Hydraulic Fracture Optimization Based on Discrete Fracture Network Model in Shale Reservoirs

Abstract

Horizontal well drilling and multistage hydraulic fracturing are two main technologies in the development of shale gas reservoirs. In this study, a multi-scale percolation mathematical model for the shale gas reservoirs was established based on the discrete fracture network (DFN) model and the derived nonlinear equations were solved based on the Newton-Raphson method through programming in C Sharp. Finally, the comprehensive evaluation was conducted to evaluate the influence of hydraulic fracture stages/spacing, half-length, aperture, orientation and natural & hydraulic fracture connectivity on the shale gas exploitation in a basic geological model with complex natural fractures network. Numerical simulation results indicate that the hydraulic fracture stages/spacing, half-length and orientation only influence the early period (the first 100 day in this model) production rate which becomes consistent after that, but the fracture aperture has a little effect because the matrix permeability is the main factor to limit the gas flow. There are a demarcation point that the cumulative production acceleration slows down when the number of hydraulic fracture stage increase (20 stages for this case) and the cumulative production acceleration speed up when the hydraulic fracture half-length increase (45m for this case). The hydraulic fracture aperture has a little effect on the cumulative production when the matrix permeability is 800nD for the presented fractures network in this work. When the angle of natural fractures and horizontal well are 45°, the angle of hydraulic fracture and natural fracture varies between 45° and 75° or the angle of hydraulic fracture and well varies between 90° and 120° is most beneficial to enhance the cumulative production. The cumulative production of shale gas well is positively correlated to the all fracture intersecting node number which is changed by only adjusting the fracture stage (or fracture length, fracture orientation), we also find that the effect of the fracture length change on cumulative production is the most obvious, and the descending order of the fracture parameters is fracture length> fracture stage> fracture orientation.