Displace Open Hole Full of High Density OBM with WBM

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.