Well infilling has become an effective approach to enhance oil recovery (EOR) for many years. Closely spaced wells and water injections can help maintain reservoir pressure and enlarge producing pressure drop, which are beneficial to the low permeability reservoir development. After more than 20 years of water injection in the Ansai low permeability oil reservoir (Changqing Oil Field, China), water cut of wells now increases rapidly and the production rate decreases greatly. The primary recovery scheme and well spacing are no longer efficient. Pilot well pattern adjustments including converting, infilling and hydraulic fracturing have been adopted. Some wells achieved good results; however, due to the imperfect fracturing design, most fractured wells water broke through along fractures and led to water channeling. Meanwhile, fracture reorientation has been monitored, indicating that an integral readjustment is necessary, and in case of the probability of fracture reorientation, the process of redesigning infill well pattern and fracturing optimization should be carefully discussed before hydraulic fracture stimulation. In this paper, reservoir simulation models have been conducted, using near wellbore modeling (NWM) technology. New fractures with different orientations are incorporated in the reservoir models to better understand the involving infill wells’ performance. Numerical simulations indicate that infill wells inside inverted nine-spot pattern can be divided into several types and that each type has different production characteristics as well as optimal fracture length scope. Furthermore, the new fracture orientation and length can change the direction of mainstream line. We also give the theoretical production type curves where all kinds of fracture orientation have been considered. According to the simulation results, concept of minimum-risk optimization (MRO) and fracturing safe interval (FSI) have been put forward to estimate the optimal fracturing design for infill wells when fracture reorientation cannot be precisely detected. This design cannot only guarantee good well pattern performance, but also minimize the risks associated with difficulty in pre-determining infill fracture reorientation.
Key words: Low permeability reservoir, infill well, fracture reorientation, fracture length, minimum-risk optimization (MRO), fracturing safe interval (FSI).