Characteristics of water alternating CO2 injection in low-permeability beach-bar sand reservoirs

Abstract

Water flooding can be ineffective in highly heterogeneous low-permeability beach-bar sand reservoirs. The introduction of CO2 flooding helps boost the oil production of the reservoirs but only in an early stage. During the late stage of flooding, gas channeling would occur. Water alternating gas (CO2) (WAG) process can be used to delay gas channeling and improve the effect of CO2 injection, though its adaptability to beach-bar sand reservoirs remains unclear. In order to clarify CO2 injection characteristics in these reservoirs, experiments were carried out in high-temperature high-pressure NMR on-line displacement experiment apparatus to simulate different flooding modes on synthetic cores that can reflect the vertical heterogeneity of beach-bar reservoirs. Different CO2 injection modes were implemented on these cores and the displacement characteristics and residual oil distribution features during both WAG injection and continuous CO2 injection were analyzed quantitatively and qualitatively. The results show that the scheme of WAG injection after continuous CO2 injection can obtain better oil displacement efficiency than that of the scheme of continuous CO2 injection after WAG injection, but there is no significant difference in respect of oil displacement efficiency of WAG flooding between the mode of bar-injection – beach-production (injection into bar sand – production from beach sand) and the mode of beach-injection – beach-production (injection into and production from beach sand), with the former mode having a higher oil recovery rate. The wider pore-size distribution range of microscopic residual oil after WAG injection shows great potential of enhancing oil recovery from subsequent continuous gas injection. When WAG injection is implemented prior to continuous CO2 injection, the displacement effect of the latter is more significant. This research may provide a theoretical basis for CO2 EOR in this type of reservoirs.