考虑毛管力对相平衡影响的致密油藏二氧化碳驱数值模拟

Nanoscale pores are predominant pore spaces in tight oil reservoirs. Small-pore sizes on the order of nanometers can lead to a large capillary pressure. A number of studies has addressed that the high capillary pressure in nanometer pores has a significant effect on the phase behavior of hydrocarbon mixtures. On the other hand, several recent investigations and numerical simulation have concluded that significant increases in ultimate oil recovery might be possible using carbon dioxide injection for tight formation. In this work, a compositional modeling is presented to simulate carbon dioxide flooding in tight oil reservoirs including effect of capillary pressure on phase equilibrium. A phase equilibrium algorithm including effect of capillary pressure is proposed for carbon dioxide-hydrocarbons-water system based on Gibbs free energy minimization. The complex fractured system around horizontal well in tight oil reservoirs are described as a set of four interacting fractures subdomains. On the basis of finite volume method, we propose a combination model which is capable of coupling continuum with discrete fracture network in one space subdomain. A compositional tight oil simulator for carbon dioxide flooding is developed on the basis of the combination model and the phase equilibrium algorithm including effect of capillary pressure. Simulation studies of concept model and carbon dioxide Huff-n-Puff process in tight oil reservoirs are done. This work can provide a better understanding of the physical mechanisms and key parameters affecting the effectiveness of carbon dioxide injection for enhanced oil recovery in tight oil reservoirs.

纳米级孔隙是致密油藏的主要储集空间。大量研究表明，纳米级孔隙中的高毛管力对烃类组分混合物的相态有很重要的影响，不可忽略。另一方面，近年来的实验观察和数值模拟结果表明，致密油藏二氧化碳驱能够显著提高油气的最终采收率。本项目主要研究能够考虑毛管力对相平衡影响的致密油藏二氧化碳驱的组分数值模拟。基于Gibbs自由能最小化法，提出能够考虑毛管力对二氧化碳-烃-水体系相态影响的相平衡算法。采用分区域描述法，将致密油藏水平井周边复杂裂缝系统划分为相互关联的四种裂缝子域。基于有限体积法，建议一个能够同时适用于多重介质和离散裂缝网络的复合模型。基于上述复合模型和能够考虑毛管力对相态影响的相平衡算法，研制致密油藏二氧化碳驱的组分数值计算模拟器，并对若干个概念模型和致密油藏二氧化碳吞吐采油进行数值模拟研究。本项目的完成能够揭示致密油藏二氧化碳驱的机理、分析影响采收率的关键因素。

二氧化碳驱能够十分有效地提高致密油藏油气的采收率。与常规油气藏不同，致密油藏具有低孔、低渗的特点，储集层中纳米级尺度孔隙发育。纳米级孔隙的高毛细管力，影响油气在孔隙中的流动过程和油气的相态平衡，这种纳米限域效应最终会影响致密油藏油气的采收率。传统的数值模拟方法无法有效地考虑纳米限域效应的影响和致密储层水平井周边的复杂裂缝系统，因而也就无法准确计算纳米孔隙中油气的相态变化和预测致密油气的产量。通过本项目的研究，将纳米级孔隙中气液的毛管力差引入到了Peng-Robinson状态方程中，基于Rachford-Rice方程的闪蒸计算法，建立了能够考虑纳米级孔隙中毛管力效应的相平衡计算方法；致密储层水平井附近存在复杂的多等级裂缝系统，提出了一种多裂缝嵌入式数值计算模型来改进对复杂裂缝系统数值模拟的精度和效率；基于多裂缝嵌入式模型，建议一个能够同时适用于连续介质和离散裂缝网络的复合模型；基于上述复合模型和能够考虑毛管力对相态影响的相平衡算法，研制了致密油藏二氧化碳驱的数值计算模拟程序；采用多裂缝嵌入模型，描述地层中的天然裂缝与水力压裂的人工裂缝构成复杂裂缝网络，对实际致密油藏二氧化碳驱进行数值模拟，分析了纳米限域效应对油藏多井产量的影响；模拟结果表明，毛细管力效应对油气组分的临界性质有明显影响，降低了最小混相压力，相平衡的毛细管力效应对于致密油气产量的准确预测不可忽略。本文的研究成果对于致密油藏二氧化碳驱的机理揭示和油气采收率的准确预测具有十分重要的意义。