Abstract
When fractured low-permeability reservoirs enter a high water cut period, injected water always flows along fractures, water cut speeds increase rapidly, and oil production decreases quickly in oil wells. It is difficult to further improve the oil recovery of such fractured low-permeability reservoirs. In this paper, based on the advantages of in-depth profile control and cyclic water injection, the feasibility of combining deep profile control with cyclic water injection to improve oil recovery in fractured low-permeability reservoirs during the high water cut stage was studied, and the mechanisms of in-depth profile control and cyclic waterflooding were investigated. According to the characteristics of reservoirs in Zone X, as well as the fracture features and evolution mechanisms of the well network, an outcrop plate fractured core model that considers fracture direction was developed, and core displacement experiments were carried out by using the HPAM/Cr3+ gel in-depth profile control system. The enhanced oil recovery of waterflooding, cyclic water injection, and in-depth profile control, as well as a combination of in-depth profile control and cyclic water injection, was investigated. Moreover, variations in the water cut degree, reserve recovery percentage, injection pressure, fracture and matrix pressure, and water saturation were monitored. On this basis, the mechanism of enhanced oil recovery based on the combined utilization of in-depth profile control and cyclic waterflooding methods was analyzed. The results show that in-depth profile control and cyclic water injection can be synchronized to further increase oil recovery. The recovery ratio under the combination of in-depth profile control and cyclic water injection was 1.9% higher than that under the in-depth profile control and 5.6% higher than that under cyclic water injection. The combination of in-depth profile control and cyclic water injection can increase the reservoir pressure; therefore, the fluctuation of pressure between the matrix and its fractures increases, more crude oil flows into the fracture, and the oil production increases.
Highlights
Reserves of low-permeability reservoirs account for 60%~70% of proven reserves in China [1,2,3]
When a fractured low-permeability reservoir is in a high water cut period, CO2 injection, hydraulic fracturing, in-depth profile control, or cyclic water injection can be used to further increase oil production
To study the enhanced oil recovery mechanism of in-depth profile control and cyclic water injection methods in fractured low-permeability reservoirs, only a single layer is considered in the fractured lowpermeability core model
Summary
Reserves of low-permeability reservoirs account for 60%~70% of proven reserves in China [1,2,3]. When a fractured low-permeability reservoir is in a high water cut period, CO2 injection, hydraulic fracturing, in-depth profile control, or cyclic water injection can be used to further increase oil production. Field tests show that in-depth profile control can effectively block fractures and high-permeability reservoirs [19, 20], and it can cause subsequently displaced fluid to flow into lowpermeability reservoirs to displace residual oil [21]. For fractured lowpermeability reservoirs with high water cuts, CO2 injection, fracturing, deep control, or cyclic water injection has a limited potential to improve oil recovery. To further improve the recovery rate of fractured low-permeability reservoirs at the high water cut stage, combined with the advantages of in-depth profile control and cyclic water injection, the feasibility of in-depth profile control and cyclic water injection has been investigated through core displacement experiments. Research conclusions can provide some references for the development of fractured low-permeability reservoirs in the high water cut stage
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