Opening Conditions and Extension Law of Natural and Hydraulic Fractures in Fractured Reservoirs

In order to reveal the opening conditions and extension law of natural and hydraulic fractures in fractured reservoirs and reduce the risk of water channeling, in-situ stress test, natural fracture observation, hydraulic fracture extension test, and simulation were used. In addition, the opening conditions and extension law of natural and hydraulic fractures, the effect of horizontal stress difference on the extension of hydraulic fractures, and the net pressure of communication between natural and hydraulic fractures were systematically studied. The results show that the vertical stress σ_v in the study area is higher than the maximum horizontal principal stress σ_H, which is higher than the minimum horizontal principal stress σ_h. The gradients of vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress are 0.025, 0.020, and 0.017 MPa/m, respectively. The critical pressure gradient of fracture sliding in the tight sandstone of the study area ranges from 0.018 to 0.020 MPa/m, with an average value of 0.019 MPa/m. When the critical pressure gradient in the natural fracture exceeds 0.020 MPa/m, all the ineffective fractures are transformed into effective fractures. As the horizontal stress difference increases from 0 to 10 MPa, the length of hydraulic fractures increases gradually. The length of hydraulic fractures formed under the stress difference of 10 MPa is about 1.52 times that of hydraulic fractures formed under the stress difference of 0 MPa and 5 MPa. The fracturing experiment results show that the hydraulic fracture will be deflected to a certain extent when encountering locally developed gravel particles and then continue to extend along the original direction. In the study area, the net pressure gradient of fracture communication under the condition that natural fractures in the target layer open to form branch fractures is 0.003 MPa /m, and the net pressure of fracture communication is 4.5–9.0 MPa. It is concluded that the construction of an unsteady extension mode of hydraulic fractures in fractured reservoirs should consider the horizontal stress difference, natural fracture development, lithology characteristics, and fracturing operation conditions.