Experimental study on threshold pressure gradient of CO2 flooding in low permeability reservoir

It is difficult to conduct water flooding in low permeability oil reservoirs, especially in ultra-low permeability oil reservoirs, and gas injection may be the only viable way. Additionally, it is crucial to determine the starting pressure for the design of the gas injection program. In this paper, the starting pressure gradient during CO2 flooding is studied through core flooding experiments. The relationship of the starting pressure gradient between single core and series or parallel connected cores is analysed. The experimental results show that there exists a good relationship of power exponent between core permeability and starting pressure gradient. The starting pressure gradient of series connected cores can be predicted with a small error using the weighted method based on the starting pressure of single core. However, the predicted starting pressure gradient of parallel connected cores has a relatively larger error. The starting pressure gradient of parallel connected cores is much closer to the minimum starting pressure gradient of single core. The understanding from the experiment is very useful for the estimation of starting pressure between injection and production wells in ultra-low permeability oil reservoirs.