Drilling Fluid Technology for Well Ming-15 in Mingbulak Oilfield, Uzbekistan

Formations drilled in the Mingbulak Oilfield in Uzbekistan have very complex structures. Wells drilled in this oilfield with depths between 800 m and 3,500 m penetrated saltwater-bearing zones, long segment of anhydrite, gypsum/shale mixed formations and highly water sensitive formations. Part of the formations also have high concentration of CO₂. Formations between 3,850 m and 5,900 m are full of fault zones, vugs and fractures, and gypsum segments that may creep and form tight hole during drilling. Coexistence of high-pressure oil, gas and water may cause well blowout and mud losses to happen simultaneously. When drilling an ultra-deep well with high temperature (≥ 170℃), high pressure (pressure gradient ≥ 2.3), high salt content (≥ 20%) formation water, high H₂S content (5%-6%), the drilling fluid should satisfy the needs of both stabilizing the high pressure formations in the upper section of the well as well as preventing mud losses and flowing of H₂S in the lower section of the well. Difficulties associated with ultra-deep well drilling include creeping of long section of gypsum/anhydrite which leads to tight hole, borehole wall collapse caused by high pressure saltwater invasion, difficulties in controlling the rheology of high-density drilling fluid, high circulation pressure and reservoir protection etc. By optimizing the composition of the drilling fluid and carefully selecting mud additives, it was decided that a highdensity compound formates drilling fluid was used to drill the extremely complex second interval and the sections beyond that. Filed operations showed that the compound formates drilling fluid had good inhibitive capacity and lubricity, strong resistance to calcium contamination and low solids content. The compound formates drilling fluid had controllable rheology even when the drilling fluid was treated with high concentration of lost circulation materials. With this drilling fluid, downhole problems previously encountered such as pipe sticking (because of high mud weight and creeping of salt/gypsum layers), coexistence of well blowout and mud losses, as well as inability to drill deeper reservoirs, have all been wiped out. The aim of drilling the well successfully has been realized, and technical support and experiences have been provided to subsequent operations.