Hydraulic Fracture Propagation Of Rock-like Material With X-type Flaws

Hydraulic fracturing technology has been widely used in coal mining,shale gas development,geothermal power generation and other fields.The hydraulic fracturing technology improves the oil and gas production by continuously injecting fracturing fluid into the wellbore to generate a pressure higher than the tensile strength of the rock at the bottom of the well,thereby generating cracks in the rock.Therefore,it is particularly important to study the propagation direction of hydraulic fractures.This work mainly focuses on the influence of preset X-type cracks on the hydraulic fracture propagation.Using a triaxial hydraulic fracturing test system,a series of hydraulic fracturing tests were carried out on rock-like specimens containing pre-existing X-type cracks under different horizontal stresses.The pre-existing X-type cracks are divided into main crack and secondary crack according to the crack length.According to the angle between the main crack and the horizontal maximum principal stress,cracks are divided into two groups(45° and 90°).In each group,the cracks are divided into 6 types according to the angle between the main crack and the secondary crack from 15° to 90° with 15° increments.The hydraulic fracture propagation with different relative positions of the main crack and the secondary crack in the X-type crack are studied.The hydraulic fracturing test was carried out under four horizontal stress difference coefficients to study the influence of horizontal stress difference on the hydraulic fracture propagation of specimens with X-type cracks.The test results show that the position of the primary and secondary crack tip in the X-type cracks has a significant effect on the propagation of hydraulic cracks.Horizontal stress difference coefficients influence the direction of hydraulic fracture propagation,also.(1)The position of the primary and secondary fracture tip in the X-type cracks has an inducing effect on the propagation of hydraulic fractures.The lower the horizontal stress difference coefficient,the stronger the inducing effect of the X-type crack tips on the hydraulic fracture propagation.(2)With the increase of the horizontal stress difference coefficient,the hydraulic fracture gradually propagates to the horizontal maximum principal stress,and the inducing effect of the primary and secondary crack tips on the hydraulic fracture propagation is weakened.(3)When the main crack tip and the secondary crack tip is close,the crack tips have a concentrated strengthening effect on the hydraulic crack propagation inducing,and the hydraulic crack is more likely to propagate toward the crack tips.(4)When the main crack tip and the secondary crack tip are almost symmetrical to the horizontal maximum principal stress,the inducing effect on the hydraulic crack propagation has been balanced by the main and secondary crack tips,and the inducing effect of the crack tip on the hydraulic crack is weakened.