Abstract:

For the first time，CO₂ flooding is attemptively applied to the BZ low permeability reservoir of Bohai Sea area. To capture the rule of CO₂ miscible flooding in the target reservoir，the minimum miscibility pressure （MMP） and displacement efficiency of the target block were determined using two methods，namely the slim tube and long core flooding tests. The results show that the MMP measured via the slim tube test is 32.65 MPa，and miscibility can be realized in the field. The long core flooding incorporates the actual characteristics of the reservoir，and the measured displacement efficiency is more consistent with the field development reality. The conceptual model was built by numerical simulation to analyze the effects of the injector-producer spacing and injection intensity on the sweep efficiency of gas injection. It is shown that during the early development with the high reservoir pressure，injected gas mainly migrates along the high permeability reservoir；it sweeps the medium-low permeability reservoir，as the reservoir pressure declines and the gravity segregation of CO₂，induced by the oil-gas density difference，intensifies. For reservoirs with relatively inferior porosity and permeability，production wells shall be stimulated （e.g. fracturing） to improve the injector-producer pressure difference and ensure sufficient production. In accordance with the development indexes，an injector-producer spacing within 350-400 m and injection intensity of 1×10⁴-2×10⁴ m³/d are preferred. For the optimized scheme，the recovery rate is predicted to be 2.7%/a，associated with an estimated recovery factor of 22.2% and cumulative CO₂ storage of 107×10⁴ t. The findings of this research provide technical support for the gas flooding scheme and well location/spacing determination and are of great practical significance for offshore gas flooding.

Key words:

CO₂ , flooding, minimum miscibility pressure, reservoir numerical simulation, gas flooding sweep efficiency, CCUS, enhanced oil recovery

摘要：

渤海海域BZ低渗油藏注CO₂驱油技术首次尝试应用，为取得目标油藏注CO₂混相驱规律认识，利用细管实验与长岩心驱替实验两种方法确定目标区块最小混相压力与驱油效率。结果表明：细管实验测得的最小混相压力32.65 MPa，能够满足矿场实现混相驱；长岩心驱替实验考虑了储层实际特征，驱油效率结果更符合实际矿场开发。应用数模方法建立概念模型，分析了注采井距、注入强度对注气波及系数的影响。结果表明：在开发初期，地层压力较高，注入气主要沿着高渗储层运移，而地层压力下降后，受油气密度差异影响导致的超覆作用加剧，注气能够波及中渗储层。对于物性较差储层，产出端应采用压裂等方式提高注采压差，保障产能贡献。结合开发指标，注采井距控制在350~400 m，注气强度控制在1×10⁴~2×10⁴ m³/d较为合理，优化后方案预测高峰采油速度2.7%，预测采收率22.2%，累计埋存107×10⁴ t CO₂。研究成果为BZ低渗油藏的注气开发方案优化以及井位井距选取提供技术支持，对海上注气开发具有重要实际意义。

关键词:

CO₂驱油, 最小混相压力, 油藏数值模拟, 注气波及系数, CCUS, 提高采收率