Shale oil horizontal well spacing lower limit optimization method based on induced stress

Abstract

The invention provides a shale oil horizontal well spacing lower limit optimization method based on induced stress. The shale oil horizontal well spacing lower limit optimization method based on induced stress comprises the steps that 1, a three-dimensional geomechanical model containing double wells is established; 2, calculating to obtain an inter-well interference stress field; 3, checking the lower limit of the seam width under inter-well stress interference so as to guarantee smooth sand adding; 4, extracting stress characteristic parameters between the fractures, and calculating to obtain an induced stress difference; 5, in-situ geological parameters and construction parameters of the model are kept unchanged, only the well spacing is changed, and the steps 1-4 are repeated; and 6, forming a chart curve by taking the induced stress difference and the distance between the fractures under different well spacing as variables. According to the shale oil horizontal well spacing lower limit optimization method based on the induced stress, the shale oil horizontal well spacing optimization lower limit can be measured, a thought is provided for reasonable design and optimization of the well pattern and the well spacing, and the fracturing transformation effect is improved.

Description

Technical Field

[0001] The invention relates to the technical field of continental shale horizontal well fracturing, and in particular to a method for optimizing the lower limit of well spacing of shale oil horizontal wells based on induced stress. Background Art

[0002] As an important energy supplement, unconventional oil and gas resources such as tight oil and gas are of great significance to ensuring the balance of supply and demand in my country's energy structure. The widespread promotion and application of horizontal well segmented multi-cluster hydraulic fracturing technology is one of the main technical means to achieve efficient development of low-permeability oil and gas resources such as tight oil and gas, shale oil and gas. The core of horizontal well segmented multi-cluster fracturing technology is to form a complex fracture grid system with a large swept volume in low-permeability reservoirs, so that the oil and gas in the bottom layer can flow rapidly into the wellbore. However, there is not much research on the interference effect of multiple superimposed induced stresses on multi-well coordinated fracturing.

[0003] In terms of fracturing design, the main goal of the three-dimensional fracturing technology of horizontal well groups is to increase the volume of reservoir transformation and achieve three-dimensional balanced transformation. The horizontal well spacing and its stress interference intensity directly determine the expansion mode of inter-well fractures. The fracturing induced stress field of the horizontal well group is affected by factors such as the fracture layout method, fracture length, fracture width, fracturing sequence and well spacing, but the degree of influence varies. The induced stress generated in the middle of the two wells is the largest, and the fracture complexity is high. Reasonably arrange the distance between two adjacent wells, and use the induced stress difference and the distance between fractures under different well spacing as variables. When the induced stress difference is greater than the original stress difference, the fracture turns, thereby increasing the complexity of the fracture, the transformation volume and the effect, so as to determine the optimal well spacing. Research on the optimization method of horizontal well spacing for shale oil based on induced stress is of great significance to the efficient development technology system of shale oil.

[0004] CN112084613A discloses a method and device for optimizing the layout of horizontal wells. The method includes selecting a relatively homogeneous reservoir from the target reservoir; establishing a reservoir geomechanical model of six horizontal wells on the same horizontal well platform; using four of the six horizontal wells as fracturing construction wells and the other two as observation wells, and fracturing the fracturing construction wells according to the same pumping procedure to obtain fracturing data; judging whether the fracturing shock is caused by the expansion of the hydraulic fracture to the observation well according to the fracturing data; judging by recording the amount of fracturing fluid and sand when the fracturing shock occurs in the fracturing construction well; and simulating the hydraulic fracture morphology under the expansion of multiple clusters of fractures according to the established geomechanical model, optimizing the optimal construction scale and the corresponding hydraulic fracture half-fracture length, and obtaining the optimal horizontal well spacing. Since the method introduced in the patent requires observation and numerical simulation of the fracturing data of multiple wells, the process of determining the optimal well spacing is too cumbersome, so it is not very practical.

[0005] CN109441422A discloses a method for optimizing the spacing between shale gas wells. The method first determines the block area based on the geological structure obtained from the survey; analyzes the connection of the reservoir to determine the minimum development area unit; calculates the available reserves in the unit to determine the candidate target; cores representative shale rocks and analyzes their physical properties; uses a multi-field, multi-flow state, and multi-scale unified mathematical model to calculate the seepage field and pressure field; calculates the effective utilization area of ​​each well; and in the process of well layout, calculates the well layout based on the principle that the well interference rate is not greater than 10%. This method can reasonably formulate the well spacing for shale gas development, but it cannot determine the optimal well spacing, and the fracturing effect is not as good as the method described in this patent.

[0006] CN104278980A discloses a method for optimizing the parameters of tight oil horizontal well fracture network by transformation volume, which belongs to the field of tight oil horizontal well reservoir. This method is aimed at tight reservoirs. First, the geological characteristics of the reservoir are evaluated, and the specific reservoir conditions for volume fracturing to form a volume fracture network system are clarified. Then, the effects of different transformation volumes and fracture layout methods of horizontal wells on production capacity are studied to optimize the well spacing. However, this patent ignores the evolution of the stress field in the actual fracturing process and the requirements of sand addition on the fracture width. This method is not comprehensive enough and cannot avoid irrelevant parameter interference.

[0007] At present, most horizontal well fracturing well spacing optimization methods start from the reservoir transformation volume, and determine the optimal well spacing by calculating the maximum reservoir transformation volume under different well spacing conditions, while ignoring the stress field evolution in the actual fracturing process and the requirements of sand addition on the fracture width. Well spacing optimization involves multiple complex fracturing effect evaluation criteria, which must not only meet the maximum scale of reservoir crushing and provide seepage channels for oil and gas, but also consider the requirements of proppant migration on the width of hydraulic fractures. In other words, well spacing optimization must not only comprehensively consider the spatial geometry of hydraulic fractures and reservoir transformation volume, but also consider factors such as the influence of induced stress difference on fracture width and tortuosity. Furthermore, the complexity of tectonic stress action in continental shale oil reservoirs, the heterogeneity of reservoir physical properties and the uncertainty of construction conditions make the interweaving of hydraulic fractures and natural fractures / stratification in actual reservoirs intricate, and the well spacing optimization method and theory based on the uniform matrix model cannot be fully and effectively applied to actual engineering design.

[0008] The above existing technologies are significantly different from the present invention and fail to solve the technical problem we want to solve. For this reason, we have invented a new method for optimizing the lower limit of the spacing of shale oil horizontal wells based on induced stress. Summary of the invention

[0009] The purpose of the present invention is to provide a method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress, which can measure the optimized lower limit of shale oil horizontal well spacing and provide ideas for reasonable design and optimization of well network spacing.

[0010] The purpose of the present invention can be achieved by the following technical measures: a method for optimizing the lower limit of the well spacing of shale oil horizontal wells based on induced stress, the method for optimizing the lower limit of the well spacing of shale oil horizontal wells based on induced stress comprises:

[0011] Step 1, establishing a three-dimensional geomechanical model containing two wells;

[0012] Step 2, calculating and obtaining the inter-well interference stress field;

[0013] Step 3, checking the lower limit of the seam width under the stress interference between wells to ensure smooth sand addition;

[0014] Step 4, extracting the characteristic parameters of the inter-slit stress and calculating the induced stress difference;

[0015] Step 5, keep the in-situ geological parameters and construction parameters of the model unchanged, only change the well spacing, and repeat the above steps 1-4;

[0016] Step 6, taking the induced stress difference and the distance between fractures at different well spacings as variables, a chart curve is formed.

[0017] The purpose of the present invention can also be achieved by the following technical measures:

[0018] In step 1, a three-dimensional geomechanical model containing two wells is established using numerical simulation software based on the underlying geological parameters such as reservoir mechanical parameters, porosity parameters, and in-situ geostress.

[0019] In step 1, the formation geological parameters referenced when establishing the model, including reservoir mechanical parameters, porosity and permeability parameters, and in-situ geostress, are established according to the actual engineering formation structure.

[0020] In step 1, the established model is shown in formulas (1)-(7):

[0021] E=f 1 (C 1 ) (1)

[0022] ν=f 2 (C 2 ) (2)

[0023] σ c =f 3 (C 3 ) (3)

[0024]

[0025] σ t =f 5 (C 5 ) (5)

[0026] φ=f 6 (C6 ) (6)

[0027] κ=f 7 (C 7 ) (7)

[0028] Where: E is the elastic modulus of shale, GPa; ν is the Poisson's ratio of shale; σ c is the uniaxial compressive strength of shale, MPa; is the internal friction angle of shale, °; σ t is the tensile strength of shale, MPa; φ is the porosity of shale; κ is the permeability of shale, mD; f i (i=1,…,7) are different logging interpretation functions; C i (i=1, ..., 7) are different types of logging data.

[0029] In step 2, combined with relevant construction parameters, including fracture length, fracture spacing, and well spacing, numerical simulation of zipper-type synchronous fracturing of horizontal wells is carried out to calculate the interference stress field between wells.

[0030] In step 3, the main fracture width that meets the sand addition condition is taken as the lower limit to determine the optimized well spacing value range; the maximum fracture width calculation formula is:

[0031]

[0032] Where: w 0 is the maximum width of the fracture, mm; E is the elastic modulus of shale, MPa; v is Poisson's ratio; Δp(x) is the net pressure in the fracture, MPa. In step 3, the net pressure in the fracture is calculated as:

[0033] Δp(x)=p(x)-σ h

[0034] Where: Δp(x) is the net pressure in the fracture, MPa; p(x) is the bottom hole pressure, MPa; σ h is the minimum horizontal principal stress, MPa.

[0035] In step 4, the extracted inter-fracture stress characteristic parameter is the induced stress difference at the middle position of the two wells.

[0036] In step 6, the induced stress difference and the distance between fractures under different well spacings are used as variables to form a chart curve, and the threshold value is determined by taking the induced stress difference greater than or equal to the original horizontal stress difference as the lower limit of the well spacing to determine the optimal well spacing.

[0037] The method for optimizing the lower limit of the spacing between horizontal wells of shale oil based on induced stress also includes, after step 6, step 7, verifying the reservoir permeability of the stress interference zone between wells.

[0038] In step 7, the permeability of the stress extraction position is checked according to the following formula, where the stress σ′ ii and p are calculated, k 0 is the known initial reservoir permeability:

[0039]

[0040] Where: k is the permeability at the stress extraction location, md; k 0 is the initial reservoir permeability, md; σ′ ii is the induced stress at the middle position, MPa; p is the tensile strength, MPa.

[0041] The purpose of the present invention can also be achieved through the following technical measures: a shale oil horizontal well spacing lower limit optimization system based on induced stress, which adopts a shale oil horizontal well spacing lower limit optimization method based on induced stress to determine the optimal well spacing of shale oil horizontal wells.

[0042] The method for optimizing the lower limit of the spacing of shale oil horizontal wells based on induced stress in the present invention belongs to the technical field of continental shale horizontal well fracturing. According to the formation characteristics such as reservoir mechanical parameters, porosity and permeability parameters, and in-situ geostress conditions, the present invention establishes an in-situ scale full three-dimensional geomechanical model containing double wells; carries out numerical simulation of zipper-type synchronous fracturing of horizontal wells, calculates the interference stress field between wells; verifies the lower limit of the fracture width under the interference of well stress; extracts the characteristic parameters of the fracture stress, and calculates the induced stress difference; takes the induced stress difference and the fracture distance under different well spacing as variables, forms a plate curve, and determines the optimal well spacing; verifies the permeability of the "stress extraction position" according to the formula. The method described in the present invention can measure the optimized lower limit of the spacing of shale oil horizontal wells, and provide ideas for the reasonable design and optimization of well network spacing.

[0043] The present invention comprehensively considers the formation characteristics, characterizes and models the actual continental shale reservoir, and effectively overcomes the shortcomings of conventional cluster spacing optimization methods that fail to comprehensively consider construction factors and formation factors. The optimal well spacing is determined based on the interference effect of multiple superimposed induced stresses on multi-well collaborative fracturing, and the numerical calculation results are more rigorous. The physical meaning of each energy parameter is clear and easy to obtain. It comprehensively considers the reservoir mechanical parameters, porosity and permeability parameters, in-situ geostress and natural structural surface distribution characteristics, net pressure in the fracture, reservoir transformation volume and fracture width, which is helpful for field application and promotion. The research results are of great significance to the efficient development technology system of shale oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a flow chart of a specific embodiment of the method for optimizing the lower limit of the well spacing of shale oil horizontal wells based on induced stress of the present invention;

[0045] Figure 2The natural fractures and bedding diagram in the continental shale described in Example 2 of the present invention;

[0046] Figure 3 The diagram of the extraction position of the inter-fracture stress characteristic parameters and the change of the well spacing described in Example 2 of the present invention;

[0047] Figure 4 The graph is formed by taking the induced stress difference and the distance between fractures under different well spacings as variables as described in Example 2 of the present invention;

[0048] Figure 5 This is a curve chart of the reservoir permeability in the inter-well stress interference zone verified in Example 2 of the present invention. DETAILED DESCRIPTION

[0049] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0050] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations and / or combinations thereof.

[0051] like Figure 1 As shown, Figure 1 The flowchart of the method for optimizing the lower limit of the well spacing of shale oil horizontal wells based on induced stress of the present invention. The method for optimizing the lower limit of the well spacing of shale oil horizontal wells based on induced stress includes:

[0052] Step 1. Use numerical simulation software to establish a three-dimensional geomechanical model containing two wells based on reservoir mechanical parameters, porosity and permeability parameters, in-situ geostress and other underlying geological parameters;

[0053] Step 2. Combined with relevant construction parameters, such as fracture length, fracture spacing, well spacing, etc., numerical simulation of zipper-type synchronous fracturing of horizontal wells is carried out to calculate the interference stress field between wells;

[0054] Step 3. Check the lower limit of the seam width under the stress interference between wells to ensure smooth sand addition;

[0055] Step 4. Extract the characteristic parameters of the inter-slit stress and calculate the induced stress difference;

[0056] Step 5. Keep the in-situ geological parameters and construction parameters of the model unchanged, only change the well spacing, and repeat the above steps 1-4;

[0057] Step 6. Using the induced stress difference and the distance between fractures at different well spacings as variables, a chart curve is formed.

[0058] Step 7. Check the reservoir permeability in the inter-well stress interference zone.

[0059] As a preferred embodiment of the present invention, the method is characterized in that the optimized well spacing value range is determined by taking the main fracture width that meets the sand addition condition as the lower limit. The maximum fracture width calculation formula is:

[0060]

[0061] Where: w 0 is the maximum width of the fracture, mm; E is the elastic modulus of shale, MPa; v is Poisson's ratio; Δp(x) is the net pressure in the fracture, MPa.

[0062] The calculation formula of the net pressure in the crack is:

[0063] Δp(x)=p(x)-σ h

[0064] Where: Δp(x) is the net pressure in the fracture, MPa; p(x) is the bottom hole pressure, MPa; σ h is the minimum horizontal principal stress, MPa.

[0065] As a preferred embodiment of the present invention, the formation geological parameters referred to when establishing the model in step 1 include reservoir mechanical parameters, porosity parameters, in-situ geostress, etc., which are established according to the actual engineering formation structure; the inter-fracture stress characteristic parameters extracted in step 4 are the induced stress difference at the middle position of the two wells.

[0066] As a preferred embodiment of the present invention, the on-site fracturing construction parameters include crack length, crack spacing, well spacing, etc.

[0067] As a preferred embodiment of the present invention, the formation geological condition parameters include reservoir mechanical parameters, porosity and permeability parameters, in-situ geostress, and the like.

[0068] As a preferred embodiment of the present invention, the characteristic parameter of inter-fracture stress extracted in step 4 is the induced stress difference at the middle position of the two wells.

[0069] As a preferred embodiment of the present invention, in step 6, the optimal well spacing is determined, and the threshold for determining the lower limit of the well spacing is: the induced stress difference is greater than or equal to the original horizontal stress difference.

[0070] As a preferred embodiment of the present invention, in step 7, the reservoir permeability of the inter-well stress interference zone is checked. The permeability of the "stress extraction position" is checked according to the following formula, where the stress σ′ ii and p are calculated, k0 is the known initial reservoir permeability.

[0071]

[0072] Where: k is the permeability at the stress extraction location, md; k 0 is the initial reservoir permeability, md; σ′ ii is the induced stress at the middle position, MPa; p is the tensile strength, MPa.

[0073] The numerical model is shown in formulas (1)-(7):

[0074] E=f 1 (C 1 ) (1)

[0075] ν=f 2 (C 2 ) (2)

[0076] σ c =f 3 (C 3 ) (3)

[0077]

[0078] σ t =f 5 (C 5 ) (5)

[0079] φ=f 6 (C 6 ) (6)

[0080] κ=f 7 (C 7 ) (7)

[0081] Where: E is the elastic modulus of shale, GPa; ν is the Poisson's ratio of shale; σ c is the uniaxial compressive strength of shale, MPa; is the internal friction angle of shale, °. σ t is the tensile strength of shale, MPa; φ is the porosity of shale; κ is the permeability of shale, mD; f i (i=1,…,7) are different logging interpretation functions respectively; C i (i=1, ..., 7) are different types of logging data respectively;

[0082] The model stress boundary and seepage boundary are assigned according to the on-site fracturing construction parameters and ground stress conditions.

[0083] As a preferred embodiment of the present invention, in step 7, the reservoir permeability of the inter-well stress interference zone is checked. The permeability of the "stress extraction position" is calculated according to the following formula, where the stress σ′ ii and p are calculated, k 0 is the known initial reservoir permeability.

[0084]

[0085] Where: k is the permeability at the stress extraction location, md; k 0 is the initial reservoir permeability, md; σ′ ii is the induced stress at the middle position, MPa; p is the tensile strength, MPa.

[0086] The present invention uses numerical simulation software to establish a three-dimensional geomechanical model of dual wells; conducts numerical simulation of zipper-type synchronous fracturing of horizontal wells, calculates the interference stress field between wells; verifies the lower limit of the fracture width under the stress interference between wells; raises the characteristic parameters of the stress between fractures, and calculates the induced stress difference; keeps the in-situ geological parameters and construction parameters of the model unchanged, only changes the well spacing, and repeats the above steps; takes the induced stress difference and the distance between fractures under different well spacings as variables, forms a plate curve, and determines the well spacing; calculates the permeability of the "stress extraction position" according to the formula to verify the optimal well spacing. The method described in the present invention can measure the optimized lower limit of the well spacing of shale oil horizontal wells, provide ideas for the reasonable design and optimization of the well spacing of the well network, and improve the effect of fracturing transformation.

[0087] The following are several specific embodiments of the present invention.

[0088] Example 1

[0089] In a specific embodiment 1 of the present invention, the method for optimizing the lower limit of the spacing of shale oil horizontal wells based on induced stress includes the following steps:

[0090] Step 1. According to reservoir mechanical parameters, porosity parameters, in-situ geostress and natural structural surface distribution characteristics, a three-dimensional geomechanical model containing two wells is established using numerical simulation software; the numerical model is shown in formulas (1)-(7):

[0091] E=f 1 (C 1 ) (1)

[0092] ν=f 2 (C 2 ) (2)

[0093] σ c =f 3 (C 3 ) (3)

[0094]

[0095] σ t =f 5 (C 5 ) (5)

[0096] φ=f 6 (C 6 ) (6)

[0097] κ=f 7 (C 7 ) (7)

[0098] Where: E is the elastic modulus of shale, GPa; ν is the Poisson's ratio of shale; σ c is the uniaxial compressive strength of shale, MPa; is the internal friction angle of shale, °. σ t is the tensile strength of shale, MPa; φ is the porosity of shale; κ is the permeability of shale, mD; f i (i=1,…,7) are different logging interpretation functions; C i (i=1, ..., 7) are different types of logging data respectively;

[0099] The stratigraphic characteristics are the distribution structure of actual continental shale formations obtained from logging interpretation, and the numerical model includes reservoirs and interlayers; the distribution characteristics of natural structural planes include the strike, dip and inclination of bedding and natural fractures; the stratigraphic characteristics in the numerical model are established according to the actual stratigraphic structure, and the distribution of natural structural planes is randomly generated according to the statistical characteristic parameters that satisfy the bedding and natural fractures.

[0100] The field fracturing construction parameters include fracture length, fracture spacing, well spacing, etc. The geostress conditions include the vertical stress of the reservoir, the maximum and minimum horizontal principal stresses, and the initial pore pressure.

[0101] Step 2. Based on the three-dimensional geomechanical model, combined with actual construction parameters such as fracture length, fracture spacing, and well spacing, conduct a simulation of fracture expansion in horizontal wells by synchronous fracturing, and calculate the inter-well interference gravity field;

[0102] Step 3. Check the lower limit of the seam width under stress interference between wells to ensure smooth sand addition. Calculated based on 6 times the diameter of 40-mesh quartz sand / ceramsite, 2.55mm, only proceed to the next step if the calculated seam width is greater than 2.55mm, otherwise adjust the well spacing.

[0103] The maximum crack width calculation formula is:

[0104] as follows

[0105]

[0106] Where: w0 is the maximum width of the fracture, mm; E is the elastic modulus of shale, MPa; v is Poisson's ratio; Δp(x) is the net pressure in the fracture, MPa.

[0107] The calculation formula of the net pressure in the crack is:

[0108] Δp(x)=p(x)-σ h

[0109] Where: Δp(x) is the net pressure in the fracture, MPa; p(x) is the bottom hole pressure, MPa; σ h is the minimum horizontal principal stress, MPa.

[0110] Step 4. Extract the characteristic parameters of the inter-fracture stress at the middle position of the two wells and calculate the induced stress difference;

[0111] Step 5. Keep the in-situ geological parameters and construction parameters of the model unchanged, only change the well spacing, and repeat the above steps 1-4;

[0112] Step 6. Take the induced stress difference and the distance between fractures under different well spacings as variables to form a chart curve, take the induced stress difference greater than or equal to the original horizontal stress difference as the lower limit of the well spacing to determine the threshold and determine the optimal well spacing.

[0113] Step 7. Check the reservoir permeability in the inter-well stress interference zone. Check the permeability of the "stress extraction position" according to the following formula. The stress σ′ in the formula ii and p are calculated, and k0 is the known initial reservoir permeability.

[0114]

[0115] Where: k is the permeability at the stress extraction location, md; k 0 is the initial reservoir permeability, md; σ′ ii is the induced stress at the middle position, MPa; p is the tensile strength, MPa.

[0116] Example 2

[0117] In order to further improve the technical quality of Shengli shale oil development and ensure the stability of shale oil production capacity, it is urgent to carry out relevant research on three-dimensional multi-well coordinated fracturing. At present, most horizontal well fracturing well spacing optimization methods start from the reservoir transformation volume, and determine the optimal well spacing by calculating the maximum reservoir transformation volume under different well spacing conditions, while ignoring the requirements of sand addition on fracture width and the influence of induced stress difference during actual fracturing. Well spacing optimization involves multiple complex fracturing effect evaluation criteria, which must not only meet the maximum scale of crushing the reservoir and provide a seepage channel for oil and gas, but also consider the requirements of proppant migration on the width of hydraulic fractures. The fracturing induced stress field of the horizontal well group is affected by factors such as fracture layout, fracture length, fracture width, fracturing sequence and well spacing. The induced stress generated in the middle of the two wells is the largest, and the fracture complexity is high. Reasonably arrange the distance between two adjacent wells, taking the induced stress difference and the distance between fractures under different well spacing as variables. When the induced stress difference is greater than the original stress difference, the fracture turns, thereby improving the complexity, transformation volume and effect of the fracture.

[0118] Taking the continental shale reservoir in Block 1 of Shengli Oilfield as an example, the method for optimizing the lower limit of horizontal well spacing of shale oil based on induced stress described in Example 1 is adopted, which specifically includes the following steps:

[0119] Step 1: According to the stratigraphic characteristics shown by the well logging interpretation of the continental shale reservoir, the continental shale reservoir thickness is 30m, the thickness of the upper and lower interlayers is 30m, the model length is set to 1100m, and the width is set to 180m. Figure 2 The horizontal distribution of bedding and two groups of natural fractures with an angle of 75° to the minimum horizontal principal stress are randomly distributed in the reservoir. The mechanical parameters obtained from the indoor test of continental shale are input into the numerical model to establish a three-dimensional hydraulic fracturing geomechanical finite element numerical model of shale reservoir, as shown in Figure 2. Figure 2 As shown in Figure 2. The model stress boundary and seepage boundary are assigned according to the on-site fracturing construction parameters and ground stress conditions. Ground stress conditions: σ v =91MPa,σ H =79MPa,σ h =72MPa, horizontal stress difference is 7MPa. Construction displacement 18m 3 / min, viscosity 0.015Pa·s, oil layer thickness 30m, total liquid volume 1000m 3 The designed segment length is 60m, the cluster spacing is 12m, and the crack expansion process of a single-stage fracturing is simulated.

[0120] Step 2: Use numerical simulation software to carry out numerical simulation of zipper-type synchronous fracturing in horizontal wells and calculate the interference stress field between wells; the fracture length is 300m, that is, the half-fracture length is 150m, the fracture spacing is 12m, and the initial value of the well spacing is 400m.

[0121] Step 3: Check the lower limit of the seam width under stress interference between wells to ensure smooth sand addition. The seam width is calculated as 6 times the diameter of 40 mesh quartz sand / ceramsite, 2.55mm. Only when the calculated seam width is greater than 2.55mm, proceed to the next step, otherwise adjust the well spacing.

[0122] Step 4: If Figure 3 As shown in Figure 1, the characteristic parameters of inter-fracture stress are extracted from the middle position of the two wells, and the induced stress difference is calculated.

[0123] Step 5: Keep the in-situ geological parameters and construction parameters of the model unchanged, change the well spacing to 500m, 450m, 350m, and 300m respectively, and repeat the above steps 1-4. Figure 3 As shown;

[0124] Step 6: Taking the induced stress difference and the distance between fractures under different well spacing as variables, a chart curve is formed, such as Figure 4 As shown in the figure, when the distance between fractures is 60m, the induced stress difference reaches the maximum value, and then gradually decreases. The threshold for determining the lower limit of the well spacing is: the induced stress difference is greater than or equal to the original horizontal stress difference, and the original horizontal stress difference is 7MPa. Therefore, it can be seen from the figure that the lower limit of the well spacing is reached when the well spacing is 400m.

[0125] Step 7: Check the reservoir permeability in the inter-well stress interference zone. Figure 5 The curve is drawn as shown. The permeability of the "stress extraction position" is checked according to the following formula. The stress σ′ in the formula ii and p are calculated, k 0 is the known initial reservoir permeability. Only k ≥ 10k 0 , it meets the requirements, otherwise further adjust the well spacing and repeat steps 1-6.

[0126]

[0127] Where: k is the permeability at the stress extraction location, md; k 0 is the initial reservoir permeability, md; σ′ ii is the induced stress at the middle position, MPa; p is the tensile strength, MPa.

[0128] Example 3

[0129] Taking the continental shale reservoir in Block 2 of Shengli Oilfield as an example, the method for optimizing the lower limit of horizontal well spacing of shale oil based on induced stress described in Example 1 is adopted, which specifically includes the following steps:

[0130] Step 1: According to the stratigraphic characteristics shown by the well logging interpretation of the continental shale reservoir, the continental shale reservoir thickness is 30m, the thickness of the upper and lower interlayers is 30m, the model length is set to 1100m, and the width is set to 180m. Figure 2 The horizontal distribution of bedding and two groups of natural fractures with an angle of 75° to the minimum horizontal principal stress are randomly distributed in the reservoir. The mechanical parameters obtained from the indoor test of continental shale are input into the numerical model to establish a three-dimensional hydraulic fracturing geomechanical finite element numerical model of shale reservoir, as shown in Figure 2. Figure 2 As shown in Figure 2. The model stress boundary and seepage boundary are assigned according to the on-site fracturing construction parameters and ground stress conditions. Ground stress conditions: σ v =91MPa,σ H =80MPa,σ h =70MPa, horizontal stress difference is 10MPa. Construction displacement 18m 3 / min, viscosity 0.015Pa·s, oil layer thickness 30m, total liquid volume 1000m 3 The designed segment length is 60m, the cluster spacing is 12m, and the crack expansion process of a single-stage fracturing is simulated.

[0131] Step 2: Use numerical simulation software to carry out numerical simulation of zipper-type synchronous fracturing in horizontal wells and calculate the interference stress field between wells; the fracture length is 200m, that is, the half-fracture length is 100m, the fracture spacing is 12m, and the initial value of the well spacing is 300m.

[0132] Step 3: Check the lower limit of the seam width under stress interference between wells to ensure smooth sand addition. The seam width is calculated as 6 times the diameter of 40 mesh quartz sand / ceramsite, 2.55mm. Only when the calculated seam width is greater than 2.55mm, proceed to the next step, otherwise adjust the well spacing.

[0133] Step 4: Extract the inter-fracture stress characteristic parameters from the middle position between the two wells and calculate the induced stress difference.

[0134] Step 5: Keep the in-situ geological parameters and construction parameters of the model unchanged, change the well spacing to 400m, 350m, 250m, and 200m respectively, and repeat the above steps 1-4;

[0135] Step 6: Taking the induced stress difference and the distance between fractures under different well spacing as variables, a chart curve is formed. When the distance between fractures is 55m, the induced stress difference reaches the maximum value, and then gradually decreases. The threshold for determining the lower limit of the well spacing is: when the induced stress difference is greater than or equal to the original horizontal stress difference of 10MPa, it can be known that the lower limit of the well spacing is reached when the well spacing is 250m.

[0136] Step 7: Check the reservoir permeability in the inter-well stress interference zone. Check the permeability of the "stress extraction position" according to the following formula. The stress σ′ in the formula ii and p are calculated, k 0 is the known initial reservoir permeability. Only k ≥ 10k 0 , it meets the requirements, otherwise further adjust the well spacing and repeat steps 1-6.

[0137]

[0138] Where: k is the permeability at the stress extraction location, md; k 0 is the initial reservoir permeability, md; σ′ ii is the induced stress at the middle position, MPa; p is the tensile strength, MPa.

[0139] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0140] Except for the technical features described in the specification, all other technical features are known technologies to those skilled in the art.

Claims

1. Optimization method for lower limit of horizontal well spacing of shale oil based on induced stress, It is characterized in that The induced stress-based shale oil horizontal well spacing lower limit optimization method includes: Step 1, establishing a three-dimensional geomechanical model containing two wells; Step 2, calculating and obtaining the inter-well interference stress field; Step 3, checking the lower limit of the seam width under the stress interference between wells to ensure smooth sand addition; Step 4, extracting the characteristic parameters of the inter-slit stress and calculating the induced stress difference; Step 5, keep the in-situ geological parameters and construction parameters of the model unchanged, only change the well spacing, and repeat the above steps 1-4; Step 6, taking the induced stress difference and the distance between fractures at different well spacings as variables, a chart curve is formed.

2. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 1, It is characterized in that In step 1, a three-dimensional geomechanical model containing two wells is established using numerical simulation software based on the underlying geological parameters such as reservoir mechanical parameters, porosity parameters, and in-situ geostress.

3. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 2, It is characterized in that In step 1, the formation geological parameters referenced when establishing the model, including reservoir mechanical parameters, porosity and permeability parameters, and in-situ geostress, are established according to the actual engineering formation structure.

4. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 3, It is characterized in that In step 1, the established model is shown in formulas (1)-(7): E=f 1 (C 1 ) (1) n=f 2 (C 2 ) (2) σ c =f 3 (C 3 ) (3) σ t =f 5 (C 5 ) (5) φ=f 6 (C 6 ) (6) k=f 7 (C 7 ) (7) Where: E is the elastic modulus of shale, GPa; ν is the Poisson's ratio of shale; σ c is the uniaxial compressive strength of shale, MPa; is the internal friction angle of shale, °; σ t is the tensile strength of shale, MPa; φ is the porosity of shale; κ is the permeability of shale, mD; f i (i=1,…,7) are different logging interpretation functions; C i (i=1,…,7) are different types of logging data.

5. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 1, It is characterized in that In step 2, combined with relevant construction parameters, including fracture length, fracture spacing, and well spacing, numerical simulation of zipper-type synchronous fracturing of horizontal wells is carried out to calculate the interference stress field between wells.

6. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 1, It is characterized in that In step 3, the main fracture width that meets the sand addition condition is taken as the lower limit to determine the optimized well spacing value range; the maximum fracture width calculation formula is: Where: w 0 is the maximum width of the fracture, mm; E is the elastic modulus of shale, MPa; v is Poisson's ratio; Δp(x) is the net pressure in the fracture, MPa.

7. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 6, It is characterized in that In step 3, the net pressure inside the fracture is calculated as: Δp(x)=p(x)-σ h Where: Δp(x) is the net pressure in the fracture, MPa; p(x) is the bottom hole pressure, MPa; σ h is the minimum horizontal principal stress, MPa.

8. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 1, It is characterized in that In step 4, the extracted inter-fracture stress characteristic parameter is the induced stress difference at the middle position of the two wells.

9. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 1, It is characterized in that In step 6, the induced stress difference and the distance between fractures under different well spacings are used as variables to form a chart curve, and the threshold value is determined by taking the induced stress difference greater than or equal to the original horizontal stress difference as the lower limit of the well spacing to determine the optimal well spacing.

10. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 1, It is characterized in that The method for optimizing the lower limit of the spacing between horizontal wells of shale oil based on induced stress also includes, after step 6, step 7, verifying the reservoir permeability of the stress interference zone between wells.

11. The method for optimizing the lower limit of shale oil horizontal well spacing based on induced stress according to claim 10, It is characterized in that In step 7, the permeability of the stress extraction position is checked according to the following formula, where the stress σ′ ii and p are calculated, k 0 is the known initial reservoir permeability: Where: k is the permeability at the stress extraction location, md; k 0 is the initial reservoir permeability, md; σ′ ii is the induced stress at the middle position, MPa; p is the tensile strength, MPa.

12. Shale oil horizontal well spacing lower limit optimization system based on induced stress, It is characterized in that The induced stress-based shale oil horizontal well spacing lower limit optimization system adopts the induced stress-based shale oil horizontal well spacing lower limit optimization method described in any one of claims 1-11 to determine the optimal well spacing of shale oil horizontal wells.

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