致密油藏碳化水驱提高采收率机理研究

Carbonated water injection (CWI) is a promising method for oil reservoir development, however, no investigation has been conducted to study the enhanced oil recovery (EOR) of CWI in tight oil reservoirs at present in literatures. Experiments conducted by our team have proven the feasibility of EOR by CWI in tight sandstones. Due to the complicity of CWI process, its EOR mechanisms remain unclear and flow mechanics of CWI in tight oil reservoirs have not been established. The project aims to optimize CWI strategy, to understand the mechanisms of Carbon dioxide transport and EOR through core displacement experiments, Nuclear Magnetic Resonance (NMR) Imaging, CT scanning, high pressure imbibition experiments. The Carbon dioxide transport model is established by measuring Carbon dioxide partitioning coefficients and diffusion coefficients under the situation of coexistence of oil and water. The mechanisms of CWI EOR are investigated by considering wettability alteration, relative permeability variation, and mobility ratio variation. Based on Carbon dioxide transport model and experimental results, a three-dimension three-phase CWI compositional model is established and then solved by the developed computer program. This model is able to consider the mechanisms of oil swelling, oil viscosity reduction, water-blocking effect, wettability alteration, and to predict the development performance of CWI. This research will provide the theoretical foundation for tight oil reservoirs development in China.

目前国内外尚未有文献报道关于碳化水在致密储层的研究，本研究团队开展的先导性实验已经证明碳化水提高致密砂岩油藏采收率是可行的。由于碳化水驱过程复杂性，其提高采收率机理还没有认清，致密油藏碳化水驱渗流理论尚未建立。本项目以致密砂岩油藏为对象，通过岩心驱替、核磁共振、CT扫描、高温高压渗吸等先进测试手段，优选碳化水驱替方式，认清油水体系二氧化碳传递机理和碳化水驱增油机理，建立碳化水驱渗流模型。考虑油水同时存在情况下，测定碳化水-原油体系中二氧化碳分配系数和扩散系数，建立二氧化碳传递模型；综合考虑储层润湿性变化、储层相对渗透率变化、原油降粘流度比变化的作用机理，研究碳化水驱增油机理；基于二氧化碳传递模型、相对渗透率实验成果，建立碳化水驱三维、三相渗流模型并自主编程求解，模拟碳化水驱原油膨胀、原油降粘、水锁效应、润湿性等变化规律，预测碳化水驱开发效果。本研究成果对于我国致密油藏高效开发提供理论依据。

碳化水驱是指将CO2溶解在水中既而注入地层驱油的方法。碳化水驱结合了CO2驱和水驱的优势，具有较好的驱油效率和波及效率，是一种具有广阔前景的提高油藏采收率方法。本项目针对致密油藏，通过岩心驱替、高温高压渗吸、CT在线扫描等实验手段，探究碳化水驱提高采收率的效果和机理。在实验研究的基础上，考虑CO2由水相至油相的传递，建立了碳化水驱数值模拟方法，进行开发参数优化。研究结果表明，碳化水驱能够有效提高致密油藏采收率，比水驱增加7%的采收率，比CO2水气交替注入增加3%的采收率。在表面活性剂存在的情况下，碳化水的驱油效率大幅提高，碳化水+表活剂驱比水驱增加了10%的采收率。对于不同的渗吸流体，渗吸采收率由高到低依次为：碳化水+表面活性、碳化水、表面活性剂、地层水。碳化水+表面活性剂的渗吸效果最好，渗吸采收率达46.32%，分别比地层水和表面活性剂高15.2%和8.58%。致密油藏碳化水驱的效果受碳化水浓度、注入时机和注入速度等因素的影响：注入碳化水浓度越大，产油量越高，研究区存在最佳注入浓度约1.0mol/L；碳化水注入时机越早，产油量越高；在不发生水窜的前提下，碳化水+表活剂驱注入速度越大，水平井产油量越高，在10-15m3/d处存在拐点。本研究首次揭示了致密油藏碳化水驱提高采收率的可行性，为我国致密油藏高效开发提供了依据。