页岩气扩散渗流机理及产气规律研究

The successful development of the shale gas must be based on deep understanding of the discipline of gas production, but the transport mechanism of shale gas through matrix pores is the bottleneck of conquering the difficulties in shale gas development. The scientific problem, transport mechanism of shale gas through matrix pores and gas production discipline, can be divided into: (1) What’s the wettability of shale matrix? (2) What’s the distribution of the water in nanoscale shale matrix pores? (3) What’s gas occurrence mechanism and state in organic and inorganic matrix pores? (4) Is the transport mechanism of shale gas through matrix pores diffusion, seepage, or other mode of motion? (5) How to forecast the shale gas production? The influence of organic and inorganic mass ratio was not considered in the present transport models of shale gas through matrix pores, which results in an unclear recognizing of the shale gas production discipline and the large derivation between prediction results by the present models and actual performance of shale gas wells. Therefore, in this work, the size distribution of shale matrix pores and the water distribution in shale matrix pores are investigated first, then diffusion-slippage-flow coupled transport mechanism of shale gas through matrix pores is proposed, and the transport model considering the organic content and the range of Knudsen number is established. Finally, the relevant gas production model is established and the shale gas production discipline is revealed. Our study will provide a more accurate technique for forecasting shale gas production, and moreover, it will provide scientific evidence to the rational development of shale gas reservoirs.

页岩气的成功开发须建立在认清产气规律的基础上，而页岩气在基质孔隙中的传递机理是攻克产气规律的瓶颈。页岩气传递机理及产气规律这一科学问题可分解为：（1）页岩基质块的润湿性如何？（2）页岩有机质-无机质孔隙中气水分布及差异性？（3）页岩有机质-无机质孔隙中甲烷的赋存机理及赋存状态？（4）页岩气在基质纳米孔隙中的传递方式是扩散、渗流还是其他运动方式？（5）页岩气井产气规律如何预测？目前页岩气在基质孔隙中的传递模型没有考虑有机质与无机质含量对传递的影响，造成气井产气变化规律认识不清，现有模型预测结果与实际生产动态偏差较大。因此，本课题拟从页岩基质孔隙分布和气水分布入手，研究页岩气在基质孔隙中扩散、渗流多机理耦合的传递机理，建立考虑有机质含量和努森数范围的传递模型，进而建立相应的产气模型，揭示页岩气井的产气规律。本研究将为页岩气界提供一套更准确的生产动态预测方法，为页岩气的合理开发提供科学依据。

随着油气源的不断开采，各大主力油气田采出程度升高，含水率上升，全球目光逐渐转移向非常规资源，其中页岩气是目前最受关注的资源之一，但是目前中国页岩气开发效果仍不理想，源于理论认识不清，第一，对页岩储层的认识，包括孔隙类型、气水分布等；第二，对赋存方式的理解，因为页岩基质类型不同，气体吸附的机理也不相同，尤其加上含水的存在，使得吸附机理变得更加复杂；第三，对页岩气气体传输规律的认识不清楚，页岩孔隙结构复杂，小到纳米级的基质孔隙，大到毫米级的裂缝，再加上应力敏感、吸附解吸、真实气体效应的影响，使得传输规律更加复杂。.本研究主要也分为三个方面，首先基于页岩成藏理论，分析了气水分布的特征，解释了页岩含水饱和度的规律，包括“超低含水饱和度的原因”，页岩储层孔隙初始都被水填充，由于有机质生烃排水作用，有机质孔隙仅存在少量束缚水，而无机质孔隙普遍含水饱和度较高；第二，分析页岩气吸附的类型，在不同的孔隙基质中，有机质孔隙吸附类型主要以气-固界面为主，而无机质孔隙气-液吸附为主要形式，并建立了相应的吸附模型；然后推导了不同孔隙形状的表观渗透率模型，进行敏感因素分析，孔隙形状越扁平渗透率越低，应力敏感和吸附作用对渗透率影响较明显，而真实气体效应可以忽略；最后建立页岩气双孔双渗模型并用有限差分方法进行数值求解，得到页岩气产能预测曲线，进行敏感因素分析，应力敏感早期影响产量，解吸作用晚期影响产量，而真实气体效应对产量的影响可以忽略。.本基金研究成果可为页岩基质孔隙中气-水分布特征、气体传输机理和页岩产气规律的认识提供理论指导和参考依据，对页岩气藏合理开发具有指导意义。