A high-control design method for fractures under multi-layer interference in unconventional oil and gas reservoirs

Abstract

The present invention relates to a method for designing high-control fractures under multi-layered geological interference in unconventional oil and gas reservoirs, and belongs to the technical field of hydraulic fracturing in oil and gas reservoir production enhancement and transformation. The present invention discloses a method for designing high-control fractures under multi-layered geological interference in unconventional oil and gas reservoirs, comprising obtaining geological parameters of a target horizontal well, preliminarily designing a fracturing fluid flow rate according to a production capacity target; calculating the hydraulic fracturing process of the target horizontal well, and evaluating the fracture height; comparing the fracture height with the expected control height, and if the fracture height is greater than the expected control height, reducing the fracturing fluid flow rate or the total hydraulic fracturing time and re-evaluating until the evaluated fracture height is less than the expected control height; and performing hydraulic fracturing construction on the target horizontal well based on the updated fracturing fluid flow rate or the total hydraulic fracturing time. The present invention comprehensively considers the effects of interlayer filtration and bedding tensile strength on fracture expansion, and effectively evaluates the fracture height size to optimize the overall hydraulic fracturing effect.

Description

Technical Field

[0001] The invention relates to a high-control design method for pressure fractures under multi-layer interference in unconventional oil and gas reservoirs, belonging to the technical field of hydraulic fracturing in oil and gas reservoir production increase and transformation. Background Art

[0002] Hydraulic fracturing technology, as a key measure to increase production from unconventional low-permeability oil and gas reservoirs, plays an important role in improving oil and gas recovery rates. Unconventional oil and gas reservoirs, such as shale gas reservoirs, have a large number of near-horizontal bedding layers. These structural weaknesses interfere with the vertical extension of hydraulic fractures, affecting the final fracture height. Fracture height control is crucial to improving the effectiveness of fracturing: when multiple oil and gas layers are distributed vertically, the fracture height needs to be appropriately increased to connect multiple small layers and activate the production potential of multiple layers. However, when the oil and gas reservoir is thin or contains bottom water, excessive fracture height will greatly damage the production-enhancing effect, so the fracture height needs to be strictly controlled.

[0003] Currently, fracture height control relies primarily on parameter trial and error and optimization based on prior engineering experience, or on fracture height prediction and parameter optimization based on conventional reservoir hydraulic fracturing simulation technology. However, these predictions and control are often unstable. Therefore, there is an urgent need to develop new technologies for reasonable fracture height prediction and effective control, thereby improving the effectiveness of hydraulic fracturing in unconventional oil and gas reservoirs. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for designing highly controlled fractures under multi-layer interference in unconventional oil and gas reservoirs in order to solve the problems existing in the prior art.

[0005] The present invention solves the above technical problems and provides a technical solution: a method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs, comprising the following steps:

[0006] Step S10: Obtain geological parameters of the target horizontal well, select fracturing fluid according to the production capacity target of the target horizontal well, and preliminarily set construction parameters;

[0007] Step S20: Calculate the hydraulic fracturing process of the target horizontal well based on the hydraulic fracturing model considering multi-layer physical interference, and evaluate the fracture height;

[0008] Step S30: Compare the fracture height with the expected control height. If the fracture height is greater than the expected control height, reduce the fracturing fluid flow rate or the total hydraulic fracturing time. Repeat steps S20-S30 until the estimated maximum fracture height is less than the expected control height, and then proceed to the next step.

[0009] Step S40: hydraulically fracture the target horizontal well based on the updated construction parameters.

[0010] A further technical solution is that the geological parameters in step S10 include the Young's modulus of the reservoir rock E , Poisson's ratio of reservoir rock n , reservoir fracture toughness K IC-1 , minimum horizontal principal stress s h , vertical stress s v , bedding tensile strength T o , bedding fracture toughness K IC-2 , bedding thickness w r , bedding permeability k r , bedding density C k , equivalent filtration coefficient C L .

[0011] A further technical solution is that the construction parameters in step S10 include the number of perforation clusters N , fracturing fluid viscosity m , fracturing fluid density r f , fracturing fluid discharge q 0 and total hydraulic fracturing time T a .

[0012] A further technical solution is that in step S20, the fluid pressure in the fracture is calculated based on the hydraulic fracturing model considering multi-layer interference. p f , crack width w , crack length l , crack height h .

[0013] A further technical solution is that the calculation process in step S20 specifically includes:

[0014] Step S21: Calculate the fluid pressure in the hydraulic fracture according to geological parameters, current time, and set construction parameters;

[0015] Step S22, calculating the hydraulic fracture width;

[0016] Step S23, calculating the length of the hydraulic fracture;

[0017] Step S24: Calculate the stress intensity factor and maximum principal stress at the upper and lower tips of the hydraulic fracture;

[0018] Step S25: judging whether the crack tip encounters bedding at the current moment according to the bedding density;

[0019] If the crack tip does not encounter bedding, the stress intensity factor at the crack tip is compared with the reservoir fracture toughness. If the critical extension condition is met, the crack height is updated; if the critical extension condition is not met, the crack height remains unchanged.

[0020] If the crack tip encounters bedding, the stress intensity factor at the crack tip is compared with the fracture toughness of the bedding. At the same time, the maximum principal stress at the crack tip is compared with the tensile strength of the bedding. If the critical condition for interlayer penetration is met, the crack height is updated. If the critical condition for interlayer penetration is not met, the crack height remains unchanged.

[0021] Step S26: If bedding is encountered, calculate the current accumulated bedding loss volume, and calculate and update the equivalent loss coefficient based on the accumulated bedding loss volume;

[0022] Step S27: Based on the total hydraulic fracturing time T a and cumulative fracturing time t The relationship between the two determines whether the fracturing construction is completed;

[0023] if t <T a , it means that the fracturing operation has not been completed, and the cumulative fracturing time is updated. t for t +Δ t , then repeat the calculation of S21~S27;

[0024] if t ≥ T a , it means that the fracturing construction is completed, the calculation is completed and the final crack height is obtained.

[0025] A further technical solution is that the critical condition for expansion in step S25 is: the fracture stress intensity factor is greater than or equal to the reservoir fracture toughness.

[0026] A further technical solution is that the critical condition for penetration in step S25 is: the crack stress intensity factor is greater than or equal to the bedding fracture toughness, and the maximum principal stress at the crack tip is greater than or equal to the bedding tensile strength.

[0027] A further technical solution is that in step S25, the fracture stress intensity factor is selected as the fracture upper tip stress intensity factor or the fracture lower tip stress intensity factor according to the position of the fracture tip in contact with the bedding.

[0028] Beneficial effects of the present invention: Based on a hydraulic fracturing simulation model that takes into account multi-layer interference, the present invention calculates the stress intensity factor and maximum principal stress at the tip of the hydraulic fracture during the fracturing process, comprehensively considers the effects of interlayer filtration and bedding tensile strength on fracture propagation, and effectively evaluates the fracture height size to optimize the overall hydraulic fracturing effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a calculation flow chart of a hydraulic fracturing simulation model based on multi-layer interference considerations;

[0030] Figure 2 A three-dimensional plot of the simulation results of multiple clusters of fracture sizes after hydraulic fracturing is completed. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0032] like Figure 1 As shown, the present invention provides a method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs, comprising the following steps:

[0033] Step S10: Obtain geological parameters of the target horizontal well, select fracturing fluid according to the production capacity target of the target horizontal well, and preliminarily set construction parameters;

[0034] The geological parameters include Young's modulus of reservoir rock E , Poisson's ratio of reservoir rock n , reservoir thickness H , reservoir fracture toughness K IC-1 , minimum horizontal principal stress s h , vertical stress s v , bedding tensile strength T o , bedding fracture toughness K IC-2 , bedding thickness w r , bedding permeability k r , interlayer friction coefficient l , bedding density C k , equivalent filtration coefficient C L .

[0035] Operation parameters include the number of perforation clusters N , fracturing fluid viscosity m , fracturing fluid density r f , fracturing fluid discharge q 0 and total hydraulic fracturing time T a .

[0036] Step S20: Simulate the hydraulic fracturing process of the target horizontal well based on the hydraulic fracturing model considering multi-layer physical interference, and evaluate the fracture height:

[0037] Initialize the cumulative fracturing time t =0s, crack encounter coefficient α =0, cumulative bedding loss volume V leak =0m 3 / s, the bedding number has been encountered M = 0. Then execute the following steps.

[0038] Step S21: Calculate the fluid pressure in the hydraulic fracture according to the geological parameters, the accumulated fracturing time, and the set construction parameters:

[0039] (1)

[0040] Where: p f is the fluid pressure in the crack, MPa; E’ is the plane strain modulus, MPa; q is the displacement of single cluster fracturing fluid, m 3 / s; m is the fracturing fluid viscosity, Pa·s; C L is the equivalent filtration coefficient, m / s 0.5 ; h is the crack height, m; t is the cumulative fracturing time, s.

[0041] Among them, the plane strain modulus According to formula (2), we can get:

[0042] (2)

[0043] Where: E is Young's modulus, MPa; n is Poisson's ratio, dimensionless.

[0044] Among them, the displacement of single cluster fracturing fluid q According to formula (3), we can get:

[0045] (3)

[0046] Where: q 0 is the displacement of fracturing fluid, m 3 / s; N is the number of perforation clusters, dimensionless.

[0047] Step S22: Calculate the hydraulic fracture width:

[0048] (4)

[0049] Where: w is the crack width, m; q is the displacement of single cluster fracturing fluid, m 3 / s; E’ is the plane strain modulus, MPa; m is the fracturing fluid viscosity, Pa·s; C L is the equivalent filtration coefficient, m / s 0.5 ; h is the crack height, m; t is the cumulative fracturing time, s.

[0050] Step S23: Calculate the length of the hydraulic fracture:

[0051] (5)

[0052] Where: l is the crack length, m; q is the displacement of single cluster fracturing fluid, m 3 / s; C L is the equivalent filtration coefficient, m / s 0.5 ; h is the crack height, m; t is the cumulative fracturing time, s.

[0053] Step S24: Calculate the stress intensity factor and maximum principal stress at the upper and lower tips of the hydraulic fracture:

[0054] (6)

[0055] Where: K Iu 、 K Il are the stress intensity factors at the upper and lower crack tips, MPa·m 1 / 2 ; h is the crack height, m; p f is the fluid pressure in the crack, MPa; s h,nis the horizontal minimum principal stress at the crack tip, MPa; s h , i For the i Minimum horizontal stress at the layer, MPa; s h , i+1 For the i Minimum horizontal stress at the +1 layer, MPa; r f is the density of fracturing fluid, kg / m 3 ; h cp is the perforation height, m; h i From the bottom tip of the crack to the i distance from the top of the stratum, m; g is the acceleration due to gravity.

[0056] For the upper crack tip, K I = K Iu ; For the lower crack tip, let K I = K Il ; then:

[0057] (7)

[0058] Where: s zz and s yy are the normal stresses in the vertical and horizontal directions at the hydraulic fracture tip, MPa; t zy is the shear stress in the hydraulic fracture tip region, MPa; s v and s h are the vertical stress and minimum horizontal principal stress at the hydraulic fracture tip, MPa; r 、 i is the coordinate of the polar coordinate system with the hydraulic fracture tip as the origin; K I is the stress intensity factor at the upper and lower crack tips.

[0059] The maximum principal stress can then be calculated as follows:

[0060] (8)

[0061] Where: szz and s yy are the normal stresses in the vertical and horizontal directions at the hydraulic fracture tip, MPa; t zy is the shear stress in the hydraulic fracture tip region, MPa; s v and s h are the vertical stress and minimum horizontal principal stress at the hydraulic fracture tip, MPa; s 1 is the maximum principal stress, MPa.

[0062] Step S25: Calculate the crack encounter coefficient at the current moment according to the bedding density to determine whether the crack encounters bedding at the current moment:

[0063] (9)

[0064] Where: α n is the crack encounter coefficient at the current moment, dimensionless; α n-1 is the crack encounter coefficient at the previous moment, dimensionless; Δ h is the crack height growth step, m; C k is the bedding density, 1 / m.

[0065] If the current crack encounter coefficient is less than 1, it is determined that the crack tip has not encountered bedding. K I Reservoir fracture toughness K IC-1 For comparison, if K I Greater than or equal to K IC-1 , then update the crack height h n = h n-1 +Δ h ,like K I Less than K IC-1 , then the crack height h constant.

[0066] If the current crack encounter coefficient is ≥1, the crack tip is judged to have encountered bedding. K I Bedding fracture toughness K IC-2 For comparison, the maximum principal stress at the crack tip s1 and bedding tensile strength T o For comparison, if the crack stress intensity factor K I Greater than or equal to bedding fracture toughness K IC-2 , while the maximum principal stress at the crack tip s 1 Greater than or equal to the bedding tensile strength T o , then update the crack height h n = h n-1 +Δ h Otherwise, the crack height h After that, set the crack encounter coefficient again α =0, the number of bedding layers encountered M n = M n-1 +1.

[0067] in h n is the crack height at the current moment, h n-1 is the crack height at the previous moment; M n is the number of beddings encountered currently; M n-1 is the number of layers encountered at the previous moment;

[0068] Step S26: If the crack tip encounters bedding, calculate the cumulative bedding loss volume, and calculate and update the equivalent loss coefficient based on the cumulative bedding loss volume:

[0069] (10)

[0070] Where: V leak is the cumulative bedding loss volume, m 3 ; M is the total number of beddings encountered, dimensionless; m is the number of beddings encountered, dimensionless; k r is the bedding permeability, m 2 ; t 、 t 0 is the cumulative fracturing time and bedding loss time, s; w r is the bedding thickness, m; l is the crack length, m.

[0071] (11)

[0072] Where: C L is the equivalent filtration coefficient, m / s 0.5 ; V leak is the cumulative bedding loss volume, m 3 ; t is the cumulative fracturing time, s; h is the crack height, m; l is the crack length, m.

[0073] Step S27: Based on the total hydraulic fracturing time T a and cumulative fracturing time t The relationship between the two can be used to determine whether the fracturing construction is completed.

[0074] if t <T a , it means that the fracturing operation has not been completed, and the cumulative fracturing time is updated to t +Δ t , where Δ t is the time step, and then steps S21 to S27 are repeated;

[0075] if t ≥ T a , it means the fracturing operation is completed, the calculation is completed and the final crack height is obtained h .

[0076] Step S30: The crack height h Control height with expectation h c After comparison, if the fracture height is greater than the expected control height, the fracturing fluid flow rate is reduced by 5% or the total fracturing time is reduced by 10%, and steps S20-S30 are repeated until the estimated fracture height is less than the expected control height.

[0077] Step S40: hydraulically fracture the target horizontal well based on the updated construction parameters.

[0078] Example

[0079] The unconventional low-permeability horizontal well W2 was hydraulically fractured using the high-control design method for multi-layer hydraulic fracturing in unconventional oil and gas reservoirs under pressure interference described in the present invention. The engineering and geological data of the W2 well were collected, as shown in Table 1.

[0080] Table 1 Engineering and geological data of the first fracturing stage of the unconventional low permeability horizontal well W2

[0081]

[0082] Initialization settings: cumulative fracturing time t = 0s, fracture encounter coefficient α = 0, cumulative bedding loss volume V leak =0m 3 / s, the bedding number has been encountered M =0. Set the simulation time step to Δ t =2s, Δ h =0.1m, expected control height h c =30m.

[0083] Calculate according to steps S20-S30, and the final simulation results are shown in Figure 2 Final crack height h =19.6m, lower than the expected control height h c Therefore, the design parameters of the hydraulic fracturing rate and total hydraulic fracturing time shown in Table 1 can effectively control the fracture height. Ultimately, the hydraulic fracturing operation of the target horizontal well was completed according to the design parameters in Table 1.

[0084] The above description does not limit the present invention in any form. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any technician familiar with the profession can use the technical content disclosed above to make some changes or modifications to equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are within the scope of the technical solution of the present invention.

Claims

1. A method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs, characterized in that: The following steps are involved: Step S10: Obtain geological parameters of the target horizontal well, select fracturing fluid according to the production capacity target of the target horizontal well, and preliminarily set construction parameters; Step S20: Calculate the hydraulic fracturing process of the target horizontal well based on the hydraulic fracturing model considering multi-layer physical interference, and evaluate the fracture height; Step S21, calculating the fluid pressure in the hydraulic fracture according to geological parameters, accumulated fracturing time and set construction parameters; Step S22, calculating the hydraulic fracture width; Step S23, calculating the length of the hydraulic fracture; Step S24, calculating the stress intensity factors at the upper and lower tips of the hydraulic fracture and the stress in the hydraulic fracture tip area; Step S25, calculating the crack encounter coefficient according to the bedding density, and determining whether the crack encounters bedding at the current moment; If the fracture tip does not encounter bedding, the stress intensity factor at the fracture tip is compared with the fracture toughness of the reservoir. If the critical condition for expansion is reached, the fracture height is updated. If the critical condition for expansion is not reached, the crack height remains unchanged; If the crack tip encounters bedding, the stress intensity factor at the crack tip is compared with the fracture toughness of the bedding, and the maximum principal stress at the crack tip is compared with the tensile strength of the bedding; If the critical condition of layer penetration is reached, the crack height is updated; if the critical condition of layer penetration is not reached, the crack height remains unchanged; Step S26: If bedding is encountered, calculate the current accumulated bedding loss volume, and calculate and update the equivalent loss coefficient based on the accumulated bedding loss volume; Step S27: Based on the total hydraulic fracturing time T a and cumulative fracturing time t The relationship between the completion of the fracturing construction is judged as follows: if t <T a , it means that the fracturing operation has not been completed, and the cumulative fracturing time is updated. t for t +Δ t , then repeat the calculation of S21~S27; if t ≥ T a , it means that the fracturing operation is completed, the calculation is completed and the final crack height is obtained; Step S30: Compare the fracture height with the expected control height. If the fracture height is greater than the expected control height, reduce the fracturing fluid flow rate or the total hydraulic fracturing time. Repeat steps S20-S30 until the estimated fracture height is less than the expected control height, and then proceed to the next step. Step S40: hydraulically fracture the target horizontal well based on the updated construction parameters.

2. The method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs according to claim 1, characterized in that: The geological parameters include Young's modulus of reservoir rock E , Poisson's ratio of reservoir rock ν , reservoir fracture toughness K IC-1 , minimum horizontal principal stress σ h , vertical stress σ v , bedding tensile strength T o , bedding fracture toughness K IC-2 , bedding thickness w r , bedding permeability k r , bedding density C k , equivalent filtration coefficient C L .

3. The method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs according to claim 1, characterized in that: The construction parameters include the number of perforation clusters N , perforation height h cp , fracturing fluid viscosity μ , fracturing fluid density ρ f , fracturing fluid discharge q 0 and total hydraulic fracturing time T a .

4. The method for designing high-pressure fractures under multi-layer interference in unconventional oil and gas reservoirs according to claim 1, characterized in that: In step S20, the fluid pressure in the fracture is calculated based on the hydraulic fracturing model considering multi-layer interference. p f , crack width w , crack length l , crack height h .

5. The method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs according to claim 1, characterized in that: The critical condition for expansion in step S25 is: the fracture stress intensity factor is greater than or equal to the reservoir fracture toughness.

6. The method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs according to claim 1, characterized in that: The critical conditions for penetration in step S25 are: the stress intensity factor of the crack is greater than or equal to the bedding fracture toughness, and the maximum principal stress at the crack tip is greater than or equal to the bedding tensile strength.

7. The method for designing high-control fractures under multi-layer interference in unconventional oil and gas reservoirs according to claim 1, characterized in that: In step S25, the fracture stress intensity factor is selected as the fracture upper tip stress intensity factor or the fracture lower tip stress intensity factor according to the position of the fracture tip in contact with the bedding.

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