A shale oil through-layer volume fracturing design method and system

Abstract

The application discloses a shale oil through-layer volume fracturing design method and system, collects logging acoustic wave data of different lithological reservoirs, and obtains stress profile of a target section, rock mechanics characteristics and rock mechanics differences; weak structural plane characteristics of the target section are characterized through mechanical experiments and simulation tests; fracture breakthrough conditions among different lithological reservoirs are simulated respectively, and the fractures are optimized through displacement and viscosity of the different lithological reservoirs, so that the fractures are longitudinally expanded and transversely extended; according to the longitudinal expansion and transverse extension of the fractures, the fractures are propped with proppants, and the scheme is realized through the stress profile of the target section, the rock mechanics characteristics, the rock mechanics differences and the stress profile of the target section, the rock mechanics characteristics and the rock mechanics differences, longitudinal multi-lithology reconstruction, fracture optimization through displacement and viscosity of different lithological reservoirs, longitudinal expansion and transverse extension of the fractures, effective improvement of longitudinal sweep area of the fractures and three-dimensional dynamic use of the reservoir.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field exploration and development technology, specifically to a design method and system for cross-layer volumetric fracturing of shale oil. Background Technology

[0002] Currently, my country's economy is developing rapidly, and the demand for energy is increasing. However, our country's dependence on oil imports is gradually rising. There is an urgent need to accelerate the exploration and development of oil. A lot of work has been carried out on conventional oil reservoirs, and the development of conventional oil reservoirs has reached a high level. It is necessary to carry out work on unconventional oil reservoirs to achieve the goal of increasing oil reserves.

[0003] Unlike the marine shale oil reservoirs in North America, my country's oil reservoirs are mainly terrestrial shale oil reservoirs with more complex lithology. High-quality reservoirs include shale, sandstone, and limestone, making them more difficult to stimulate.

[0004] Current conventional fracturing design methods mainly target single lithological bodies, such as single limestone, dolomite, sandstone, shale, etc. All design ideas and methods are aimed at the same lithological body object, and there are very few fracturing designs that simultaneously target multiple lithologies. Summary of the Invention

[0005] The technical problem this invention aims to solve is that current conventional fracturing design methods mainly target single lithological bodies, such as single limestone, dolomite, sandstone, and shale. All design ideas and methods are aimed at the same lithological body, which cannot be applied to scenarios with more complex lithologies, such as continental shale oil reservoirs. The purpose of this invention is to provide a shale oil cross-layer volumetric fracturing design method and system. By utilizing the target section stress profile, rock mechanical characteristics, and rock mechanical differences, vertical multi-lithological transformation is achieved. The displacement and viscosity of different lithological reservoirs are optimized to improve fractures, enabling longitudinal and lateral fracture extension, effectively increasing the longitudinal sweep area of ​​fractures, and realizing three-dimensional reservoir utilization.

[0006] This invention is achieved through the following technical solution:

[0007] This solution provides a design method for cross-layer volumetric fracturing in shale oil, including the following steps:

[0008] Step 1: Collect well logging sonic data of reservoirs with different lithologies, and obtain stress profiles, rock mechanical characteristics and rock mechanical differences of the target section;

[0009] Step 2: Characterize the weak structural surface features of the target segment through mechanical experiments and simulation tests;

[0010] Step 3: Based on Step 1 and Step 2, simulate the breakdown of fractures between reservoirs of different lithologies. Optimize the fractures by adjusting the displacement and viscosity of different reservoirs to enable both longitudinal and lateral fracture propagation.

[0011] Step 4: Based on the longitudinal and lateral extension patterns of the cracks, apply a proppant to the cracks.

[0012] The working principle of this scheme is as follows: Current conventional fracturing design methods are mainly aimed at single lithological bodies, such as single limestone, dolomite, sandstone, shale, etc. All design ideas and methods are for the same lithological body object, which cannot be applied to more complex lithological scenarios such as continental shale oil reservoirs. The purpose of this invention is to provide a shale oil cross-layer volumetric fracturing design method and system. By using the target section stress profile, rock mechanical characteristics, and rock mechanical differences, vertical multi-lithological transformation is achieved. The displacement and viscosity of different lithological reservoirs are optimized to improve the fractures, enabling the fractures to extend both vertically and laterally, effectively increasing the vertical sweep area of ​​the fractures and realizing three-dimensional reservoir utilization.

[0013] After acquiring the logging sonic data, data correction is required to remove erroneous data and obtain the vertical stress, shear stress, cohesion, and internal friction angle of each lithological reservoir.

[0014] A further optimized scheme is that the reservoirs of different lithologies have at least two layers with different lithologies, including: limestone, dolomite, sandstone, and shale.

[0015] A further optimization scheme is that the rock mechanical characteristics are obtained through the stress profile of the target segment;

[0016] The method for obtaining rock mechanical differences includes: first, determining the Young's modulus and Poisson's ratio of different lithological reservoirs through rock mechanics experiments; then, calibrating the stress profile of the target section based on the Young's modulus and Poisson's ratio of different lithological reservoirs to obtain the rock mechanical differences. The stress profile of the target section is calculated using well logging sonic data to clarify the rock mechanical characteristics of upper and lower different lithological reservoirs. Rock mechanics experiments are used to determine the Young's modulus and Poisson's ratio of individual sandstone and shale reservoirs, thereby calibrating the stress profile and accurately obtaining the mechanical differences.

[0017] When characterizing the weak structural surface features of the target segment through mechanical experiments and simulations, the weak structural surface exhibits a compressive-shear effect under stress conditions. That is, its shear strength increases with the increase of the normal compressive stress on the weak surface. This effect affects the initial stress equilibrium and the change in shear strength during subsequent fracturing, directly influencing the formation of shear slip on the weak structural surface. The weak structural surface is characterized based on cohesive elements in ABAQUS. The conditions for the formation of shear cracks on the weak structural surface are:

[0018]

[0019] In the formula: τ—shear force on the weak surface, Pa;

[0020] p′ n —Effective normal compressive stress on the weak surface, Pa;

[0021] — Shale friction angle, °;

[0022] β—Weak surface shear dilatation angle, °, related to JRC (weak surface roughness), JCS (compressive strength), and p′. n related.

[0023] Based on the Barton model, the initial damage criterion for shale bedding plane compression and shear is expressed as follows:

[0024]

[0025] As shown in the above equation, the shear behavior of weak structural surfaces has a significant impact during hydraulic fracturing. First, fracturing affects the distribution of the geostress field, altering the effective normal stress of the weak structural surface and even reducing the effective compressive stress on the bedding plane. Shear strength decreases as the effective compressive stress decreases, which is conducive to the further formation of shear fractures due to dilatation of the weak structural surface. Second, the fracturing fluid may cause softening damage to the weak structural surface, further reducing the friction coefficient and leading to a decrease in shear strength, making it more prone to shear slip. After the weak structural surface enters the residual shear stage, its participation in the shear effect can be achieved by changing the maximum degradation value of the cohesive element damage factor, which can be expressed as:

[0026]

[0027] The further optimized scheme is as follows: the mechanical experiments and simulation tests in step two include: matrix rock mechanics experiment, matrix rock in-situ stress experiment, matrix rock cohesion experiment, rock mechanics of weak structural planes, cohesion of weak structural planes and tensile strength of weak structural planes.

[0028] A further optimized solution is that step three includes the following sub-steps:

[0029] S31. Select a fluid distribution consistent with the lithological reservoir of the target section;

[0030] S32. By changing the liquid displacement, observe the longitudinal and transverse extension of the crack to obtain the breakthrough displacement Q1 and peak displacement Q. t ;

[0031] S33. At the breakthrough displacement Q1, the breakthrough viscosity M is obtained by observing the longitudinal and transverse extension of the cracks by changing the viscosity of the liquid. c and peak viscosity M k .

[0032] The further optimized solution is that S32 includes the following sub-steps:

[0033] Gradually increase the fluid discharge rate and observe the longitudinal propagation of the fracture. When the fracture breaks through the interface between the upper and lower lithological reservoirs, record the breakthrough discharge rate Q1.

[0034] If the crack does not effectively propagate longitudinally, continue to increase the displacement and obtain the peak displacement Qt of the liquid according to the law of conservation of energy.

[0035] By gradually increasing the discharge rate, observing the extension of the crack, breaking through the lithological interface, and recording the breakthrough discharge rate Q1, if the crack cannot effectively extend longitudinally, continue to increase the discharge rate. Because of the law of conservation of energy, if the crack height is out of control, then the transverse extension of the crack will inevitably be limited, and the effect will be reduced, thus obtaining a peak discharge rate Qt.

[0036] The further optimized solution is that S33 includes the following sub-steps:

[0037] At the breakthrough displacement Q1, the fracture extension was observed using liquids of different viscosities. When the fracture broke through the interface between the upper and lower lithological reservoirs, the breakthrough viscosity M of the liquid was recorded. c ;

[0038] Continue increasing the viscosity until the fractures extend to all lithological reservoirs, at which point the peak viscosity M of the liquid is obtained. k .

[0039] The propagation of the fracture was observed using liquids of different viscosities. When the fracture could effectively break through the lithological interface, the breakthrough viscosity M of the liquid was recorded. c As viscosity continues to increase, it cannot be increased indefinitely when fractures can affect all lithological reservoirs; a viscosity peak M is required. k Because the viscosity is too high, it affects normal construction.

[0040] Under different lithologies and different weak cementation strengths, step three was repeated to obtain the optimal discharge rate Q of shale through sandstone. a Viscosity M1, sandstone displacement through shale Q b Viscosity M2, Shale-Limestone Displacement Q c Shale through limestone displacement Q d The viscosity of limestone through shale is M3.

[0041] The further optimization scheme includes step four: configuring the branch joint shape and main joint shape of the crack according to the longitudinal expansion and transverse extension shape of the crack.

[0042] Further optimization methods include configuring the branch and main crack shapes, such as:

[0043] Determine the particle size L1 of the branch joint and the particle size L2 of the main joint of the crack;

[0044] The pumping rate of the fracturing fluid with a set viscosity is determined based on the lithological interface.

[0045] The fracturing fluid was injected sequentially into the branch fracture proppant with a particle size of L1 and the main fracture proppant with a particle size of L2.

[0046] This solution also provides a shale oil cross-layer volumetric fracturing design system, which is applied to the methods described in the above solution, including: a data acquisition module, an experimental module, an optimization module, and a deployment module;

[0047] The acquisition module is used to acquire well logging sonic data of reservoirs with different lithologies, and to obtain the stress profile, rock mechanical characteristics and rock mechanical differences of the target section;

[0048] The experimental module is used to characterize the weak structural surface features of the target segment through mechanical experiments and simulation tests;

[0049] The optimization module is used to simulate the breakdown of fractures between different lithological reservoirs based on the target section stress profile, rock mechanical characteristics, rock mechanical differences and weak structural surface characteristics. It optimizes the fractures by adjusting the displacement and viscosity of different lithological reservoirs, so that the fractures can extend longitudinally and laterally.

[0050] The propping module is used to propp up cracks according to their longitudinal and lateral extension patterns.

[0051] This scheme accurately calibrates the longitudinal lithological stress differences and rock mechanics differences through formula calculations and indoor experimental calibration. Through experiments and simulations, it clarifies the influence of weak cementation surfaces of different strengths and proposes parameter selection under different lithological interfaces and weak cementation surface strengths, including displacement, fluid, and pumping program selection. Compared with conventional fracturing design methods, this scheme can effectively improve the longitudinal sweep area of ​​fractures and realize three-dimensional reservoir utilization.

[0052] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0053] This invention provides a shale oil cross-layer volumetric fracturing design method and system, suitable for fracturing of multiple lithologies. This invention achieves vertical multi-lithology transformation by using the target section stress profile, rock mechanical characteristics, rock mechanical differences, and target section stress profile, rock mechanical characteristics, and rock mechanical differences. It optimizes the fractures by adjusting the discharge and viscosity of reservoirs of different lithologies, enabling the fractures to extend both vertically and laterally, effectively increasing the vertical sweep area of ​​the fractures and realizing three-dimensional utilization of the reservoir. Attached Figure Description

[0054] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0055] Figure 1 A schematic diagram of the design process for cross-layer volumetric fracturing in shale oil;

[0056] Figure 2 This is a schematic diagram of the stress profile in Example 3;

[0057] Figure 3 This is a schematic diagram of the stress profile in Example 3;

[0058] Figure 4 This is a schematic diagram illustrating the effect of different viscosities on the fracture breakthrough lithological interface in Example 3. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0060] Example 1

[0061] This embodiment provides a design method for cross-layer volumetric fracturing in shale oil, such as... Figure 1 As shown, the steps include:

[0062] Step 1: Collect well logging sonic data of reservoirs with different lithologies, and obtain stress profiles, rock mechanical characteristics and rock mechanical differences of the target section;

[0063] Step 2: Characterize the weak structural surface features of the target segment through mechanical experiments and simulation tests;

[0064] Step 3: Based on Step 1 and Step 2, simulate the extension of fractures between reservoirs of different lithologies. Optimize the fractures by adjusting the discharge rate and viscosity of different reservoirs to achieve both longitudinal and lateral fracture extension.

[0065] Step 4: Based on the longitudinal and lateral extension patterns of the cracks, use a proppant to simulate the laying of the cracks.

[0066] Different lithological reservoirs have at least two layers with different lithologies, including: limestone, dolomite, sandstone, and shale.

[0067] Rock mechanical characteristics are obtained through stress profiles of the target segment;

[0068] Methods for obtaining rock mechanics differences include: first, determining the Young's modulus and Poisson's ratio of reservoirs with different lithologies through rock mechanics experiments; and then, calibrating the stress profile of the target section based on the Young's modulus and Poisson's ratio of reservoirs with different lithologies to obtain the rock mechanics differences.

[0069] The mechanical experiments and simulations in step two include: matrix rock mechanics experiments, matrix rock in-situ stress experiments, matrix rock cohesion experiments, rock mechanics of weak structural planes, cohesion of weak structural planes, and tensile strength experiments of weak structural planes.

[0070] Step 3 includes the following sub-steps:

[0071] S31. Select a fluid distribution consistent with the lithological reservoir of the target section;

[0072] S32. By changing the liquid displacement, observe the longitudinal and transverse extension of the crack to obtain the breakthrough displacement Q1 and peak displacement Q. t ;

[0073] S33. At the breakthrough displacement Q1, the breakthrough viscosity M is obtained by observing the longitudinal and transverse extension of the cracks by changing the viscosity of the liquid. c and peak viscosity M k .

[0074] S32 includes the following sub-steps:

[0075] Gradually increase the fluid discharge rate and observe the longitudinal propagation of the fracture. When the fracture breaks through the interface between the upper and lower lithological reservoirs, record the breakthrough discharge rate Q1.

[0076] If the crack does not effectively propagate longitudinally, continue to increase the displacement and obtain the peak displacement Qt of the liquid according to the law of conservation of energy.

[0077] S33 includes the following sub-steps:

[0078] At the breakthrough displacement Q1, the fracture extension was observed using liquids of different viscosities. When the fracture broke through the interface between the upper and lower lithological reservoirs, the breakthrough viscosity M of the liquid was recorded. c ;

[0079] Continue increasing the viscosity until the fractures extend to all lithological reservoirs, at which point the peak viscosity M of the liquid is obtained. k .

[0080] Step four includes configuring the branch crack shape and main crack shape according to the longitudinal expansion and transverse extension pattern of the crack.

[0081] Methods for configuring the branch and main crack patterns include:

[0082] Determine the particle size L1 of the branch joint and the particle size L2 of the main joint of the crack;

[0083] The pumping rate of the fracturing fluid with a set viscosity is determined based on the lithological interface.

[0084] The fracturing fluid was injected sequentially into the branch fracture proppant with a particle size of L1 and the main fracture proppant with a particle size of L2.

[0085] Example 2

[0086] This embodiment provides a shale oil cross-layer volumetric fracturing design system, applied to the method described in the previous embodiment, including: a data acquisition module, an experimental module, an optimization module, and a deployment module;

[0087] The acquisition module is used to acquire well logging sonic data of reservoirs with different lithologies, and to obtain the stress profile, rock mechanical characteristics and rock mechanical differences of the target section;

[0088] The experimental module is used to characterize the weak structural surface features of the target segment through mechanical experiments and simulation tests;

[0089] The optimization module is used to simulate the breakdown of fractures between different lithological reservoirs based on the target section stress profile, rock mechanical characteristics, rock mechanical differences and weak structural surface characteristics. It optimizes the fractures by adjusting the displacement and viscosity of different lithological reservoirs, so that the fractures can extend longitudinally and laterally.

[0090] The propping module is used to propp up cracks according to their longitudinal and lateral extension patterns.

[0091] Example 3

[0092] This embodiment is implemented according to the above embodiment, and rock mechanics experiments and acoustic data correction are performed according to step one, as shown in the table:

[0093]

[0094] The stress profile diagram was obtained by correcting the stress profile based on the data in the table above. Figure 3 As shown; according to step two characterization of weak interfaces: weak structural surfaces exhibit compressive-shear effects under stress conditions, that is, their shear strength increases with the increase of normal compressive stress on the weak surface. This effect affects the initial geostress equilibrium and the change in shear strength during the later fracturing process, directly affecting the formation of shear slip on the weak structural surface.

[0095] The weak structural surfaces are characterized using cohesive elements in ABAQUS; the condition for the formation of shear cracks on the weak structural surfaces is:

[0096]

[0097] In the formula: τ—shear force on the weak surface, Pa;

[0098] p′ n —Effective normal compressive stress on the weak surface, Pa;

[0099] — Shale friction angle, °;

[0100] β—Weak surface shear dilatation angle, °, related to JRC (weak surface roughness), JCS (compressive strength), and p′. n related.

[0101] Referring to the Barton model, the initial damage criterion for shale bedding plane compression and shear is expressed as:

[0102]

[0103] As shown in the above equation, the shear behavior of weak structural surfaces has a significant impact during hydraulic fracturing. First, fracturing affects the distribution of the geostress field, altering the effective normal stress of the weak structural surface and even reducing the effective compressive stress on the bedding plane. Shear strength decreases as the effective compressive stress decreases, which is conducive to the further formation of shear fractures due to dilatation of the weak structural surface. Second, the fracturing fluid may cause softening damage to the weak structural surface, further reducing the friction coefficient and leading to a decrease in shear strength, making it more prone to shear slip. After the weak structural surface enters the residual shear stage, its participation in the shear effect can be achieved by changing the maximum degradation value of the cohesive element damage factor, which can be expressed as:

[0104]

[0105] Step 3: Based on the results of Steps 1 and 2, the effects of different fluid displacement rates and different fluid viscosities on fracture penetration of the lithological interface under different lithologies are analyzed. Specific results are as follows: Figure 3 and Figure 4 As shown: Figure 3 The displacement of liquids Q1-Q4 gradually increases. Figure 4 The displacement of liquids M1-M3 gradually increases, thus determining that displacement Q4 and viscosity M3 are the optimal choices;

[0106] Step 4: Based on the crack morphology in Step 3, different angle crack combinations were constructed, and proppant experiments were conducted. The optimal proppant combinations were determined to be 70 / 140 mesh quartz sand, 40 / 70 mesh quartz sand, and 30 / 50 mesh quartz sand.

[0107] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A design method for cross-layer volumetric fracturing in shale oil, characterized in that, Including the following steps: Step 1: Collect well logging sonic data of reservoirs with different lithologies, and obtain the stress profile, rock mechanical characteristics and rock mechanical differences of the target section; Step 2: Characterize the weak structural surface features of the target segment through mechanical experiments and simulation tests; Step 3: Based on Step 1 and Step 2, simulate the extension of fractures between reservoirs of different lithologies. Optimize the fractures by adjusting the discharge rate and viscosity of different reservoirs to achieve both longitudinal and lateral fracture extension. Step 4: Based on the longitudinal and lateral extension patterns of the cracks, simulate the laying of cracks using a proppant. The mechanical experiments and simulation tests in step two include: matrix rock mechanics experiment, matrix rock in-situ stress experiment, matrix rock cohesion experiment, rock mechanics of weak structural planes, cohesion of weak structural planes, and tensile strength experiment of weak structural planes. Step three includes the following sub-steps: S31. Select a fluid distribution consistent with the lithological reservoir of the target section; S32. By changing the liquid displacement, observe the longitudinal and transverse extension of the crack to obtain the breakthrough displacement Q1 and peak displacement Qt; S33. Under the breakthrough displacement Q1, the viscosity of the liquid is changed to observe the longitudinal and transverse extension of the crack, and the breakthrough viscosity Mc and peak viscosity Mk are obtained. S32 includes the following sub-steps: Gradually increase the fluid discharge rate and observe the longitudinal propagation of the fracture. When the fracture breaks through the interface between the upper and lower lithological reservoirs, record the breakthrough discharge rate Q1. If the crack does not effectively propagate longitudinally, continue to increase the displacement and obtain the peak displacement Qt of the liquid according to the law of conservation of energy; S33 includes the following sub-steps: At the breakthrough displacement Q1, the fracture extension was observed using liquids of different viscosities. When the fracture broke through the interface between the upper lithological reservoir and the lower lithological reservoir, the breakthrough viscosity Mc of the liquid was recorded. Continue to increase the viscosity until the fractures can reach all lithological reservoirs to obtain the peak viscosity Mk of the liquid; Step four includes: configuring the branch joint shape and main joint shape of the crack according to the longitudinal expansion and transverse extension pattern of the crack; Methods for configuring the branch and main crack patterns include: Determine the particle size L1 of the branch joint and the particle size L2 of the main joint of the crack; The pumping rate of the fracturing fluid with a set viscosity is determined based on the lithological interface. The fracturing fluid was injected sequentially into the branch fracture proppant with a particle size of L1 and the main fracture proppant with a particle size of L2.

2. The shale oil cross-layer volumetric fracturing design method according to claim 1, characterized in that, The reservoirs of different lithologies have at least two layers with different lithologies, including: limestone, dolomite, sandstone, and shale.

3. The shale oil cross-layer volumetric fracturing design method according to claim 1, characterized in that, The rock mechanical characteristics were obtained through the stress profile of the target section; The method for obtaining rock mechanical differences includes: first, determining the Young's modulus and Poisson's ratio of different lithological reservoirs through rock mechanical experiments; and then, calibrating the stress profile of the target section based on the Young's modulus and Poisson's ratio of different lithological reservoirs to obtain the rock mechanical differences.

4. The shale oil cross-layer volumetric fracturing design system according to claim 1, characterized in that, The method applied to any one of claims 1-3 includes: a data acquisition module, an experimental module, an optimization module, and a deployment module; The acquisition module is used to acquire well logging sonic data of reservoirs with different lithologies, and to obtain the stress profile, rock mechanical characteristics and rock mechanical differences of the target section; The experimental module is used to characterize the weak structural surface features of the target segment through mechanical experiments and simulation tests; The optimization module is used to simulate the extension of fractures between different lithological reservoirs based on the target section stress profile, rock mechanical characteristics, rock mechanical differences and weak structural surface characteristics. It optimizes the fractures by adjusting the displacement and viscosity of different lithological reservoirs, so that the fractures can extend longitudinally and laterally. The proppant module is used to simulate the proppant layering of cracks based on their longitudinal and lateral extension patterns.

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