Investigation on the pressure behavior of injectors influenced by waterflood-induced fractures: field cases in Huaqing reservoir, Changqing Oilfield, China

Abstract

Pressure transient behaviors of water injectors with the influence of Waterflood-Induced Fractures (WIFs) in unconventional tight oil reservoirs are much more complicated than hydraulic fracturing producers due to the variation of the WIF properties in the field-testing period. In this paper, we present the interpretation on five different types of Bottom-Hole Pressure (BHP) responses for water injectors by combining a commercial simulator tool and our proposed models, where the Shrinking Fracture Length (SFL) and Decreasing Fracture Conductivity (DFC) effects are captured for single long WIF while the multiple fracture closures effect is characterized for multiple WIFs. Results show that the commonplace phenomena of prolonged storage effect, bi-storage effect, and interpreted considerably large storage coefficient suggest that WIF(s) may initiate by long-time water injection. Based on this interpreted large storage coefficient, the fracture half-length can be estimated. The upward buckling of pressure derivative curve may not necessarily be caused by closed outer boundary condition, but the Decreasing of Fracture Conductivity (DFC). As for multiple WIFs, they would close successively after shutting in the well due to the different stress conditions perpendicular to fracture walls, which behaves as several unit slopes on the pressure derivative curves in log–log plot. Aiming at different representative types of BHP responses cases in Huaqing reservoir, Changqing Oilfield, we innovatively analyze them from a different perspective and get a new understanding of water injector behaviors, which provides a guideline for the interpretation of water injectors in tight oil reservoirs.