Drilling fluid technology for well Chengu-1

Well Chengu-1 is an important scientific exploration well, with a total depth of 4264 m and the main target of the Proterozoic Era. The lithology consists of conglomerate, shale, carbonolite, hard brittle shale, basalt, pelitized basalt, quartzite and granite, etc. Multiple pressure layers are penetrated, thus drilling fluid technology is the key for the successful drilling of this well. Proper drilling fluid systems arc selected based on the formation characteristics and drilling requirements for reservoir protection. In the surface 562 mm hole, bentonite mud is used. In the 444. 5 mm hole, MMH drilling fluid is adopted. This system has good cuttings transportation capability and without washing the wall, which is good for the drilling in large size borehole. Non-toxic polyol drilling fluid system is used in the 311 mm hole, this minimize such troubles as sloughing, sticking and orientation correcting in the complicated deep well, and ensures the safety of completion, especially the case of heavy load casing cementing. In the 216 mm hole. KCl-lime dispersing drilling fluid system is used, this system has good inhibition and stability, and it is easy of field conditioning Using of it in this section results good well stability and downholc safety, the hole is regular and few sloughing problems are encountered. In the bottom 177. 8 mm hole, oil-in-water drilling fluid is used. Its density is adjustable between 0. 95-0. 99 g/cm3,and has better emulsifying and rheologi-cal property, and high temperature stability in 150 C . which meets the requirements of under-balanced drilling of ultra-deep wells of 4000 - 5000 m. Field drilling shows that this drilling fluid technology meets the requirements of both under-balanced drilling and conventional drilling operations, and achieves good formation protection results.