南海北部水合物水平井降压开采储层力学响应特征与稳定性研究

According to existing gas hydrate exploitation experiences, the recovery efficiency of gas hydrate is low. The horizontal well technology is an important method to realize the efficient exploitation of gas hydrate. However, the application of this method is restricted by the problem of reservoir stability because of the shallow-buried and low-strength reservoir in the northern South China Sea. Due to the limitations of numerical simulation, the mechanism of reservoir instability in the extraction of gas hydrate by horizontal wells and depressurization is still lack of systematic and in-depth understanding. In this study, a fully coupled hydro-thermo-chemo-geomechanical model is proposed based on the developed constitutive model in the previous work to study the fluid flow and geomechanical behavior of gas hydrate-bearing sediment. The reservoir deformation and wellbore stability under multiple factors are studied. Based on the above research, the mechanism of reservoir and wellbore instability of gas extraction by depressurization of horizontal wells is revealed. The relationship between gas production capacity, parameters of horizontal well and reservoir stability is clarified. The research results of this project will provide technical reserves and scientific basis for the successful application of horizontal well in marine gas hydrate exploitation and safe efficient hydrate production.

国际上试采表明水合物的开采效率与工业应用标准尚有较大差距，水平井技术是实现水合物高效开采的重要途径之一。然而，由于南海水合物储层浅、地层强度低，水平井面临的储层稳定性问题限制了该方法的应用。受数值模拟手段的限制，目前水合物水平井降压开采储层失稳的机理还缺乏系统深入的认识。本项目结合前期力学实验和数值模拟技术，建立水平井降压开采条件下的热-流-固-化多场全耦合数学模型，通过发展高效的数值计算方法，获取水平井开采条件下的气水运移规律和储层力学响应特征，揭示水平井降压开采条件下储层和井筒失稳的机理，阐明产气能力、水平井参数和储层稳定性三者之间的关系。解决多重因素控制下的储层和井筒稳定性问题，为水合物安全高效开采提供技术储备和科学依据。

现场试采的结果表明，当前天然气水合物开采面临的两大难题是产量低和存在储层失稳风险。水平井是一种提高天然气水合物开采产量的有效手段，但由于储层埋藏浅、弱固结等特点，水平井开采面临的储层失稳风险相较于直井更高。数值模拟是进行水合物开采产能和储层力学稳定性分析的重要手段，但目前的数值模拟主要关注产能分析，而耦合产能和力学稳定性的流固耦合同步计算方法还不成熟。本项目针对天然气水合物水平井开采的储层稳定性问题，建立了描述传热、渗流、相变和固体变形的多物理场耦合模型，揭示了各物理过程的耦合机理并建立了耦合关系模型。针对水合物开采流固耦合问题，发展了基于非结构网格的控制体有限元-有限元混合的数值算法，实现了复杂问题的建模和分析，同时解决了流固耦合计算时网格不相容和插值的问题。针对储层稳定性分析，创新提出了稳定系数的概念和计算方法，实现了储层稳定性的全区域的定量化分析。项目研发了具备完全自主知识产权的数值模拟软件QIMGHyd-THMC。针对水平井开采开展数值模拟研究，揭示了水平井开采时失稳的高风险区域为水平井井筒的顶底部，水平井越长、位置越靠近水合物层顶面失稳风险越大，水平井开采时的储层失稳风险主要发生在开采早期。本项目建立的数值模拟软件和揭示的水平井开采失稳机理将为水合物开采过程的传热传质规律分析、现场开采方案的设计和开采技术评价提供有力支撑。