Evaluation and Optimization of Production Performance for Fractured Buried Hill Reservoir

The C Block of a buried hill reservoir in West Africa exhibits typical dual-porosity characteristics with well-developed natural fractures and a tight matrix. The depletion production was initially employed for the reservoir, but in recent years, water and gas injection have been implemented to supplement reservoir energy. However, due to the strong heterogeneity, the dynamic indicators of different regions have complex variations, and the controlling factors for development are not clear. Moreover, the typical development technology strategies are still incomplete. These challenges pose difficulties for optimizing on-site water and gas injection in the future. Scholars have carried out studies on the related issues; however, little consideration was given to the impact of previous high-speed development of natural energy, and there is a lack of systematic optimization and adjustment for water and gas injection. This study starts by analyzing the typical characteristics of water and gas injection development, utilizing the discrete fracture numerical simulation to extract the controlling factors in development and formulate typical development strategies. Additionally, it proposes optimization directions of water and gas injection for block C. Results indicate that C block initially underwent rapid natural energy development, resulting in a significant decrease in pressure levels in the upper portion of the reservoir. Following the implementation of water and gas injection, there were positive overall effects observed. However, due to complex natural fracture distribution, certain oil wells did not exhibit satisfactory improvement. The effects of water and gas injection are controlled by various factors, including the distribution characteristics of natural fractures, well pattern, and voidage replacement ratio. Additionally, gas injection is also influenced by the structural location. We have developed a set of development strategy chart that takes into account the injection timing. Based on our analysis, we propose that the current voidage replacement ratio of water and gas should be increased to 1.1–1.2 and 1.0–1.1, respectively, while avoiding water and gas channeling. After optimization, the oil recovery of the typical well group of water and gas injection in the C block will increase by more than 5%. This study refines the current characteristics of water and gas injection in the C block, and specifies directions for future adjustments, thereby providing a strong theoretical guidance for the efficient development of such oil reservoirs.