A calculation model and acquisition method for the wellbore fracture pressure of the horizontal section of a coal seam horizontal well

Abstract

The present invention discloses a calculation model and an acquisition method for the well wall fracture pressure of a horizontal section of a coal seam horizontal well. The well wall fracture pressure P of a horizontal section of a coal seam horizontal well is f , MPa: #imgabs0# where: σ v is the vertical principal stress of the horizontal section of the coal seam horizontal well wall, MPa; σ h is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; σ H is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; S t is the tensile strength of the wellbore rock in the horizontal section of a coal seam horizontal well, MPa. The present invention analyzes the stress on the wellbore of a coal seam horizontal well, effectively improving the accuracy of predicting the wellbore fracture pressure of the horizontal section of a coal seam horizontal well of coalbed methane, and providing theoretical support for subsequent coal seam methane horizontal well drilling and fracturing construction.

Description

Technical Field

[0001] The invention belongs to the technical field of coalbed methane horizontal well reservoir transformation, and relates to a calculation model and an acquisition method for the wellbore fracture pressure of a horizontal section of a coalbed horizontal well. Background Art

[0002] Formation fracture pressure is an important parameter for reasonably determining the wellbore structure, safe drilling and fracturing construction design of coalbed methane wells. There are two ways to obtain this parameter. One is indoor rock mechanics experimental testing or on-site hydraulic fracturing construction monitoring, and the other is to use theoretical classical models to calculate. At present, the quantitative calculation formulas of formation breakdown pressure at home and abroad mainly include: Hubbert-Willis formula, Eaton formula, Anderson formula, Stephn formula, Terzaghi formula, Haimson-Fairhurst formula, Huang Rongzun formula, Wei formula, Li Chuanliang formula, R. Douglas formula and Deng Jingen formula. These calculation models of breakdown pressure are derived based on the vertical wellbore structure, without considering the influence of well inclination, azimuth and well angle on breakdown pressure. Compared with vertical wells, horizontal wells have larger well inclination. With the change of well inclination, azimuth and well angle, the complexity of well wall stress increases. The vertical well breakdown pressure prediction model is used to calculate the horizontal section of coalbed methane horizontal well wall breakdown pressure, which has low accuracy, resulting in low matching degree of on-site drilling mud performance, frequent problems such as wellbore shrinkage, wellbore collapse, leakage and poor fracturing effect. These phenomena indicate that the fracture initiation mechanism of horizontal section of horizontal well is significantly different from that of vertical well. With the continuous development of coalbed methane development, the mining area has gradually expanded to the soft and complex structural area, the coal seam and coal body structure is complex, and the horizontal well wall safety and fracturing transformation issues are prominent. Therefore, it is of great significance to conduct research on the well wall fracture mechanism of coalbed methane horizontal wells and establish a multi-factor controlled horizontal well wall fracture pressure prediction model for coalbed methane horizontal wells, which is of great significance for guiding the drilling and completion engineering of coalbed methane horizontal wells and the optimization design of hydraulic fracturing. Summary of the invention

[0003] In order to solve the problem of low prediction accuracy of wellbore rupture pressure of horizontal section of horizontal well in coal seam, the present invention provides a wellbore rupture pressure prediction model of horizontal section of horizontal well in coal seam. The method analyzes the wellbore stress field of horizontal well and combines the maximum tensile stress theory to establish a wellbore rupture pressure prediction model of horizontal section of horizontal well in coal seam. The model is constrained by vertical principal stress of formation, maximum horizontal principal stress, minimum horizontal principal stress, formation pressure, rock tensile strength, rock Poisson's ratio, formation effective stress coefficient, horizontal wellbore orientation and well circumference angle. The present invention takes into account the influence of well inclination, wellbore orientation and well circumference angle of horizontal well in coal seam on wellbore rupture pressure, so that the prediction model of wellbore rupture pressure of horizontal section of horizontal well in coal seam is more reasonable.

[0004] Specific technical solutions include:

[0005] A calculation model for the wellbore fracture pressure of a horizontal section of a coal seam horizontal well, wherein the wellbore fracture pressure P f , MPa:

[0006]

[0007] Where: v is the vertical principal stress of the horizontal section of the coal seam horizontal well wall, MPa; σ h is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; σ H is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; S t is the tensile strength of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa;

[0008] A=1-2cos2θ-2νcos2θ;

[0009] B=sin 2 β+2sin 2 βcos2θ+cos 2 β+2νsin 2 βcos2θ;

[0010] C = cos 2 β+2cos 2 βcos2θ+sin 2 β+2νcos 2 βcos2θ;

[0011] D = 1-2cos2θ + 2νcos2θ;

[0012] E=sin 2 β+2sin 2 βcos2θ-cos 2 β-2νsin 2 βcos2θ;

[0013] F=cos 2 β+2cos 2 βcos2θ-sin 2 β-2νcos 2 βcos2θ;

[0014] G=2sinβcos 2 β;

[0015] Where: ν is the Poisson's ratio of the wellbore rock in the horizontal section of the coal seam horizontal well, dimensionless; β is the well inclination azimuth of the horizontal section of the coal seam horizontal well, °; θ is the well circumference angle of the horizontal section of the coal seam horizontal well, °.

[0016] Optionally, the well inclination angle γ of the horizontal section of the coal seam horizontal well is 90°.

[0017] Optionally, the vertical principal stress σ v for:

[0018]

[0019] Where: h 1 is the vertical depth of the coal seam horizontal well into the stratum, m; h 2 is the vertical depth of the horizontal section of the coal seam horizontal well wall stratum, m; ρ 一开地层平均 is the average bulk density of the coal seam horizontal well, g / cm 3 ; ρ is the well logging volume density of the coal seam horizontal well wall formation rock, g / cm 3 ; g is the acceleration due to gravity, generally 9.8m / s 2 ; dh is the differential of the vertical depth of the coal seam horizontal well, m.

[0020] Optionally, the σ h and σ H They are:

[0021]

[0022]

[0023] Where: v is the vertical principal stress of the wellbore in the horizontal section of the horizontal well in the coal seam, MPa; E is the Young's modulus of the wellbore rock in the horizontal section of the horizontal well in the coal seam, MPa; ν is the Poisson's ratio of the wellbore rock in the horizontal section of the horizontal well in the coal seam, dimensionless; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; ε h is the strain in the direction of the minimum horizontal principal stress of the wellbore wall in the horizontal section of the coal seam horizontal well, which is a constant in the same block; ε H It is the strain in the direction of the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, and is a constant in the same block.

[0024] Optionally, the P p for:

[0025] P p =P 压力梯度 H;

[0026] Where: P压力梯度 is the wellbore formation pressure gradient in the horizontal section of a horizontal well in a coal seam, MPa / m; H is the vertical depth of the wellbore formation in the horizontal section of a horizontal well in a coal seam, m.

[0027] Optionally, the α 煤系 for:

[0028]

[0029] Where: ρ is the wellbore rock logging volume density of the horizontal section of the coal seam horizontal well, g / cm 3 ρ ma is the mineral skeleton density of the wellbore rock in the horizontal section of the coal seam horizontal well, g / cm 3 ; DTS is the time difference of shear wave in the horizontal section of the coal seam horizontal well, μs / ft; DTC is the time difference of longitudinal wave in the horizontal section of the coal seam horizontal well, μs / ft; DTS ma DTC is the shear wave time difference of the horizontal section of the coal seam horizontal well wall stratum skeleton, μs / ft; ma is the longitudinal wave time difference of the stratum skeleton of the well wall in the horizontal section of the horizontal well in the coal seam, μs / ft; K is the rock deformation coefficient of the well wall in the horizontal section of the horizontal well in the coal seam, dimensionless, K=1 for sandstone and mudstone formations, K=0.5~1 for primary structural coal and crushed coal, and K=0~0.5 for crushed coal and mylonitic coal.

[0030] Optionally, the S t for:

[0031]

[0032] Where: E is the Young's modulus of the rock wall of the horizontal section of the coal seam horizontal well, MPa; V sh is the mud content of the wellbore wall in the horizontal section of a horizontal well in a coal seam, %.

[0033] Optionally, the ν is:

[0034]

[0035] Where: DTS is the shear wave time difference of the wellbore formation in the horizontal section of the horizontal well in the coal seam, μs / m; DTC is the longitudinal wave time difference of the wellbore formation in the horizontal section of the horizontal well in the coal seam, μs / m.

[0036] A method for obtaining the wellbore fracture pressure of a horizontal section of a coal seam horizontal well, comprising:

[0037] Step 1, establishing a calculation model for the wellbore rupture pressure of the horizontal section of a horizontal well in a coal seam, wherein the calculation model for the wellbore rupture pressure of the horizontal section of a horizontal well in a coal seam is the calculation model for the wellbore rupture pressure of the horizontal section of a horizontal well in a coal seam according to claim 1;

[0038] Step 2, obtain: vis the vertical principal stress of the horizontal section of the coal seam horizontal well wall, MPa; σ h is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; σ H is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; S t is the tensile strength of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa; ν is the Poisson's ratio of the wellbore rock in the horizontal section of the coal seam horizontal well, dimensionless; β is the wellbore inclination angle of the horizontal section of the coal seam horizontal well, °; θ is the wellbore circumference angle of the horizontal section of the coal seam horizontal well, °;

[0039] Step 3, calculate and obtain the parameters A, B, C, D, E, F and G in the calculation model of the wellbore fracture pressure of the horizontal section of the horizontal well in the coal seam;

[0040] Step 4, calculating the wellbore rupture pressure of the horizontal section of the coal seam horizontal well by the wellbore rupture pressure calculation model of the horizontal section of the coal seam horizontal well in step 1.

[0041] Optionally, the vertical principal stress σ v for:

[0042]

[0043] Where: h 1 is the vertical depth of the coal seam horizontal well into the stratum, m; h 2 is the vertical depth of the horizontal section of the coal seam horizontal well wall stratum, m; ρ 一开地层平均 is the average bulk density of the coal seam horizontal well, g / cm 3 ; ρ is the well logging volume density of the coal seam horizontal well wall formation rock, g / cm 3 ; g is the acceleration due to gravity, generally 9.8m / s 2 ; dh is the differential of the vertical depth of the coal seam horizontal well, m;

[0044] The σ h and σ H They are:

[0045]

[0046]

[0047] Where: v is the vertical principal stress of the wellbore in the horizontal section of the horizontal well in the coal seam, MPa; E is the Young's modulus of the wellbore rock in the horizontal section of the horizontal well in the coal seam, MPa; ν is the Poisson's ratio of the wellbore rock in the horizontal section of the horizontal well in the coal seam, dimensionless; α 煤系P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; ε h is the strain in the direction of the minimum horizontal principal stress of the wellbore wall in the horizontal section of the coal seam horizontal well, which is a constant in the same block; ε H It is the strain in the direction of the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, which is a constant in the same block;

[0048] The P p for:

[0049] P p =P 压力梯度 H;

[0050] Where: P 压力梯度 is the formation pressure gradient of the wellbore in the horizontal section of the horizontal well in the coal seam, MPa / m; H is the vertical depth of the formation of the wellbore in the horizontal section of the horizontal well in the coal seam, m;

[0051] The α 煤系 for:

[0052]

[0053] Where: ρ is the wellbore rock logging volume density of the horizontal section of the coal seam horizontal well, g / cm 3 ρ ma is the mineral skeleton density of the wellbore rock in the horizontal section of the coal seam horizontal well, g / cm 3 ; DTS is the time difference of shear wave in the horizontal section of the coal seam horizontal well, μs / ft; DTC is the time difference of longitudinal wave in the horizontal section of the coal seam horizontal well, μs / ft; DTS ma DTC is the shear wave time difference of the horizontal section of the coal seam horizontal well wall stratum skeleton, μs / ft; ma is the longitudinal wave time difference of the stratum skeleton of the well wall in the horizontal section of the horizontal well in the coal seam, μs / ft; K is the rock deformation coefficient of the well wall in the horizontal section of the horizontal well in the coal seam, dimensionless, K=1 for sandstone and mudstone formations, K=0.5~1 for primary structural coal and crushed coal, K=0~0.5 for crushed coal and mylonitic coal;

[0054] The S t for:

[0055]

[0056] Where: E is the Young's modulus of the rock wall of the horizontal section of the coal seam horizontal well, MPa; V sh is the mud content of the wellbore formation in the horizontal section of the coal seam horizontal well, %;

[0057] The ν is:

[0058]

[0059] Where: DTS is the shear wave time difference of the wellbore formation in the horizontal section of the horizontal well in the coal seam, μs / m; DTC is the longitudinal wave time difference of the wellbore formation in the horizontal section of the horizontal well in the coal seam, μs / m.

[0060] The present invention analyzes the stress on the wellbore of a coalbed methane coal seam horizontal well, and in combination with the characteristics of the coal system, establishes a method for calculating the wellbore rupture pressure of the horizontal section of a coalbed horizontal well based on the vertical principal stress of the formation, the maximum horizontal principal stress, the minimum horizontal principal stress, the formation pressure, the rock tensile strength, the rock Poisson's ratio, the formation effective stress coefficient, the horizontal wellbore orientation, and the wellbore angle. This method makes up for the well type mismatch problem of using a coalbed methane vertical well rupture pressure model to predict the horizontal section of a coalbed methane coal seam horizontal well, and also makes up for the technical difficulty of the long-standing lack of a wellbore rupture pressure prediction model for the horizontal section of a coalbed methane coal seam horizontal well. It also effectively improves the accuracy of the prediction of the wellbore rupture pressure of the horizontal section of a coalbed methane coal seam horizontal well, and provides theoretical support for subsequent coalbed methane coal seam horizontal well drilling and fracturing construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The accompanying drawings are used to provide a further understanding of the present disclosure and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present disclosure but do not constitute a limitation of the present disclosure. In the accompanying drawings:

[0062] Figure 1 It is the in-situ stress coordinate transformation diagram of coalbed methane well;

[0063] Figure 2 This is a schematic diagram of the wellbore structure of the X-03 horizontal well group;

[0064] Figure 3 It is a flow chart of a method for calculating the wellbore fracture pressure of a horizontal section of a coal seam horizontal well of the present invention;

[0065] Figure 4 This is the fracturing construction curve of X-03H horizontal well. DETAILED DESCRIPTION

[0066] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0067] The purpose of the present invention is to provide a calculation model and acquisition method for the wellbore fracture pressure of a horizontal section of a coal seam horizontal well, a calculation model for the wellbore fracture pressure of a horizontal section of a coal seam horizontal well, P f , MPa:

[0068]

[0069] Where: v is the vertical principal stress of the horizontal section of the coal seam horizontal well wall, MPa; σ h is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; σ H is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; S t is the tensile strength of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa;

[0070] A=1-2cos2θ-2νcos2θ;

[0071] B=sin 2 β+2sin 2 βcos2θ+cos 2 β+2νsin 2 βcos2θ;

[0072] C = cos 2 β+2cos 2 βcos2θ+sin 2 β+2νcos 2 βcos2θ;

[0073] D = 1-2cos2θ + 2νcos2θ;

[0074] E=sin 2 β+2sin 2 βcos2θ-cos 2 β-2νsin 2 βcos2θ;

[0075] F=cos 2 β+2cos 2 βcos2θ-sin 2 β-2νcos 2 βcos2θ;

[0076] G=2sinβcos 2 β;

[0077] Where: ν is the Poisson's ratio of the wellbore rock in the horizontal section of the coal seam horizontal well, dimensionless; β is the well inclination azimuth of the horizontal section of the coal seam horizontal well, °; θ is the well circumference angle of the horizontal section of the coal seam horizontal well, °.

[0078] In an embodiment of the present disclosure, the well inclination angle γ of the horizontal section of the coal seam horizontal well is 90°.

[0079] In the embodiment of the present disclosure, the vertical principal stress σ v for:

[0080]

[0081] Where: h 1 is the vertical depth of the coal seam horizontal well into the stratum, m; h 2 is the vertical depth of the horizontal section of the coal seam horizontal well wall stratum, m; ρ 一开地层平均 is the average bulk density of the coal seam horizontal well, g / cm 3 ; ρ is the well logging volume density of the coal seam horizontal well wall formation rock, g / cm 3 ; g is the acceleration due to gravity, generally 9.8m / s 2 ; dh is the differential of the vertical depth of the coal seam horizontal well, m.

[0082] In the embodiments of the present disclosure, σ h and σ H They are:

[0083]

[0084]

[0085] Where: v is the vertical principal stress of the wellbore in the horizontal section of the horizontal well in the coal seam, MPa; E is the Young's modulus of the wellbore rock in the horizontal section of the horizontal well in the coal seam, MPa; ν is the Poisson's ratio of the wellbore rock in the horizontal section of the horizontal well in the coal seam, dimensionless; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; ε h is the strain in the direction of the minimum horizontal principal stress of the wellbore wall in the horizontal section of the coal seam horizontal well, which is a constant in the same block; ε H It is the strain in the direction of the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, which is a constant in the same block;

[0086] In the embodiments of the present disclosure, P p for:

[0087] P p =P 压力梯度 H;

[0088] Where: P 压力梯度 is the formation pressure gradient of the wellbore in the horizontal section of the horizontal well in the coal seam, MPa / m; H is the vertical depth of the formation of the wellbore in the horizontal section of the horizontal well in the coal seam, m;

[0089] In the embodiments disclosed herein, considering that compared with conventional sand and mudstone formations, coal seams have a dual pore structure and are easy to break and deform, the more broken the coal seams are, the more complex the pore structure is, the smaller the proportion of pore fluid acting on the same area is, and the smaller the contribution of formation pressure to ground stress is. In order to make the formation effective stress coefficient logging prediction model more compatible with the easy deformation characteristics of coal seams, the deformation coefficient (K) is introduced to improve the formation effective stress coefficient logging prediction model of the horizontal section of the coal seam horizontal well wall, α 煤系 for:

[0090]

[0091] Where: ρ is the wellbore rock logging volume density of the horizontal section of the coal seam horizontal well, g / cm 3 ρ ma is the mineral skeleton density of the wellbore rock in the horizontal section of the coal seam horizontal well, g / cm 3 ; DTS is the time difference of shear wave in the horizontal section of the coal seam horizontal well, μs / ft; DTC is the time difference of longitudinal wave in the horizontal section of the coal seam horizontal well, μs / ft; DTS ma DTC is the shear wave time difference of the wellbore stratum skeleton in the horizontal section of the coal seam horizontal well, μs / ft; ma is the longitudinal wave time difference of the stratum skeleton of the well wall in the horizontal section of the horizontal well in the coal seam, μs / ft; K is the rock deformation coefficient of the well wall in the horizontal section of the horizontal well in the coal seam, which indicates the degree of rock mass damage, dimensionless, K=1 for sandstone strata, K=0.5~1 for primary structural coal and crushed coal, and K=0~0.5 for crushed coal and mylonitic coal;

[0092] In the embodiments of the present disclosure, S t for:

[0093]

[0094] Where: E is the Young's modulus of the rock wall of the horizontal section of the coal seam horizontal well, MPa; V sh is the mud content of the wellbore formation in the horizontal section of the coal seam horizontal well, %;

[0095] In the embodiments of the present disclosure, ν is:

[0096]

[0097] Where: DTS is the shear wave time difference of the wellbore formation in the horizontal section of the horizontal well in the coal seam, μs / m; DTC is the longitudinal wave time difference of the wellbore formation in the horizontal section of the horizontal well in the coal seam, μs / m.

[0098] The method for obtaining the wellbore fracture pressure of the horizontal section of a coal seam horizontal well includes:

[0099] Step 1, establishing a calculation model for the wellbore rupture pressure of a horizontal section of a coal seam horizontal well, wherein the calculation model for the wellbore rupture pressure of a horizontal section of a coal seam horizontal well is the calculation model for the wellbore rupture pressure of a horizontal section of a coal seam horizontal well of the present invention;

[0100] Step 2, obtain: v is the vertical principal stress of the horizontal section of the coal seam horizontal well wall, MPa; σ h is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; σ H is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; S t is the tensile strength of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa; ν is the Poisson's ratio of the wellbore rock in the horizontal section of the coal seam horizontal well, dimensionless; β is the wellbore inclination angle of the horizontal section of the coal seam horizontal well, °; θ is the wellbore circumference angle of the horizontal section of the coal seam horizontal well, °;

[0101] Step 3, calculate and obtain the parameters A, B, C, D, E, F and G in the calculation model of the wellbore fracture pressure of the horizontal section of the horizontal well in the coal seam;

[0102] Step 4, calculating the wellbore rupture pressure of the horizontal section of the coal seam horizontal well by the wellbore rupture pressure calculation model of the horizontal section of the coal seam horizontal well in step 1.

[0103] The derivation process of the wellbore fracture pressure model for the horizontal section of a coal seam horizontal well is as follows:

[0104] Under the basic assumptions of cylindrical coordinate system and linear elastic mechanics, Fairhurst derived the stress distribution around the borehole of any coalbed methane well trajectory. Figure 1 and formula (1). In this paper, coalbed methane wells include vertical wells, directional wells and horizontal wells. Horizontal wells refer to special wells with a maximum well inclination of about 90° and a certain length of horizontal well section maintained in the target layer.

[0105]

[0106] Where: r is the wellbore radial stress, MPa; σ θ is the wellbore tangential stress, MPa; σ z is the wellbore axial stress, MPa; τ rθ is the shear stress on the r and θ planes, MPa; τ θz is the shear stress on the z and θ planes, MPa; τ rz are the shear stresses on the r and z planes, MPa; r w is the well radius, m; Pwf is the fluid column pressure in the wellbore, MPa; r is the radial distance, m; ν is the Poisson's ratio of the rock, dimensionless; They are the original stress components after the original coordinate transformation, MPa; the transformed coordinate system is as follows Figure 1 As shown, γ is the well inclination angle, °; β is the well inclination azimuth of the coalbed methane well, °; θ is the well circumference angle of the coalbed methane well, °;

[0107] The transformed in-situ stress component and the in-situ stress σ H , σ h , σ v The relationship between them is shown in formula (2):

[0108]

[0109] When r = r w When , the wellbore stress expression is formula (3):

[0110]

[0111] The effective stress of the three principal stresses at any point on the wellbore of a coalbed methane well is shown in formula (4):

[0112]

[0113] Since the horizontal section of the coal seam horizontal well has a well inclination angle of 90°, when the well inclination angle γ is 90°, σ θ , σ z , τ θz as follows:

[0114] σ θ =σ v (1-2cos2θ)+σ H (sin 2 β+2sin 2 βcos2θ)+σ h (cos 2 β+2cos 2 βcos2θ)-P wf (5);

[0115] σ z =σ H (cos z β+2νsin 2 βcos2θ)+σ h (sin 2 β+2νcos 2 βcos2θ)-2νcos2θσ v (6);

[0116] τ θz=2cosθsinβcos 2 βσ h -2cosθsinβcos 2 βσ H (7);

[0117] In the formula, let A=1-2cos2θ-2νcos2θ; B=sin 2 β+2sin 2 βcos2θ+cos 2 β+2νsin 2 βcos2θ; C=cos 2 β+2cos 2 βcos2θ+sin 2 β+2νcos 2 βcos2θ; D=1-2cos2θ+2νcos2θ; E=sin 2 β+2sin 2 βcos2θ-cos 2 β-2νsin 2 βcos2θ; F=cos 2 β+2cos 2 βcos2θ-sin 2 β-2νcos 2 βcos2θ; G = 2sinβcos 2 β.

[0118] Since the bedding in the coal rock fracturing sample is basically along the ground stress direction, the possibility of shear fracture along the bedding is small. Therefore, the wellbore fracture of coalbed methane horizontal wells is mainly controlled by the maximum tensile stress around the well. For the horizontal section of the coal seam horizontal well, the wellbore fracture pressure (P f ) is defined as the limit pressure of the wellbore fluid column pressure at which the horizontal section of the horizontal wellbore wall does not experience tensile fracture, and the critical state of the maximum tensile stress criterion is expressed by formula (8):

[0119] σ 3min =-S t (8);

[0120] Where: 3min is the maximum tensile stress of the rock on the wellbore wall in the horizontal section of the coal seam horizontal well, MPa; S t It is the tensile strength of the rock on the wall of the horizontal section of a horizontal well in a coal seam, MPa.

[0121] According to the maximum tensile stress criterion, the key to solving the fracture pressure of the horizontal section of the coal seam horizontal well is to determine the maximum tensile stress among the three principal stresses of the horizontal section of the well wall. Rock mechanics generally stipulates that compressive stress is positive, tensile stress is negative, and σ 11 , σ 22 , σ33 is the effective principal stress at any point on the horizontal section of the horizontal wellbore. When the effective principal stress at a point on the horizontal section of the horizontal wellbore is negative, the rock is in a tensile stress state. When the effective principal stress on the horizontal section of the horizontal wellbore is positive, the rock is in a compressive stress state. Since most of the coal seams in my country belong to low-pressure reservoirs or normal-pressure reservoirs, the formation pressure gradient is less than 10kPa / m, and the water column pressure gradient is 10kPa / m. Coalbed methane horizontal well drilling uses water-based drilling fluid, and the drilling fluid column pressure gradient is greater than 10kPa / m. Therefore, the drilling fluid column pressure is greater than the coal seam formation pressure, and 0<α 煤系 <1, we get σ 11 =P wf -α 煤系 P p >0,σ 11 The stress state in the coal seam is compressive stress. Analyzing formula (4), it is found that the three effective principal stresses (σ 11 , σ 22 and σ 33 ) between 22 -σ 33 >0, assuming σ 22 and σ 33 is a negative value, both belong to tensile stress, then σ 3min =σ 33 Assume σ 22 is a positive value, which belongs to compressive stress, σ 33 is a negative value, which is tensile stress, then σ 3min =σ 33 Comprehensive analysis shows that 3min =σ 33 , using σ 33 =-S t Prediction of wellbore fracture pressure in horizontal section of coal seam horizontal well:

[0122]

[0123] Combine the parameters A, B, C, D, E, F and G in formulas (5), (6) and (7), and iterate formula (9) to obtain:

[0124]

[0125] The horizontal wellbore fluid column pressure P is solved by formula (10): wf , which is the horizontal section wellbore fracture pressure P f .

[0126] Specifically, the accuracy of the calculation model of the present invention is verified through Example 1:

[0127] Embodiment 1:

[0128] In order to better illustrate the calculation process of the horizontal section well wall fracture pressure of the coal seam horizontal well, the present invention is further described in detail below in combination with the geological parameters and mechanical parameters of the coalbed methane X-03 horizontal well group in Gaoping mining area of ​​Shanxi Province. The X-03 horizontal well group consists of the X-03H horizontal well and the X-03V vertical well ( Figure 2 ).

[0129] like Figure 3 As shown, the method for calculating the wellbore fracture pressure of the horizontal section of a coal seam horizontal well in this embodiment is specifically as follows:

[0130] Step 1: Based on the maximum tensile stress criterion, a calculation model for the wellbore fracture pressure of the horizontal section of a horizontal well in a coal seam is established.

[0131]

[0132] Where: v is the vertical principal stress of the horizontal section of the coal seam horizontal well wall, MPa; σ h is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; σ H is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; α 煤系 P is the effective stress coefficient of the horizontal section of the coal seam horizontal well, dimensionless; p is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; S t is the tensile strength of the rock wall of the horizontal section of the coal seam horizontal well, MPa;

[0133] A=1-2cos2θ-2νcos2;

[0134] B=sin 2 β+2sin 2 βcos2θ+cos 2 β+2νsin 2 βcos2θ;

[0135] C = cos 2 β+2cos 2 βcos2θ+sin 2 β+2νcos 2 βcos2θ;

[0136] D = 1-2cos2θ + 2νcos2θ;

[0137] E=sin 2 β+2sin 2 βcos2θ-cos 2 β-2νsin 2 βcos2θ;

[0138] F=cos 2 β+2cos 2 βcos2θ-sin 2 β-2νcos 2 βcos2θ;

[0139] G=2sinβcos 2 β;

[0140] Where: ν is the Poisson's ratio of the wellbore rock in the horizontal section of the coal seam horizontal well, dimensionless; β is the well inclination azimuth of the horizontal section of the coal seam horizontal well, °; θ is the well circumference angle of the horizontal section of the coal seam horizontal well, °.

[0141] Step 2: Determine the vertical principal stress (σ v ) is 15.9MPa, the maximum horizontal principal stress (σ H ) is 12.6MPa and the minimum horizontal principal stress (σ h ) is 10.00MPa.

[0142] Step 3: Determine the formation pressure (P) of the X-03H horizontal well at a depth of 1580 m by interpreting the injection pressure drop test data. p ) is 1.97MPa.

[0143] Step 4: Using the longitudinal and transverse wave data of the X-03H horizontal well at a depth of 1580m, combined with the longitudinal and transverse wave and density data of the rock skeleton, the effective stress coefficient of the horizontal section of the horizontal well of the coal seam is calculated through the prediction model of the effective stress coefficient of the horizontal section of the horizontal well of the coal seam (α 煤系 ) is 0.6.

[0144] Step 5: Through on-site coring and laboratory testing, the tensile strength of the wellbore rock at the depth of 1580m (S t ) is 1.2MPa.

[0145] Step 6: Through on-site coring and laboratory measurement, the Poisson's ratio (ν) of the wellbore rock at a depth of 1580m in the X-03H horizontal well is 0.25.

[0146] Step 7, using the logging while drilling instrument, obtain the well inclination azimuth (β) of 45° at a depth of 1580m of the X-03H horizontal well, and determine the parameters A, B, C, D, E, F and G at the coal seam horizontal well angle θ=0° or θ=180° through the well inclination azimuth (β) and the rock Poisson's ratio (ν) at a depth of 1580m of the X-03H horizontal well, A=-1.5, B=2.25, C=2.25, D=-0.5, E=0.75, F=0.75, G=0.707.

[0147] Step 8: According to the values ​​of the parameters determined in steps 2 to 7 above, substitute them into the horizontal section wellbore fracture pressure prediction model of the coal seam horizontal well in step 1 to calculate and determine the wellbore fracture pressure P at a depth of 1580m of the X-03H horizontal well. f The numerical value is the calculated horizontal section wellbore fracture pressure (P f ) is 17.65MPa. The actual measured horizontal section wellbore fracture pressure (P f ) is 19.02MPa( Figure 4 ), proving that the accuracy of this model is 92.80%.

[0148] The preferred embodiments of the present disclosure are described in detail above in conjunction with the accompanying drawings; however, the present disclosure is not limited to the specific details in the above embodiments. Within the technical concept of the present disclosure, a variety of simple modifications can be made to the technical solution of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0149] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0150] In addition, various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A method for calculating the wellbore fracture pressure of a horizontal section of a coal seam horizontal well, characterized in that: The horizontal section well wall fracture pressure of the coal seam horizontal well , MPa: ； Where: is the vertical principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; is the effective stress coefficient of the wellbore formation in the horizontal section of the coal seam horizontal well, dimensionless; is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; is the tensile strength of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa; ； ； ； ； ； ； ； Where: is the Poisson's ratio of the rock mass of the horizontal section of a horizontal well in a coal seam, dimensionless; is the well inclination azimuth of the horizontal section of the coal seam horizontal well, °; is the wellbore angle of the horizontal section of the coal seam horizontal well, °; The vertical principal stress for: ； Where: is the vertical depth of the coal seam horizontal well into the stratum, m; is the vertical depth of the wellbore stratum corresponding to the horizontal section of the coal seam horizontal well, m; is the average bulk density of the coal seam horizontal well, g / cm 3 ; is the well logging volume density of the coal seam horizontal well wall formation rock, g / cm 3 ; g is the acceleration due to gravity, generally 9.8m / s 2 ; dh is the differential of the vertical depth of the coal seam horizontal well, m; The and They are: ； ； Where: is the vertical principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; E is the Young’s modulus of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa; is the Poisson's ratio of the rock mass of the horizontal section of a horizontal well in a coal seam, dimensionless; is the effective stress coefficient of the wellbore formation in the horizontal section of the coal seam horizontal well, dimensionless; is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; It is the strain in the direction of the minimum horizontal principal stress of the wellbore wall in the horizontal section of the coal seam horizontal well, which is a constant in the same block; It is the strain in the direction of the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, which is a constant in the same block; The for: ； Where: is the formation pressure gradient of the horizontal section of the coal seam horizontal well, MPa / m; is the vertical depth of the wellbore stratum in the horizontal section of the coal seam horizontal well, m; The for: ； Where: ρ is the well logging volume density of the rock on the horizontal section of the coal seam horizontal well, g / cm 3 ; ρma is the mineral skeleton density of the wellbore rock in the horizontal section of the coal seam horizontal well, g / cm 3 ; DTS is the shear wave time difference of the wellbore formation in the horizontal section of the coal seam horizontal well, μs / ft ; DTC is the longitudinal wave time difference of the well wall formation in the horizontal section of the coal seam horizontal well, μs / ft ; DTS ma is the shear wave time difference of the wellbore stratum skeleton in the horizontal section of the coal seam horizontal well, μs / ft ; DTC ma is the longitudinal wave time difference of the wellbore stratum skeleton in the horizontal section of the coal seam horizontal well, μs / ft ; K is the rock deformation coefficient of the horizontal section of the coal seam horizontal well, dimensionless, sandstone and mudstone formation K =1, primary structural coal and crushed coal K =0.5~1, crushed coal and mylonitic coal K =0~0.

5.

2. The method for calculating the horizontal section wellbore fracture pressure of a coal seam horizontal well according to claim 1, characterized in that: The well inclination angle of the horizontal section of the coal seam horizontal well is 90°.

3. The method for calculating the wellbore fracture pressure of a horizontal section of a coal seam horizontal well according to claim 1 or 2, characterized in that: The for: ； Where: E is the Young’s modulus of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa; is the mud content of the wellbore wall in the horizontal section of a horizontal well in a coal seam, %.

4. The method for calculating the horizontal section wellbore fracture pressure of a coal seam horizontal well according to claim 1 or 2, characterized in that: The for: ； Where: DTS is the shear wave time difference of the wellbore formation in the horizontal section of the coal seam horizontal well, μs / m ; DTC is the longitudinal wave time difference of the well wall formation in the horizontal section of the coal seam horizontal well, μs / m .

5. A method for obtaining the wellbore fracture pressure of a horizontal section of a coal seam horizontal well, characterized in that: include: Step 1, establishing a method for calculating the wellbore rupture pressure of a horizontal section of a horizontal well in a coal seam, wherein the method for calculating the wellbore rupture pressure of a horizontal section of a horizontal well in a coal seam is the method for calculating the wellbore rupture pressure of a horizontal section of a horizontal well in a coal seam according to any one of claims 1 to 4; Step 2, obtain: is the vertical principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; is the minimum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; is the maximum horizontal principal stress of the wellbore wall in the horizontal section of the horizontal well in the coal seam, MPa; is the effective stress coefficient of the wellbore formation in the horizontal section of the coal seam horizontal well, dimensionless; is the wellbore formation pressure in the horizontal section of the coal seam horizontal well, MPa; is the tensile strength of the wellbore rock in the horizontal section of the coal seam horizontal well, MPa; is the Poisson's ratio of the rock mass of the horizontal section of a horizontal well in a coal seam, dimensionless; is the well inclination azimuth of the horizontal section of the coal seam horizontal well, °; is the wellbore angle of the horizontal section of the coal seam horizontal well, °; Step 3, calculating and obtaining the parameters A, B, C, D, E, F and G in the calculation formula of the wellbore fracture pressure of the horizontal section of the horizontal well in the coal seam; Step 4, calculating the wellbore rupture pressure of the horizontal section of the horizontal well in the coal seam by the wellbore rupture pressure calculation method of the horizontal section of the horizontal well in the coal seam in step 1.

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