改性纳米纤维素隔离膜和温压场协同调控天然气水合物相态机理

Natural Gas Hydrates (NGH) is a potential source of alternative, clean and highly-efficient energy. The horizontal drilling is one of the effective ways to efficiently exploit NGHs. However, the prolonged contact between drilling fluids and NGHs during drilling may lead to excessive drilling fluid invasion and subsequently NGH phase transition, which accordingly causes wellbore instability. Such processes intensify difficulties in horizontal drilling operation. This project focuses on the NGH phase state stability during drilling, and analyzes the mass and heat transfer between drilling fluids and NGHs as well as the NGH dissociation front migration. A new idea of synergistic control of gas hydrate phase state by modified nano-cellulose isolation membrane and temperature-pressure field was put forward. With the help of numerical computation and laboratory experiments, the mass and heat transfer between the horizontal wellbore and NGH reservoir is modelled, so as to characterize three-dimensional temperature and pressure distributions in the wellbore and NGH reservoir. An isolation membrane of modified nano-cellulose that is able to suppress mass transfer between drilling fluids and NGHs is developed. The mechanism of the modified nano-cellulose isolation membrane stabilizing NGH phase states is clarified, and synergistic contribution degrees of temperatures, pressures and modified nano-cellulose to NGH phase state stability and their interaction mechanisms are quantitatively characterized. This research reveals the modified nano-cellulose isolation membrane-temperature-pressure NGH phase state control and adjustment mechanisms during horizontal drilling, and resultant findings provide theoretical and technical support to development of safe highly-efficient drilling fluids with respect to NGH reservoirs.

天然气水合物是一种潜在的清洁高效接替能源，水平井是实现水合物高效开采的有效途径之一。水平井钻井过程中，钻井液与水合物接触时间长，钻井液侵入量多，导致水合物相变引起井壁失稳，水平井钻探施工难度大。本项目以深海浅层水平井钻探过程中天然气水合物相态稳定性为研究目标，创新提出改性纳米纤维素隔离膜和温压场协同调控天然气水合物相态新思路，借助数值计算、室内实验等手段，分析钻井液与水合物之间的传质传热和水合物分解界面的推移规律，建立水平井井筒-水合物储层传质传热模型，得到井筒和水合物储层内三维温压场分布形态。研发阻碍钻井液与水合物传质的改性纳米纤维素隔离膜，明确改性纳米纤维素隔离膜稳定水合物相态的机理，量化表征温度、压力和改性纳米纤维素对水合物藏相态稳定的协同贡献程度和相互作用机制，揭示改性纳米纤维素隔离膜和温压场协同调控天然气水合物相态机理，为水合物储层安全高效钻井液构建提供理论与技术支撑。

天然气水合物是一种潜在的清洁高效接替能源，水平井是实现水合物高效开采的有效途径之一。水平井钻井过程中，钻井液与水合物接触时间长，钻井液侵入量多，导致水合物相变引起井壁失稳，水平井钻探施工难度大。本项目提出了一种基于改性纳米纤维素和温压场协同调控储层天然气水合物相态新思路，建立了钻井液在井筒及水合物储层多孔介质中多相多组分非稳态运移控制模型。选取二轮试采复杂结构井钻井施工工况，模拟分析了地温梯度、海水深度、钻井液排量、钻井液注入温度对井筒环空和储层温压场的影响，揭示了深海浅层水平井井筒-水合物储层非稳态传质传热机理，厘清了钻井液侵入水合物储层水合物分解界面演化规律，随着钻井时间的增加，水合物分解界面呈现向储层深部运移的趋势，在12h后水合物分解界面向储层深部运移的趋势变缓。应用氨基硅烷偶联剂改性纳米纤维素，开展了改性纳米纤维素成膜及水合物抑制性能评价，发现改性纳米纤维素溶液处理的云母片表面粗糙度Ra为16.4 nm，形成了厚度可达60 nm的吸附膜，与云母片之间具有较强的亲和力；当水合物分解率达到80%时，纯水中仅需2.1小时，而改性纳米纤维素溶液中需要3.8小时，表明改性纳米纤维素有较明显的水合物分解抑制性。钻井液中加入3%的改性纳米纤维素隔离膜，能够是的钻井液泥饼渗透率显著降低，形成隔离传质屏障，有效降低了钻井液的侵入速度；在排量30 L/s、水深1250 m、钻井液注入温度为20℃下，储层水合物几乎不发生分解。应用粒子群算法实现了钻井液添加剂和温压场对储层天然气水合物相态协同调控，回归得到了含改性纳米纤维素隔离膜和钻井液注入温度、排量关系图版，钻井液施工参数选择在图版包络线里面范围，水合物不发生分解，保障储层水合物相态稳定。