基于流-固-化耦合理论的欠平衡钻水平井井壁失稳机理研究

Horizontal underbalanced drilling technology has been one of the most efficient ways during exploration and development of oil and gas fields, whereas wellbore instability problems severely affect the application of horizontal underbalanced drilling technology in drilling and probing of oil and gas. Wellbore instability in horizontal underbalanced drilling was mostly analyzed from the aspect of mechanics in the previous studies, which ignored the chemical effect on wellbore from drilling fluid, so inaccuracies of wellbore instability analysis will exist when only mechanical analysis is implemented. In order to solve wellbore instability problems in horizontal underbalanced drilling, this study plans to combine drilling core experiment and numerical analysis to explore the mechanism and microcosmic process of drilling fluid's penetration into different lithostratigraphy during liquid phase horizontal underbalanced drilling and gas-liquid phase horizontal underbalanced drilling. Then, the exact chemical effect of drilling fluid on wellbore will be discussed, and combining the result of mechanical analysis and seepage effect, a fluid-solid-chemistry coupling model will be established. Finally, wellbore collapse pressure model for horizontal underbalanced drilling will be set up, and considering the influence of length of horizontal segment and maintaining wellbore stability on the design of appropriate negative differential pressure, methods for designing appropriate negative differential pressure will be presented. The research results will be of great academic value to fundamental researches of wellbore stability and underbalanced drilling, which also promote the use of horizontal underbalanced drilling technology.

欠平衡钻水平井技术是油气田勘探开发过程中的一种高效技术，而井壁失稳问题严重影响着其在油气钻探中的推广应用。以往的欠平衡钻水平井井壁失稳问题大多只从力学上进行研究，而忽略了钻井液对井壁岩石的化学作用，从而就造成了井壁失稳分析的不准确性。本项目拟通过岩心实验和数值模拟相结合的方式，对液相欠平衡和气液两相欠平衡钻井条件下，水平井不同岩性地层钻井过程中钻井液渗入地层的机理和微观过程进行研究；揭示钻井液化学作用对井壁失稳的影响机理，结合欠平衡钻水平井井壁力学失稳和渗流作用的分析结果，建立井壁失稳的流-固-化耦合分析模型；研究准确计算欠平衡钻水平井井壁坍塌压力模型，进一步分析水平井水平段长度和维持井壁稳定对合理负压差设计的影响，形成欠平衡钻水平井合理负压差的设计方法。研究成果对井壁稳定与欠平衡钻井的基础理论研究具有重要学术价值，对促进欠平衡钻水平井的应用具有重大实际意义。

在本基金项目的支持下，经过详实、细致地研究分析，已达到了预期的研究目标。基本明确欠平衡钻井条件下钻井液浸入地层的机理和微观过程，对钻井液对井壁岩石的化学作用、渗流作用对井壁稳定的影响有了明确的定性和定量化的规律性认识。已建立了一套基于流-固-化耦合作用针对欠平衡钻水平井的井壁坍塌压力计算模型，该模型可综合考虑欠平衡条件下，原地应力、钻井液液柱压力作用、井周岩石内部的渗流作用，钻井液与井周岩石化学作用、渗吸产生的毛管力作用对水平井钻井过程坍塌压力的影响。基于该模型的应用实例证明了该模型能够准确可靠地为欠平衡钻井合理负压差设计提供指导。