基于B-L方程的低渗透油藏CO2水气交替注入能力

注入能力的降低是制约低渗透油藏CO2水气交替（WAG）驱提高采收率的关键因素。采用特征线法，考虑CO2在原油、注入水中的溶解及CO2对原油的抽提作用，引入渗流阻滞系数，对气驱油及水驱气过程中的Buckley-Leverett（B-L）方程进行修正，确定气驱油、水驱气前缘及尾部移动速度，结合多重复合油藏渗流理论，建立可表征CO2水气交替驱注入能力变化的数学模型，并应用Laplace变换及Stehfest数值反演，求得井底压力解。通过与不同地层渗透率下的数值模拟结果进行对比，验证模型的准确性。结果表明，相比于偏油湿地层，偏水湿地层水的注入能力较低；CO2与原油间界面张力越小，注水能力越大；当地层渗透率低于5×10-3 μm2时，水的注入能力急剧减小。
The loss of injectivity has become one of the key factors restricting the recovery efficiency in CO2 water-alternating-gas (WAG) projects for low permeability reservoir. In this paper, considering the solubility of CO2 in oil and water and the evaporation of oil components by CO2, a method of characteristics was used, and a retardation factor was introduced to modify the Buckley-Leverett(B-L) equations during CO2 flooding and water displacing CO2. Then, the front and rear velocity for CO2 flooding and water displacing gas can be determined. A mathematic model to calculate the injectivity of CO2 WAG was proposed based on the flowing theory of composite reservoirs and the front and rear velocity of CO2 and water flow in the WAG process. The methods of Laplace transformation and Stehfest numerical inversion were applied to solve the model and obtain an approximate solution of the bottom-hole pressure. The analytical model was verified by numerical simulation results for reservoirs with different permeability. Sensitivity analysis shows that, in comparison with partial oil-wet reservoirs, the injectivity of water is lower for partial water-wet formations. The water injectivity increases with the reduction of CO2-oil interfacial tension. When the reservoir permeability is lower than 5×10-3μm2, the injectivity of water deceases significantly