海域天然气水合物低温抑制性钻井液体系

针对海域天然气水合物钻探所面临的深水低温、浅层窄安全密度窗口、水合物生成与分解抑制、海洋作业环保要求等工程和技术难题，开发了低温抑制性钻井液体系。研发了热力学抑制剂KCl复配动力学抑制剂A2的水合物抑制技术，优选了低温流变稳定性能良好的高分子处理剂，开发的钻井液体系流变性能良好，膨润土含量为2 %，API滤失量为4 mL，密度为1.07 g/cm3；低温流变稳定性优良，4 ℃较25 ℃时钻井液当量循环密度最大增量为0.004 g/cm3，优良的流变学性能有利于深水浅部地层窄安全密度窗口井段的井壁稳定和井眼清洁。该体系还具有优异的水合物生成抑制性能，4 ℃、20 MPa、20 h内完全抑制水合物生成，陈化10 d及被钻屑污染后抑制性能保持良好，同时具备明显的水合物分解抑制性能。各处理剂重金属含量达标，钻井液体系生物毒性半有效浓度EC₅₀值及半致死浓度LC₅₀值均大于30 000 mg/L，满足我国一级海区环保要求。该钻井液的综合技术性能满足海域天然气水合物钻探技术要求。 

A Drilling Fluid Which Inhibits Formation of Natural Gas Hydrate at Low Temperatures in Offshore Drilling

A low temperature inhibitive drilling fluid was developed for use in offshore natural gas hydrate drilling to deal with several technical challenges such as low temperature in deep waters, narrow safe drilling window in shallow formations, inhibition of the formation and decomposition of gas hydrate and requirements on environment protection etc. The low temperature inhibitive drilling fluid was formulated with a gas hydrate inhibitor, which was developed by compounding the thermodynamic inhibitor KCl and a kinetic inhibitor A2, and high molecular weight additives with superior low temperature rheology stability. This drilling fluid has good rheology. The bentonite content, API filter loss and density of this drilling fluid are 2%, 4 mL and 1.07 g/cm3, respectively. It has a stable low temperature rheology: the ECD at 4 ℃ is only 0.004 g/cm3 (max.) higher than that of the drilling fluid at 25 ℃. This excellent rheological property is beneficial to the borehole wall stability and borehole cleaning in drilling the shallow formations in deep waters with narrow safe drilling windows. This drilling fluid also shows excellent performance in inhibiting the formation of gas hydrate. At 4 ℃ and 20 MPa, gas hydrate is completely inhibited in 20 h. After aging for 10 d and being contaminated with drilled cuttings, the drilling fluid still retains good inhibitive capacity, and is able to decompose the gas hydrate formed. The contents of heavy metals in the additives used to formulate the drilling fluid meet the requirements of relevant standards, the EC₅₀ and LC₅₀ of the drilling fluid are all greater than 30 000 mg/L, satisfying the needs of first-level environment protection in the sea area in China. The overall technical performance of this drilling fluid satisfies the technical requirements of drilling natural gas hydrate in offshore area.