Production characteristics and underlying mechanisms during CH4 injection in tight oil reservoirs: Multi-scale fracture core experiments and focus on impacts of gravity segregation and pressure

In this work, we attempted to evaluate the effects of gravity segregation and back pressure on oil production characteristics during CH4 flooding under multi-scale fracture conditions. A multi-scale fracture model was employed to conduct CH4 flooding experiments. Based on the equation of state and mass conservation, gas breakthrough time (GBT) and gas channeling time (GCT) were identified. The impacts of gravity segregation and back pressure on the performance of CH4 injection were explored, and the CH4 utilizations at various phases were quantified. Results showed that the oil production process can be divided into three phases based on GBT and GCT: phase I (before GBT), phase II (from GBT to GCT), and phase III (after GCT). The top-CH4 injection had a higher oil recovery of 37.75 % in contrast to 32.33 % of bottom-CH4 injection by gravitational effect, and the incremental oil recovery mainly came from phases II and III. On the other hand, the lower back pressure exhibited superior oil recovery performance with top injection due to the later GCT. Moreover, GBT was insensitive to gravity segregation and back pressure, which is caused by the preferential flow of injected CH4 in fractures. Additionally, the back pressure significantly affected the oil production capacity (OPC). A higher back pressure exhibited considerable OPC in phases II and III (initial). However, the enormous differential pressure will introduce earlier gas channeling and shorten the effective period of CH4 flooding. Finally, the study found that a certain degree of self-sealing caused by Marangoni flow and gas-liquid slugs occurred after GCT, significantly affecting the recovery of bypassed oil under multi-scale fracture conditions.