Enhancing production from coal seam gas reservoir by hydraulic fracturing with controlled fracture vertical growth

摘要

Hydraulic fracturing is an important technique to stimulate coal seam gas extraction by enhancing connectivity between coal seams and wellbore via hydraulically induced fractures. The Efficiency of dewatering and methane production of hydraulically stimulated coal seam gas reservoir is highly depending on the geometry of induced fractures; especially the fracture vertical height which is controlled by different kinds of parameters including in-situ reservoir conditions and stimulation pumping schedules. Field practice shows that in many cases fracture stimulation was not successful due to the overgrowth of fracture height leading to longer dewatering time and delayed methane production. The purpose of this study is to perform hydraulic fracturing simulation on a specific coal seam gas reservoir stimulation case to control fracture vertical growth by optimising pumping schedule. First, mini-frac data analysis was performed to determine the in-situ stresses of the coal seams. Next, main fracture analysis was conducted through pressure history match to determine fracture geometry. The simulation was conducted using a 3D hydraulic fracturing simulator E-Stimplan 3D. The hydraulically generated fracture was then applied to a compositional reservoir simulator CMG – GEM to perform production history matching. Meantime, a detailed parametric study was also carried out to gain a deep understanding of fracture vertical growth under reservoir condition and to investigate the feasibility of controlling vertical growth by variation of pumping schedule. Based on the sensitivity analysis, an optimised pumping schedule was applied to the simulator to control fracture vertical growth and finally enhanced production estimation was performed and compared with the initial results of stimulation. It can be concluded that while the contrasts of Young’s modulus, Poisson’s ratio and fracture toughness between pay zone and surrounding layers have limited containment effects, in-situ stresses play a leading role in fracture vertical growth, it is therefore highly important to perform in-situ stress calibration in the simulation of fracture geometry; under reservoir property and in-situ stress conditions, optimising pumping schedule is an effective and feasible method to control fracture vertical growth, reduce dewatering time and enhancemethane production in specific cases.