致密储层超临界CO2动态溶解作用下气水两相渗流规律及对孔喉特征的作用机理研究

Due to the characteristic of small pore throat, high irreducible water saturation and energy supplementation problems in tight oil reservoirs, CO2 got lot of attention due to its special characters, such as diffusion, energizing, and etc. For this project, the apparent viscosity of supercritical CO2 in tightoil reservoir was obtained first. The non-linear mathematical relationship between pressure and flow rate was investigated during the process supercritical CO2 dynamic dissolving in tightoil formation water. Its impact on the flow capacity, the irreducible water saturation, residual CO2 saturation were studied. The threshold pressure gradient was characterized and the flow behavior characteristic of supercritical CO2-water as two-phase flow under different fluid saturation was clarified. Through NMR technology, the effect of water rock reaction on pore size in porous media, interaction of pore connections and limit of movable pore throat were explored. Through advanced technologies such as ICP-MS, SEM and EDS, the ion concentration, ion activity product and mineral saturation index during the process of dynamic dissolution of mineral by carbonic acid were also systematically studied. The thermodynamic parameters of water-rock reaction were obtained, and the surface micro-morphology, elements, and mineral composition characteristics was investigated. The mechanism of the dissolution of supercritical CO2 and formation water in tight oil reservoirs is revealed. This provides the scientific basis and theoretical support for the application of supercritical CO2 flooding technology on continental tight reservoirs in China.

针对致密油藏孔喉细小、束缚水含量高、能量补充困难等特点，CO2因其优异的传质、增能等特性备受关注。本项目首先获取超临界CO2在致密岩心中的表面粘度图版，探究CO2动态溶解过程中气水两相压力和流量的非线性数学关系，考察长期水岩反应对渗流能力、束缚水饱和度、残余CO2饱和度等参数的影响，明确启动压力梯度变化特征，阐明不同流体饱和度下超临界CO2-水两相渗流特征。通过核磁共振技术，探索水岩反应对多孔介质孔喉尺寸、连通性及可动孔喉下限的作用特征，明确水岩反应对孔喉特征的影响机制。借助电感耦合等离子体质谱、扫描电镜、元素分析等多种先进技术，系统研究矿物溶蚀过程中离子浓度、离子活度积和矿物饱和度指数的变化规律，获得水岩反应热动力学参数，阐明表面微观形貌、元素以及矿物成分变化特征，进而揭示超临界CO2和地层水对致密储层的溶蚀作用机理，为超临界CO2驱技术在我国陆相致密油藏的应用提供科学依据和基础理论支持。

针对致密油藏孔喉细小、束缚水含量高、能量补充困难等特点，CO2因其优异的传质、增能等特性备受关注。本项目以鄂尔多斯盆地长6储层为研究对象，建立了致密岩心超临界CO2-水两相相对渗透率稳态测量方法，考察了溶蚀作用对岩心孔隙度、渗透率、最小可动孔喉尺寸的作用特征，明确了溶蚀前后岩心和单矿物的组成和溶蚀液离子成分变化特征，获得了不同矿物对碳酸的敏感性差异，揭示了岩心和单矿物溶蚀反应程度的相关性。研究发现，两相渗流过程中，溶蚀后CO2相对渗透率减小，水相相对渗透率增大。束缚水饱和度降低22.3%-33.8%，两相同流区增大26.2%-42.4%。注入压力从15 MPa上升至25 MPa，岩心气相渗透率提高了12.0%-18.8%，液相渗透率上升了27.5%-34.4%，T2截止值减小了9.7%-26.7%。目标储层中方解石和白云石对碳酸敏感性最强，溶蚀率最大。钾长石和钠长石对碳酸敏感性适中。矿物敏感性越强，在储层中的溶蚀程度和单独被碳酸所溶蚀的程度越接近。研究可为致密油藏超临界CO2驱技术应用提供基础理论支持。