基于多场耦合理论的页岩气水平井井壁失稳机理研究

Horizontal wells have been one of the most important technologies during the development of shale gas, while the problem of wellbore instability always reduces the efficiency of horizontal wells in shale gas drilling. In the former study of wellbore stability of shale gas drilling in horizontal wells, there are few systematic researches about multi-field coupling study of wellbore stability and its effects on structures of formation rocks, which induce the deviation from the actual situations and the inaccuracy of results from wellbore stability analysis. This study starts with the theory of multi-field coupling, and with oil based drilling fluid and water based drilling fluid considered, invasion of filtration from drilling fluid, solute diffusion and ion transport in micro structures of shale will be studied, after which the change of parameters of micro structures will be obtained; meanwhile, effects of fluid in shale formation on wellbore stability will also be quantified, and combined with the change of parameters of weak planes and rock mass in shale, mechanism of multi-field coupling in wellbore stability is to be explored when effects of weak planes are considered, and multi-field coupling model for wellbore stability analysis will also be established in shale gas drilling of horizontal wells; finally, collapse pressure model will be obtained, and the model for limit extension length of the horizontal section will also be established. Results of the study have a great academic value for development of basic theories in shale gas drilling and wellbore stability analysis, which is also of great practical significance.

水平井是页岩气开发过程中的一种重要技术，而井壁失稳问题严重影响着页岩气水平井的开发效率。在以往的页岩气水平井井壁失稳分析过程中，系统考虑多场耦合作用及其对页岩储层结构影响的研究较少，从而造成了与实际情况的偏差和分析结果的不准确。本项目拟从多场耦合理论出发，在油基钻井液和水基钻井液条件下，研究页岩微观结构中钻井液滤液侵入、溶质扩散及离子运移等规律，分析多场耦合下微观结构参数的变化；揭示页岩储层所含流体对井壁稳定的定量化影响，结合页岩弱面和本体参数在多场耦合下的变化，探索弱面影响下井壁失稳的多场耦合机理，建立页岩气水平井井壁失稳的多场耦合分析模型；研究多场耦合下的水平井井壁坍塌压力计算模型，进一步建立水平段极限延伸长度的设计模型。研究成果对于丰富页岩气钻井和井壁稳定的基础理论具有重要学术价值和实际意义。

在本基金项目的支持下，经过本项目研究团队详实、细致地研究，现已基本达到了预期的研究目标。通过本项目研究，主要取得了以下成果：（1）研究了页岩气水平井应力、化学和渗流耦合作用下，钻井液浸入对页岩微观结构的影响，分析了化学势差作用下的压力扩散规律和含水饱和度变化规律；（2）考虑弱面影响条件下，分析了弱面产状、强度以及页岩地层层理各项异性对钻井井壁稳定的影响；（3）建立了综合考虑地应力差异、毛管力、含水饱和度、温度、渗流效应的流固热化多场耦合井壁稳定模型，基于该模型对页岩气钻井井眼方位和井斜角进行了优化。