南海西部超浅层气田水平井EZFLOW无固相弱凝胶钻井液研究与应用

南海西部L气田埋深浅、地层破裂压力低、泥岩含量高，水平井钻进过程中因泥岩造浆导致钻井液黏度、切力升高，循环当量密度（ECD）升高导致压漏地层问题频发。为此，利用岩心驱替试验结果和软件模拟计算ECD相结合的方法优化了EZFLOW钻井液的流变性，并采用高浓度泥岩造浆的方法优化了其抑制性能，用自制的高压填砂承压仪评价了其承压能力。室内评价试验显示，EZFLOW钻井液低剪切速率黏度为15 000~30 000 mPa·s时，在直接返排渗透率恢复率达到85%以上的前提下，ECD小于地层破裂压力；优化后的EZFLOW钻井液抑制性和封堵性强，能够抗25%现场泥岩侵污。现场应用发现，EZFLOW钻井液表观黏度随井深波动幅度小，ECD附加值最大仅为0.07 g/cm3，钻进及砾石充填过程中均未发生漏失，产气量均超过配产的10×104m3/d，最高达16×104m3/d。研究结果表明，优化后的EZFLOW无固相弱凝胶钻井液能够解决超浅层水平井破裂压力低导致压漏地层和保护储层的问题。 

Research and Application of EZFLOW Solid-Free Weak Gel Drilling Fluid in Horizontal Wells in Shallow Gas Fields in the Western South China Sea

The L Gas Field in the Western South China Sea has a shallow depth and low fracture pressure as well as high mudstone content,which increases viscosity and shear force due to the high rheology of drilling fluid and mudstone mud-making,as well as frequent lost circulation caused by high ECD in horizontal well drilling.Hence,the rheological properties of drilling fluid were optimized by combining the core displacement test results with software-simulated ECD calculations.The method of high-concentration mudstone mud-making was used to optimize the inhibition performance of the drilling fluid,and the self-made high-pressure sand-filled pressure meter was used to evaluate the pressure bearing capacity of drilling fluid.When the EZFLOW drilling fluid had a viscosity of 15 000-30 000 mPa·s at low shear rate,and when the recovery rate of direct flow back permeability was higher than 85%,the ECD was less than the formation fracture pressure;the optimized drilling fluid demonstrated strong inhibitory and plugging properties,with an ability to resist 25% of in-situ mudstone contamination.Field application results showed that the apparent viscosity of the drilling fluid was insensitive to well depth change,the maximum ECD added value was only 0.07 g/cm3,no leakage occurred during drilling and gravel packing,and the gas production exceeded the allocated rate of 10×104m3/d,with the maximum rate of 16×104m3/d.The study results indicate that an optimized EZFLOW solid-free weak gel drilling fluid can solve the problem of formation break-down caused by low fracture pressure in ultra-shallow horizontal wells as well as reservoir protection.