Abstract
According to the theory of plane mechanics involving the interaction of hydraulic and natural fractures, the law of hydraulic fracture propagation under the influence of natural fractures is verified using theoretical analysis and RFPA2D-Flow numerical simulation approaches. The shear and tensile failure mechanisms of rock are simultaneously considered. Furthermore, the effects of the approach angle, principal stress difference, tensile strength and length of the natural fracture, and elastic modulus and Poisson’s ratio of the reservoir on the propagation law of a hydraulic fracture are investigated. The following results are obtained: (1) The numerical results agree with the experimental data, indicating that the RFPA2D-Flow software can be used to examine the hydraulic fracture propagation process under the action of natural fractures. (2) In the case of a low principal stress difference and low approach angle, the hydraulic fracture likely causes shear failure along the tip of the natural fracture. However, under a high stress difference and high approach angle, the hydraulic fracture spreads directly through the natural fracture along the original direction. (3) When natural fractures with a low tensile strength encounter hydraulic fractures, the hydraulic fractures likely deviate and expand along the natural fractures. However, in the case of natural fractures with a high tensile strength, the natural fracture surface is closed, and the hydraulic fracture directly passes through the natural fracture, propagating along the direction of the maximum principal stress. (4) Under the same principal stress difference, a longer natural fracture corresponds to the easier initiation and expansion of a hydraulic fracture from the tip of the natural fracture. However, when the size of the natural fracture is small, the hydraulic fracture tends to propagate directly through the natural fracture. (5) A smaller elastic modulus and larger Poisson’s ratio of the reservoir result in a larger fracture initiation pressure. The presented findings can provide theoretical guidance regarding the hydraulic fracturing of reservoirs with natural fractures.
Highlights
In recent years, hydraulic fracturing has been widely used in the engineering practices of petroleum, natural gas, shale gas, and coal mining [1,2,3,4,5]
In the case of fractured reservoirs, the presence of natural fractures can change the path of the hydraulic fracture propagation, leading to the formation of a complicated fracture propagation system with multibranch fractures, which increases the complexity of the hydraulic fracture network[10, 11]
The homogeneity, size, number of cells, borehole aperture, and basic mechanical parameters of the model were kept unchanged, and the single variable method was used to examine the influence of the approach angle, principle stress difference, tension strength and length of the natural fracture, elastic modulus, and Poisson’s ratio on the hydraulic fracture propagation
Summary
Hydraulic fracturing has been widely used in the engineering practices of petroleum, natural gas, shale gas, and coal mining [1,2,3,4,5]. The hydraulic fracture in this case directly passes through the natural fracture and extends along the direction of the maximum horizontal principal stress At this time, the fluid pressure in the hydraulic fracture can be expressed as follows [27, 28]. RFPA2D-Flow software, a seepage-stress coupling analysis system for rock fracture instability, is used to analyze the interaction mechanism between natural fractures and hydraulic fractures based on the damage mechanics theory, in which both tensile and shear failure criteria of the rock are chosen[35, 36] In this numerical simulation, a two-dimensional plane stress model (Figure 3) sized 0:5 m × 0:5 m is adopted. To ensure that the numerical calculation can more closely simulate the real physical experiment, the actual physical parameters of the experimental sample are adopted in the numerical simulation as much as possible
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