Application of a High Temperature High Density Oil Base Drilling Fluid in Well Dabei-12X in Tarim Oilfield

The Well Dabei-12X, located at the east high of the Dabei 12^# structure in the Kuche depression, is a high temperature high pressure (HTHP) appraisal well drilled in Tarim oilfield in 2018. This well penetrated the gypsum and salt zones of the Kumugeliemu formation (4267-5287 m) in which high pressure and ultra-high pressure prevail. Also exist in this section are high-pressure saltwater zones and mud loss zones. In previous drilling operation, borehole wall collapse, mud losses and saltwater invasion have been occurring all the time. Based on the geologic characteristics and operational requirements in this area, the technical difficulties in using oil base drilling fluids at HTHP were analyzed and a high temperature high density oil base drilling fluid was formulated. Laboratory experiments were performed to simulate the possible risks in drilling the HTHP sections. Experimental results showed that the high temperature high density oil base drilling fluid had stable properties; the electric stability was 1562 V, and the HTHP filter loss was 1 mL. When contaminated with 30% (Vol.%) nearly saturated saltwater, percent change in the apparent viscosity of the oil base drilling fluid was less than 10%, the filter loss was less than 2 mL and the electric stability was 1002 V. The oil base drilling fluid had good high temperature stability; after being aged at 170 ℃ for 10 d, the rheology of the drilling fluid was still stable. The settling factor of the drilling fluid was 0.522. Field application indicated that this oil base drilling fluid can be used to solve the problems encountered in drilling the HTHP and ultra-high-pressure salt/gypsum zones in the Kelasu structural belt in Kuche depression, Tarim Basin. The density of the drilling fluid in the fourth interval was 2.43 g/cm³ and the drilling fluid still retained good flow property, low viscosity, low gel strengths and ECD, no mud losses resulted from high viscosity and high gel strengths were encountered. In drilling the 2135 m salt/ gypsum zones, the rheology of the mud was very stable because of the high resistance of the mud to salt and gypsum contamination.