Study on the optimization of injection modes and injection parameters for natural gas in ultra-low permeability reservoir

Abstract

The heavy gas breakthrough is a serious problem when developing the ultra-low permeability reservoir by natural gas, one available way to deal with this difficulty is to adjust the injection mode, such as the water alternating gas (WAG) injection and the intermittent gas (IG) injection. In order to slow down the gas breakthrough and to enhance oil displacement efficiency in ultra-low permeability reservoir better, the injection modes are compared and the injection parameters are optimized. Based on the ultra-low permeability core experiments, the WAG flooding and the IG flooding after gas fingering are simulated. The displacement efficiencies and the injection differential pressures of these two injection modes are compared. And the suitable injection mode in ultra-low permeability reservoir is confirmed. Furthermore, the experiments with the selected injection mode for several flooding rounds and different flooding periods are made, and the optimized injection parameters are achieved. The results show that both the WAG injection and the IG injection can slow down the gas breakthrough and improve the oil displacement efficiency. The oil displacement efficiency improvement of WAG injection and the IG injection is close, but the pressure of WAG injection is much higher than the one of IG injection. The IG injection can be confirmed as the optimized injection mode in the ultra-low permeability reservoir after gas fingering as the pressure requirement. The economic IG flooding round is 7 at most as the oil displacement efficiency stops improving till the flooding round increases to 7. The optimized IG flooding period after gas breakthrough in ultra-low permeability reservoir is 6 h under the differential pressure of 2MPa limited by the dissolving ability of natural gas.