation of the elliptical hole.

[0233] As Figure 7 shown, within the range of 0 - 90° of the stress azimuth angle, the fracture initiation pressure increases with the increase of the stress azimuth angle, and the increasing amplitude first increases and then decreases gradually, i.e., the fracture initiation azimuth is in the direction of the maximum principal stress.

[0234] As Figure 8 shown, with the increase of the aspect ratio, the fracture initiation pressure decreases significantly. When the aspect ratio exceeds 8 (the pore is approximately a crack), the fracture initiation pressure is very close to the sum of the maximum principal stress and the tensile strength.

[0235] As Figure 9 shown, the fracture initiation pressure is linearly and positively correlated with the tensile strength, and the increase value of the fracture initiation pressure is equal to the increase value of the tensile strength.

[0236] As Figure 10 shown, the water content decreases with the increase of the imbibition front distance, and the decreasing trend becomes slower with more imbibition days.

[0237] As Figure 11 shown, the swelling stress decreases rapidly first, then increases slowly, and finally decreases to about 40 MPa with the increase of the imbibition front distance; the more imbibition days, the greater the initial value of the swelling stress, the smoother the change trend, and the swelling stress corresponding to the end of imbibition is the same under different imbibition days finally.

[0238] As Figure 12 shown, within the range of the simulated days, the more imbibition days, the greater the invasion depth of the fracturing fluid and the longer the length of the formed fracture. From Figure 12 it can be seen that within the range of the simulated days, the more imbibition days, the longer the fracture length; but with the increase of the imbibition days, the growth rate of the fracture length gradually decreases. Therefore, the soaking time cannot be increased blindly. In order to minimize the production delay and cost loss caused by the soaking construction, the value of the hydration fracture length at each soaking time can be determined according to the change curve of the hydration fracture length, and the soaking transformation volume can be qualitatively estimated based on the length value to determine the stimulation effect; then combined with the cost losses caused by production delay, equipment maintenance, and personnel management at each soaking time, the soaking time can be determined based on the principle of maximizing the production benefit after transformation, forming a set of soaking time optimization methods.

[0239] The present invention has been specifically described above through embodiments. It is necessary to point out here that these embodiments are only the preferred embodiments of the present invention, and do not impose any limitations on the present invention, nor are they limited to the forms disclosed herein. It should not be regarded as excluding other embodiments. Any modifications and simple changes made by those skilled in the art without departing from the technical idea and scope of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. An optimization method for soaking time based on hydration expansion stress, characterized in that, It includes the following steps: Collect the basic parameters required for calculation; Obtain the fracture pressure and pore pressure through the two-dimensional pore model and well logging in-situ stress data, combined with the rock tensile strength failure criterion; Determine the capillary pressure, use the Hagen-Poiseuille's law to calculate the spontaneous imbibition path length, consider the tortuosity to obtain the curved liquid column distance, establish a forced spontaneous imbibition model in combination with the capillary distribution frequency to obtain the forced imbibition depth in the shale bedding direction, and combine with the water diffusion equation to obtain the dynamic profile of water content; Obtain the rock mechanical parameters after rock hydration through the water content data, use the semi-analytical method to process the strain boundary conditions of the hole boundary and far field, use the analytical expression of the trial function to construct the relationship between the radial displacement and the spontaneous imbibition distance, establish a three-dimensional stress-strain mathematical model in combination with the constitutive equation, and calculate the three-dimensional stress of different water absorption profiles of the rock, where the circumferential stress is regarded as the swelling stress; Combine the crack generation conditions in the force field and the force field changes under different soaking days to obtain the optimal crack length and the corresponding shortest soaking time.

2. The optimized soaking time method based on hydration expansion stress according to claim 1, characterized in that The basic parameters required for calculation include formation parameters and fracturing construction parameters.

3. The optimized soaking time method based on hydration expansion stress according to claim 2, wherein The formation parameters include in-situ stress parameters, rock mechanical parameters, and formation fluid parameters.

4. The method for optimizing the soaking time based on the hydration expansion stress according to claim 2, wherein The fracturing construction parameters include fracturing fluid parameters and construction time parameters.

5. The method for optimizing the soaking time based on the hydration expansion stress according to claim 1, wherein The obtaining of the fracture pressure through the two-dimensional pore model and well logging in-situ stress data, combined with the rock tensile strength failure criterion, further includes: According to the superposition principle of linear elasticity theory, decompose the stress field model of the elliptical hole under the combined action of the far-field maximum principal stress, far-field minimum principal stress, and uniform pressure into the superposition of the stress fields under the action of each force alone. After determining the geometric relationship of the elliptical hole, conduct discriminant analysis on the circumferential stress at the hole edge to obtain the fracture pressure.

6. A computer software product, including computer-executable code stored on a computer-readable storage medium, where the computer-executable code is configured to execute the soaking time optimization method based on hydration swelling stress according to one of claims 1-5.

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