A Semi-Analytical Model for Water Injection Wells in Tight Reservoir Considering the Multi-Dynamic Closure Phenomenon- Case Studies in X Oilfield, China

Abstract

Abstract It is well known that waterflooding will induce fractures. The extensive direction, length, and conductivity of the induced fractures will largely determine the performance of producing wells. The quantitative characterization of the fractures can help prevent water breakthrough in time. This paper proposes the waterflooding-induced dynamic fracture model (WIDF) to monitor the fracture half-length at any time and extend the stable production period. The rock mechanics principles are applied to characterize the dynamic extension and closure of fractures during water injection and build-up periods. The model was solved using the point source function approach. The phenomenon of multi-dynamic closure causes the fluid to be squeezed several times, creating a storage effect and a non-constant conductivity within the fracture. Experimental results show that the conductivity of water injection-induced fracture follows an exponential function. Finally, Duhamel's principle was used to couple the pressure response of the wellbore and the fracture. The field case is shown in the paper to verify the accuracy and practicality of the WIDF model. Multi-peak appears in the pressure derivative curves. The actual data match well, and the parameter values obtained are close to the actual values. However, the conventional finite-conductivity model treats pressure response data with multiple peaks as incorrect values. This behavior will result in the length of dynamic closed fracture being ignored and the interpreted fracture half-length being smaller than the actual value. Misidentification of fracture lengths will affect the determination of reasonable injection volumes and even cause producing water early, which can severely impact the performance of production wells. The storage effect caused by multiple dynamic fracture closures reasonably explains the significant storage effect obtained by the conventional model. The WIDF model enables researchers can monitor induced fracture half-length at any time, allowing them to take measures in time. The model has been successfully applied to the X oilfield in China and has significantly improved the performance of injection wells and increased the stable production period of production wells. The fluid compressed by dynamic fractures reasonably explains the significant storage effect in injection wells. The identification and interpretation of multiple dynamic fracture closure phenomenon make us obtain more accurate fracture half-length parameters, which monitors and suppresses the occurrence of water breakthrough effectively and help researchers take measures to increase the stable production period of production wells.