水合物分解对固井二界面胶结质量的影响机理

Natural gas hydrates are widely distributed in the ocean sediments, permafrost regions and lake sediments. The large amounts of gas hydrate reserves make them significant to solving the growing tension of traditional fossil fuel crisis, if they are successful exploited. However, there are still many key technical problems need to be solved during the process of commercial exploitation, one of the most important is safe and efficient gas production, and well cementing is the essential to achieving this goal..While well cementing is the important method to maintain the stability of gas hydrate formation, improve gas production capacity and avoid gas channeling and blowout, the cement hydration heat will lead to the hydrate dissociation in the formation around the wellbore, decomposed high pressure free gas and water invasion into cement slurry, which cause the decreased or even loss of the cementation strength and the sealing ability, and often make major drilling accidents happened. .This project will be based on the doctoral dissertation， the effects of high pressure free gas and water invasion on the development of micro cracks in the cement sheath, the cementing quality and sealing ability of the second cementing interface will be studied by experiment tests and numerical simulations. The criteria for judging the critical conditions of high pressure free gas and water reversely invade into cement slurry will be built, the invasion mechanism of high pressure free gas and water, the influence on internal micro cracks and macro mechanical parameters of cement sheath will be studied, the relationship between the initial temperature and pressure difference of cement slurry, the developmental degree of micro cracks in cement sheath, the cementation and sealing strength of the second cementing interface will be built, so as to lay a theoretical foundation for the design of low temperature cementing slurry system and cementing technology..The studies above undoubtedly have important theoretical significance and application value on hydrate resource efficient production, also have important strategic significance to deal with the energy shortage in our country, the protection of national energy security and sustainable development of the society.

当传统油气固井遇到天然气水合物时，固井水泥浆水化热很可能导致水合物分解，产生局部高压。水合物分解产生的游离气和水在局部高压作用下会向井壁运移，影响二界面胶结质量，导致储层封隔能力下降或失效、甚至固井报废等严重钻井事故。基于此，本项目将在博士学位论文研究的基础上，运用实验测试和数值模拟的方法研究水合物分解产生的高压游离气、水的侵入对水泥环微裂隙发育、二界面胶结强度与封隔能力的影响。建立高压游离气、水侵入水泥浆的临界条件判别准则，掌握高压游离气、水对水泥石内部裂隙结构及其力学强度和二界面胶结质量的影响规律，建立水泥环内部裂隙发育程度—水泥环力学性质与二界面胶结强度和封隔能力的响应关系，从而为水合物地层低热固井水泥浆体系与固井工艺的设计奠定理论基础。上述研究对水合物资源安全高效开采具有重要的理论意义和应用价值，同时对我国应对能源短缺、保障国家能源安全和社会可持续发展具有重要的战略意义。

天然气水合物广泛地分布于陆地冻土和海洋沉积物中，当传统油气固井（尤其是深水环境）遇到天然气水合物时，固井水泥浆水化放热极易导致近井壁地层水合物分解，产生局部高压游离气水，在压差的驱替下易反向越过井壁，侵环空水泥浆中，从而影响二界面和水泥环密封、力学性能，导致固井质量下降或失效，甚至产生报废等严重事故。基于此，本项目针对固井过程中水泥浆水化放热对近井壁水合物地层物性的影响，水合物分解产生的高压气水反向侵入环空水泥浆的临界判别条件，以及高压游离气水反侵产生的水泥环内部裂隙结构对其力学性质与二界面胶结质量的影响规律等研究内容，开展了一系列物理试验模拟和数值计算分析。.通过本项目的研究，获得以下重要认识：（1）开发出了一套基于吉洪诺夫正则化的人造天然气水合物地层骨架数字化制备技术，建立了涵盖制备工艺、宏观物性与微观孔隙3方面参数的关系模型和多元回归方程，在已知一方参数的条件下可以快估算出另外两方，并且在对冻土区储层骨架的模拟中取得了良好的应用效果。（2）采用“连续分段模拟”，分析评价了不同固井水泥浆水化热和固井压差条件下水泥浆对近井壁地层物性的影响以及气水反侵临界条件，得出“压有差、内有气”的气水反侵临界判别条件，建立了水泥浆水化热（0.10-0.25J/g/s）和固井压差（0-10MPa）范围内气水反侵临界条件定量判别曲线，总结出“浅低热深高压”的固井工艺参数选取原则。（3）得出了不同程度（反侵平均速率0-0.38 L/min，反侵量0-3.59%，反侵压差0-0.05 MPa）的高压气水反侵行为对水泥环内部微裂隙结构和形态，以及不同特征裂隙（裂隙长度0-3 mm，长宽比2.5-10，倾角0-90o）对水泥环与二界面力学和密封性能的影响，掌握高压气水反侵对水泥石内部裂隙结构及其力学强度和二界面胶结质量的影响规律。.本项目研究成果对深水油气开采的现场固井工艺参数优选，提高固井质量和降低成本具有良好的指导作用，同时对我国应对能源短缺和保障国家能源安全具有重要的战略意义。