Pressure-transient analysis of water injectors considering the multiple closures of waterflood-induced fractures in tight reservoirs: Case studies in Changqing Oilfield, China

摘要

There is growing evidence showing that water injection may induce formation fracturing, and the injection well may experience multiple fracture closures during fracture falloff period. Such dynamic behavior of waterflood-induced fractures (WIFs) needs to be carefully assessed and controlled through a waterflooding program. The objective of this study is to develop a pressure transient analysis procedure to analyze the bottom-hole pressure (BHP) responses of water injection wells with the consideration of the dynamic behavior of WIFs. We propose a rigorous physical process that integrates mini-fractures initiation, communication, propagation and closure throughout waterflooding. The piecewise function is used to characterize the consecutive closure of fractures during the shut-in period. Based on the proposed model, the pressure-transient solutions are obtained and the type curves are plotted. Results indicate that the consecutive fractures closure lowers the rate of pressure drop and results in the fluctuations of the pressure derivative curve. During the field-testing period, the water that stored in the WIFs would be squeezed into the matrix since there are no proppants inside, which behaves similar with wellbore storage effect. Meanwhile, due to the different stress conditions in the fracture walls, they close successively after shutting in the well, which results in the multiple storage effect that characterized by several unit slopes in the pressure derivative curve. Finally, several cases studies from Changqing Oilfield have validated the practicability of the models to evaluate the BHP performances of water injectors.