Displace Open Hole Full of High Density OBM with WBM
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摘要: 塔里木油田库车山前克深区块“库姆格列木群”层位岩性属于复合盐膏层，该盐膏层具有压力系数高、裸眼长、易塑性变形等特点，均用油基钻井液钻进。KS603井钻遇该层位中途完钻时发生井漏，为避免油基钻井液在下套管过程中出现恶性井漏，采用欠饱和盐水钻井液替换油基钻井液完成中完作业。施工过程中，在隔离浆中加入高碱比抗盐钙降黏剂碱液，利用碱液的稀释特性与OH-离子对油基钻井液中Ca2+的中和作用，使隔离浆始终保持较低黏切；因隔离浆中不能加入堵漏材料，在隔离浆之后的第一罐井浆中加入8%堵漏剂，提高井浆的防漏能力，避免替浆过程中发生井漏；为保证盐膏层井壁稳定，替浆密度为2.39 g/cm3，严格控制钻井液中的Cl-含量不小于1.8×105 mg/L，避免盐晶颗粒过多地溶入钻井液中，同时使用盐重结晶抑制剂NTA-2；下套管前进行承压堵漏作业。钻井液性能稳定，在中完作业过程中光钻铤、单扶、三扶、四扶通井、测盐层蠕变静止72 h后四扶通井、下套管、固井作业均一次性成功，井壁稳定、井下安全。Abstract: The lithology of the "Kumugeliemu Group" in the Block Keshen (Kuche piedmont structure) of the Tarim Oilfield is of compound salt and gypsum having high pressure coefficients. Long open hole drilling and plastic deformation of the formations penetrated made drilling very difficult. Oil base drilling fluids have long been used in the drilling operation. The well KS603 drilled in this area has experienced mud losses at the end of salt and gypsum drilling. To avoid losses of the expensive oil base drilling fluids during casing running, unsaturated brine muds have been used at the end of the drilling of the salt and gypsum formations. In displacing the oil base mud, a thinner solution with high concentration of alkalis is added into the spacer fluid in an effort to maintain a low viscosity of the spacer through the thinning effect of the alkali solution and the neutralization of calcium in the oil base drilling fluid with OH-. Since lost circulation material (LCM) cannot be added directly into the spacer, the first pit of mud displaced into the hole will be treated with 8% of LCM to avoid losses of the mud. To maintain the borehole stability of the salt and gypsum formations, the displacing mud will be weighted to 2.39 g/cm3, and the chloride maintained at no less than 1.8×105 mg/L to avoid the dissolution of formation salts. NTA-2 is used in the displacing mud to avoid salts from recrystallizing. Before running the casing strings, the compressive capacity of the weak points along the borehole is increased with LCMs to avoid mud losses during casing running. The properties of the displacing fluids have been proved stable by field operations. Wiper trips using drill strings with no centralizers, one centralizer, three centralizers, or four centralizers have been performed without a hitch. After 72 hours of waiting for logging the creeping of the salt and gypsum formations, a wiper trip with four-centralizer drill string, the casing running, and the well cementing have all been done right the first time. The borehole wall has remained stable during the operation, with no downhole troubles occurred.
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