This presentation provides a technical overview of a project that evaluated the feasibility of natural gas injection in a Wennan low-permeability sandstone reservoir. An advantage of gas injection is high injectivity. Relative high injectivity combining with near miscible condition between injection gas and reservoir fluid offers a potentially attractive prospect in the Wennan low-permeability reservoir. Zhongyuan Oil Field and Southwest Petroleum Institute(SWPI) sponsored the project at Wennan field, including reservoir characterization, experiments, flow simulation, and nitrogen injection, which proved that a high gas injectivity and reservoir pressure was 44.35 MPa.

Main attention in this paper is paid to experimental research, including the regular PVT test. The slim-tube tests were used to determine minimum enriched gas composition(MMC) for miscible displacement at present reservoir pressure. The experiment was conducted at pressure 44.35 MPa and 393K. These slim-tube tests showed that a lean gas with reservoir fluids can reach near-miscible condition. Only a few LPG are needed to enrich the the lean gas.Then a set of experiments to quantify displacement efficiency and provide parameters for composition simulations.The first experiment evaluated the recovery efficiency in sandstone by enriched gas displacements, and the second looked at the recovery in sandstone by lean gas. These experiments showed the enriched gas breakthrough as 0.92HCPV in the typical miscible condition, final recovery to be 87.01%, and lean gas breakthrough(0.437HCPV) with recovery at 65.29%, as in immiscible displacement. However, substantial recovery after breaking(0.5HCPV with final recovery 85.29%) suggests an efficient extraction of intermediate components in the typical volatile oil. Additionally, the experiments in the project was

conducted to evaluate water flooding and WAG flooding. WAG flooding showed that WAG could significantly increase oil recovery, but the pressure difference would increase greatly when gas was injected. Therefore, WAG is not suggested in Wennan oilfield.

Further investigation was made at field with CMG 3D compositional simulation. Compositional simulation suggested slow breakthrough and high recovery at the Wennan field. These evaluations will lead to a pilot test in the Wennan low-permeability reservoir.

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