Ultra-Low Density Drilling Fluid Technology for Drilling Ultra-Deep Fractured Carbonate Reservoirs

Ultra-deep wells with pressure coefficient less than 1.0 in Tarim oilfield are generally faced with lost circulation of mud in drilling the low-pressured carbonate reservoirs. These wells, with an average depth of over 6,000 m, have high risks of well control, thus, apart from drilling with no return when lost circulation is encountered, no other better means can be used to drill the wells to the designed depths, and in many cases, the wells can only be completed before drilling to the designed depths. To extend the lengths of the horizontal sections of the ultra-deep wells, researchers have developed high strength hollow micro glass beads for formulating water-based drilling fluids with density in a range of 0.93 g/cm³ to 1.07 g/cm³. This paper describes in detail the operation process of the well Zhonggu 262-H4C in which lost circulation was encountered and a low density drilling fluid was then used to regain circulation and drilling was resumed directionally to the designed depth. In the field operation, the density of the drilling fluid was reduced to 0.98 g/cm³, and additional 373 m was drilled after the circulation, realizing the geological goal of one well to penetrate two fractured-vuggy bodies. This measure ensures that the well to be drilled to the designed depth after encountering lost circulation, setting a precedent of drilling an ultra-deep well with vertical depth over 6,000 m using low density drilling fluid formulated with hollow micro glass beads, and providing a technical reference for drilling low-pressured formations in old oilfields in China.