Numerical simulation of hydraulic fracture propagation in naturally fractured formations using the cohesive zone model

摘要

This paper presents an integrated methodology that utilizes the cohesive zone model (CZM) to simulate propagation of hydraulic fractures, and their interactions with pre-existing natural fractures. CZM has the advantage of capturing the non-elastic behavior of shales; usually induced by total organic carbon (TOC) as well as dissimilar mechanical properties of cemented natural fractures. At the intersection of an advancing hydraulic fracture and a stationary natural fracture, the hydraulic fracture may arrest, cross, or divert into a pre-existing natural fracture depending on the rock mechanical properties, magnitude and direction of rock principal stresses, and fracture intersection angle. The activation of natural fractures during hydraulic fracture treatments improves fracture complexity and expands reservoir drainage area, making stimulation treatments more effective. In this work, triaxiality effects are incorporated into the cohesive zone model. Utilizing triaxiality makes the traction separation law (TSL) tied to confining pressure and this ensures a more reliable transition from laboratory test environment to bottomhole conditions. We present a methodology to determine the cohesive properties or TSL characteristics of rock, after performing semicircular bending tests (SCBT). Finite element analysis (FEA) is then used to calibrate the cohesive properties of both rock and natural fractures. The calibrated parameters were utilized in a field-scale FEA to simulate the growth of complex fracture networks. The results show how fracture intersection angle and the nature of cemented materials inside the natural fractures might divert a hydraulic fracture initially propagating in a direction perpendicular to the minimum horizontal stress. The sensitivity analysis of primary parameters such as fluid viscosity, natural fracture distribution, fracture intersection angle, and differential stresses is implemented to provide a better insight into the performance of hydraulic fracturing jobs in naturally fractured reservoirs. Results indicate the importance of nonlinear fracture tip effects as in-situ stress differences increase.