海上低渗储层防水锁强封堵钻井液技术

南海西部北部湾盆地乌石区块低渗储层硬脆性页岩发育，孔隙喉道小，水敏性强。前期钻井过程中水锁伤害严重，表皮系数高，且井壁失稳问题突出，井眼扩径严重，钻井事故频发。在前期使用的钻井液基础上，通过优选氟碳防水锁剂，降低油水界面张力，使用成膜封堵剂及优化碳酸钙粒径配比，在加强井壁稳定的同时可显著降低钻井液滤液及固相颗粒侵入储层，进一步保护低渗储层。室内评价优化后的钻井液滤失量及砂床侵入深度得到显著降低，高温高压滤失实验滤液固相粒度分布显示固相侵入低，高温高压滤失实验泥饼表面扫描电镜观察显示致密光滑，水锁伤害率降低至17.5%，岩心渗透率恢复值大于80%。乌石区块现场应用表明，优化后的钻井液井壁稳定性好，井径扩大率仅为5.4%，储层保护效果较好，测试表皮系数仅为1.2。该钻井液技术对其他低渗储层钻井液设计有一定的借鉴意义。 

A Drilling Fluid with Water Block Preventive Capacity and Strong Plugging Capacity for Offshore Low Permeability Reservoir Drilling

The formation rocks of the reservoir in Block Wushi in Beibu Gulf, west of South China Sea, have low permeability and are developed with hard and brittle shales. Shales, plus small-sized pore throats and severe water sensitivity resulted in serious formation damage by water block, and high skin factors. Borehole wall instability and washout during drilling resulted in frequent drilling accidents. To resolve these problems, a new drilling fluid has been formulated based on the composition of the drilling fluid previously used, through selection of a fluorocarbon water block preventing agent, reduction of oil/water interfacial tension, using flming plugging agents and optimizing the size distribution of CaCO3 particles. This new drilling fluid not only helped stabilize borehole wall, it also had low fltration rate and was able to prevent drilling fluid particles from entering the reservoir formations, thereby protect the reservoir from being damaged. Laboratory evaluation results showed that the fltration rate of the drilling fluid and the invasion depth of solid particles and fltrate were all remarkably reduced. HTHP experiment and solid particle distribution in mud fltrate showed that the invasion depth of solid particles was reduced. SEM experiment showed that HTHP mud cakes were dense and smooth. The percent damage by water block was reduced to 17.5%, and the percent recovery of core permeability was greater than 80%. Field application of the new drilling fluid in Block Wushi showed that it has the ability of better stabilizing borehole wall; the rate of hole enlargement was only 5.4%. Skin factor was only 1.2, indicating that the drilling fluid is capable of protecting the reservoir. The drilling fluid technology used on this well is of reference signifcance in designing drill-in fluids for low permeability reservoir drilling.