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CO2 enhanced oil recovery (CO2-EOR) technology is a competitive strategy to improve oil field economic returns and reduce greenhouse gas emissions. However, the arbitrary emissions or combustion of the associated gas, which mainly consists of CO2 and CH4, will cause the aggravation of the greenhouse effect and a huge waste of resources. In this paper, the high-performance facilitated transport multilayer composite membrane for CO2/CH4 separation was prepared by individually adjusting the membrane structure of each layer. The effect of test conditions on the CO2/CH4 separation performance was systematically investigated. The membrane exhibits high CO2 permeance of 3.451 × 10−7 mol·m−2·s−1·Pa−1 and CO2/CH4 selectivity of 62 at 298 K and 0.15 MPa feed gas pressure. The cost analysis was investigated by simulating the two-stage system. When the recovery rate and purity of CH4 are 98%, the minimum specific cost of separating CO2/CH4 (45/55 vol%) can be reduced to 0.046 $·Nm−3 CH4. The excellent short-to-mid-term stability indicates the great potential of large industrial application in the CH4 recovery and CO2 reinjection from oilfield associated gas.

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The pore connectivity and distribution of moveable fluids, which determines fluid movability and recoverable reserves, are critical for enhancing oil/gas recovery in tight sandstone reservoirs. In this paper, multiple techniques including high-pressure mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and microcomputer tomography scanning (micro-CT) were used for the quantitative characterization of pore structure, pore connectivity, and movable fluid distribution. Firstly, sample porosity and permeability were obtained. Pore morphology and the 3D distribution of the pore structures were analyzed using SEM and micro-CT, respectively. The pore-size distribution (PSD) from NMR was generally broader than that from MIP because this technique simply characterized the connected pore volume, whereas NMR showed the total pore volume. Therefore, an attempt was made to calculate pore connectivity percentages of pores with different radii (<50 nm, 50 nm–0.1 μm, and 0.1 μm–1 μm) using the difference between the PSD obtained from MIP and NMR. It was found that small pores (r<0.05 μm) contributed 5.02%–18.00% to connectivity, which is less than large pores (r>0.05 μm) with contribution of 36.60%–92.00%, although small pores had greater pore volumes. In addition, a new parameter, effective movable fluid saturation, was proposed based on the initial movable fluid saturation from NMR and the pore connectivity percentage from MIP and NMR. The results demonstrated that the initial movable fluid saturation decreased by 14.16% on average when disconnected pores were excluded. It was concluded that the effective movable fluid saturation has a higher accuracy in evaluating the recovery of tight sandstone reservoirs.