Estimating compressibility of complex fracture networks in unconventional reservoirs

摘要

Previous studies show that fracture closure is the primary drive mechanism for fracture cleanup during flowback process in hydraulically stimulated reservoirs. Estimating fracture compressibility is practically essential to calculate effective fracture volume, evaluate fracture volume change, and forecast ultimate hydrocarbon recovery. However, limited experimental data are available for evaluating compressibility of fracture networks in unconventional reservoirs. In this paper, we categorize induced fractures into unpropped and propped fractures, and estimate their compressibilities from fracture conductivity measurements and Hertzian contact theory, respectively. We also investigate the effects of rock and proppant parameters on fracture compressibility. Finally, we propose a workflow to estimate compressibility of complex fracture networks and investigate the roles of propped and unpropped fractures during fracture closure. The results show that fracture compressibility depends on how fracture porosity and aperture change with effective stress. For propped fractures, the rate of porosity change primarily controls fracture compressibility. In addition, compressibility of complex fracture networks approximates that of unpropped fractures at low effective stress and that of propped fractures at high effective stress. Overall, the results highlight the role of unpropped fractures in hydrocarbon recovery from stimulated unconventional reservoirs.