The impact of fracture network on CO2 storage in shale oil reservoirs

Abstract

CO2 flooding or huff-n-puff has been recognized as a practicable EOR and carbon storage method. Since the shale rocks are very tight, it is preferable to inject CO2 into shale reservoirs for storage to avoid CO2 leakage. In this paper, we investigated the CO2 storage capacity in a hydraulically fractured horizontal well in Xinjiang shale oil reservoirs. A comparison of CO2 storage capacity was performed by stimulating different fracture networks. Three types of fracture network were created: 2 clusters, 3 clusters and 6 clusters in per 90-m stage. In the multi-cluster hydraulic fracturing treatment, injected frac fluids and proppants in each stage remain the same. It is found that perforating more clusters in a stage results in a shorter length of hydraulic fractures. However, for the case of 30 m cluster spacing, a considerable portion of the created fracture length does not contribute to production. Oil recovery factor with 15 m fracture spacing is greater than that of 45 m and 30 m. In the case of CO2 injection for EOR purpose, this process has less pronounced effect on CO2 storage. However, fracture network plays an important role for CO2 storage in depleted reservoirs. Smaller fracture spacing provides a larger SRV near the horizontal wellbore which is critical to CO2 storage. The SRV near the wellbore for 15-m fracture spacing is larger than that of 30-m and 45-m, which is critical to improving horizontal well deliverability. This study will provide a perspective on improving cluster efficiency and increasing CO2 storage capacity in horizontal wells.