纳米颗粒和防聚剂对钻井液中水合物形成的影响研究

As potential alternative energy sources, well drilling operations are integral to the exploration and development of natural gas hydrates buried in sediments. To maintain the stability of a borehole and the safety of a well, the drilling operations in gas hydrate-bearing sediment (GHBS) is typically performed under the overpressure conditions. In these conditions, drilling fluid would penetrate into the GHBS due to differential pressure. In addition to damaging the hydrate reservoir, the invasion of the drilling fluid might also induce the hydrate dissociation and reformation under certain conditions and cause wellbore instability and distortion of the data from well logging. To eliminate the adverse effects while maintaining the wellbore stability, the filter loss of drilling fluid should be reduced as much as possible. Increasing the anti-filtration and pore or fracture plugging ability of the drilling fluid is an effective method for reducing the invasion of the drilling fluid and enhancing the wellbore stability. The application of nanoparticles is an effective means for implementing this method. However, the addition of nanoparticles may also introduce new risk factors, as studies have indicated; the addition of solid particles, especially nanoparticles, can facilitate hydrate formation and growth. Thus, reducing the invasion level of drilling fluid while preventing the large-scale formation and aggregation of hydrates in wellbores is a critical issue to be resolved for the application of nano-based hydrate drilling fluid..According to this issue, this project will investigate the characteristics of gas hydrate formation in fluids with nanoparticles by using molecular simulations and micro/macro measurements and focusing on the properties of nanoparticle surfaces and interactions of solid-liquid-gas interface. The goal of this project is to understand the influence of surface property, particle size, amount of nanoparticles and its combination with anti-agglomerant on hydrate formation and accumulation, and finally reveal the mechanism for this influence. Our work can provide a guidance for selecting type and addition amount of nanoparticles, designing a drilling fluid system with low filtration, high instability prevention and hydrate inhibition. This is of great significance to the application of nanoparticles in borehole stability, reservoir protection, sealing fractures caused by hydrate dissociation etc., and even the successful implementation of natural gas hydrates production tests in near future in the South China Sea.

研究和实践表明，钻井液侵入会严重影响含水合物地层井壁稳定和测井识别与评价。加入纳米固体颗粒是降低钻井液侵入并增强井壁稳定的有效方法之一。但在钻井液中添加纳米颗粒却有可能促进井内水合物形成和聚集进而诱发井内安全事故。因此，如何降低水合物风险是纳米材料应用于非常规水合物钻探需要解决的关键问题。本项目将在承担完成的两个国家自然科学基金项目研究基础上，以纳米颗粒表面特性及界面相互作用为突破口，运用微观分子模拟和宏微观实验测试手段来深入研究含纳米颗粒流体中水合物形成特征，掌握纳米颗粒表面性质、粒径、含量及其与水合物防聚剂共存条件下对水合物形成和聚集的影响规律，揭示其内在机理，为后续优选纳米颗粒类型和加量、设计低滤失强护壁高水合物抑制性钻井液体系以及确定合理的机械转速提供理论和技术指导。上述研究对水合物地层井壁稳定、水合物储层测井评价和储层保护乃至后续南海水合物试开采都具有重要的理论意义和应用价值。

天然气水合物是重要的潜在清洁能源。而勘探开发天然气水合物离不开钻井与钻井液，钻井液的侵入会严重影响含水合物地层井壁稳定性以及测井识别和评价。海洋区域水合物储层大多渗透性较差，裂隙较发育，因此可在钻井液中添加适量纳米颗粒以提高体系裂隙封堵和稳定井壁的能力，这也是弱化钻井液侵入储层的有效方法之一。.但是纳米颗粒加入到钻井液中也可能带来负面影响，比如可能改变水合物在钻井液中的形成特性，从而诱发水合物相关风险。针对这一重大技术难题，本项目首先对不同类型、不同粒径、不同添加量的纳米颗粒对CH4水合物形成的影响规律进行了系统研究，在此基础上利用多种实验方法并引入分子动力学模拟，多角度、多尺度、深层次地揭示了纳米颗粒（SiO2和CaCO3颗粒）影响水合物形成的内在机理。进而研究了纳米颗粒对水合物热力学相平衡的影响规律以及纳米颗粒与水合物抑制剂复配后对水合物形成和聚集的影响，最后评价了所设计新型水合物钻井液体系性能，进而提出了相应的钻井液方案。研究结果发现亲水型纳米颗粒在一定颗粒粒径及添加量下能够抑制CH4水合物的形成；而疏水型纳米颗粒而言，其在不同粒径和添加量条件下均对水合物的形成起到促进作用。亲水纳米颗粒因其亲水特性，能够对周围水分子产生束缚作用，从而阻碍水分子定向排列形成包裹CH4分子的笼型结构。而且受亲水纳米颗粒影响，CH4分子会更容易聚集形成“纳米气泡”。疏水纳米颗粒在其疏水特性的影响下，会促使溶入流体中的气体分子在其附近快速扩散，并有一部分气体分子吸附和聚集于其表面，使得纳米颗粒附近气-液接触面积有所增大。最终确定了模拟钻井液的最佳基础配方为：1.0-2.0%低黏CMC + 0.1-0.2%PAM + 0.1-0.2%XC + 5.0-10.0%NaCl +5.0-8.0%KCl + 1.0-2.0%PVCap + 1.0-6.0%纳米CaCO3。该体系性能优良，成本较低，无毒无害，可以用于海洋水合物钻井。.上述研究为水合物钻井液体系的构建提供了全新的思路和极具价值的理论和技术参考，对水合物安全高效钻采有重要意义。