Numerical simulation of hydraulic fracture deflection influenced by slotted directional boreholes using XFEM with a modified rock fracture energy model

Abstract

Directional hydraulic fracturing (DHF) has been used in conductivity improvement of oil, gas and geothermal energy reservoirs and hard rock roof breaking in coal mines. Although the new DHF method that integrates fracturing borehole and slotted directional borehole (SDB) has been recently introduced, the mechanism and effectiveness of SDB on hydraulic fracture guiding remain open issues. In this study, numerical simulation based on extended finite element method (XFEM) is performed to investigate the effect of SDB on hydraulic fracture deflection. A modified rock fracture energy model is proposed and performed by a user subroutine USDFLD with XFEM, which considers the hardening property of rock shear fracture energy as confining pressure increases. The effects of stress field and borehole line azimuth on hydraulic fracture deflection and bottomhole pressure are investigated. Hydraulic fracture morphologies of three kinds of multi-borehole fracturing technology are compared, the control mechanism of SDBs on hydraulic fracture is explained, and the bottomhole pressure relief effect, borehole spacing and potential application prospects are discussed. Result indicates that the modified model shows good accuracy and applicability in predicting hydraulic fracture propagation, and the hardening property of shear fracture energy should be considered when using XFEM and cohesive element method. The feature “first drop and then rise” of bottomhole pressure curve is an important sign that imply the hydraulic fracture propagates into the influence radius of SDB and can be used as an indicator to evaluate the effectiveness of DHF. Slot increases the influence radius of directional borehole and changes the local stress direction, thus allowing SDB-based DHF technique to be applied to the conditions where the stress difference ratio does not exceed one. When the stress difference ratio is greater than one, the pressurized SDB is more suitable.