岩心尺度静态自发渗吸的数值模拟

Numerical simulation of static spontaneous imbibition at the core scale

基于渗流理论、化学势以及嵌入式离散裂缝模型,推导了新的静态渗吸数学模型,模型考虑了毛细管力、重力、溶质浓度差导致的渗透压3种渗吸驱动力对渗吸采油的影响,并利用现有研究验证了模型的准确性。通过数值计算,着重研究了不同驱动机制对渗吸采出程度的贡献,分析了浸泡液矿化度、岩块壁面膜效率、岩块尺寸对采出程度的影响。结果表明:静态渗吸采油过程中,毛细管力、重力以及渗透压都为重要的渗吸驱动力;受多种渗吸驱动力影响,岩块内部含水饱和度呈一定规律变化;早期渗吸中,毛细管力、重力对采出程度的贡献较大;长远来看,渗透压主导的持续渗吸作用对采出程度的贡献也不可忽视。浸泡液矿化度与岩块采出程度呈近线性负相关;岩块尺寸与采出程度非线性负相关;岩块壁面膜效率与采出程度非线性正相关,并且膜效率的增加可以延长高渗透压主导渗吸采油的时长。在水力压裂作业和注水开发过程中,适当降低压裂液和注入流体矿化度值以及增加水力改造裂缝网络复杂程度,可以强化初期渗吸采油效果,缩短渗吸时间,提高最终采收率。

A new mathematical model of static imbibition has been derived based on flow theory, chemical potential, and embedded discrete fracture model. The model considers the effects of three imbibition drivers caused by solute concentration difference, i.e., capillary force, gravity, and osmotic pressure, on imbibition oil recovery. The accuracy of the model has been verified using published studies. Through numerical calculations, this study focuses on the contribution of different driving mechanisms to the imbibition recovery rate, and analyzes the effects of soaking fluid mineralization, rock wall membrane efficiency, and rock block size on the recovery rate. The results show that capillary force, gravity and osmotic pressure are all important driving forces in static imbibition oil recovery;the water saturation inside the rock mass varies in a certain pattern under the influence of various imbibition driving forces;the contribution of capillary force and gravity to the recovery rate is great during early imbibition;in the long run, the contribution of continuous imbibition dominated by osmotic pressure to the recovery rate cannot be ignored. Soaking fluid mineralization is nearly linearly and negatively correlated with the block recovery rate;block size is nonlinearly and negatively correlated with the recovery rate;rock wall membrane efficiency is nonlinearly and positively correlated with the recovery rate, and the increase of membrane efficiency can prolong the duration of imbibition oil recovery dominated by high osmotic pressure. During hydraulic fracturing operation and water injection development, to appropriately decrease the mineralization value of fracturing fluid and injected fluid as well as increase the complexity of hydraulically modified fracture network can enhance the initial imbibition recovery efficiency, shorten the imbibition time, and improve the final recovery rate.