Causes Analysis And Measures Research Of Drilling Complex Problems In The Second Hole Section Of The Center Tarim Oilfield

In order to reduce the drilling complex occurrences, this paper did experiments and referred to drilling data to analysis the reasons and to research measures of the two main problems, bit balling in Triassic formation and wellbore instability in Permian formation, during the second section drilling in Middle Traim Oil Field. The main research contents and results are as follows.(1)Analyzed the mineral composition,physicochemical and microstructure of Triassic and Permian shale in laboratory. The red shale in Triassic has high clay content and strong dispersion,which caused the PDC bit balling. The brown shale in Permian has the nearly composition and physicochemical. It is the objective conditions of wellbore instability. A lot of micro pore and inter-granular pore are in the grey shale, which aggravated the Shale hydration and caused the wellbore collapse.(2)Calculated the rock density and shear wave curve, and calculated the rock mechanical characteristic parameters, overburden pressure, pore pressure and horizontal principle stress according to the theoretical calculation model. These parameters could provide basic data for further wellbore stability research. The results show that the upper Permian brown shale strength is obviously lower than the others. The Poisson rate, modulus of elasticity, cohesion and the friction angle are 0.30-0.35,7000-35000MPa, 3-13MPa,3-30MPa and 20-35degrees respectively. The formation overburden pressure equivalent mud density is nearly 2.20-2.22g/cm3. and the pore pressure equivalent mud density is 1.05-1.10 g/cm3. The maximum principle stress is 2.35-2.55 g/cm3, the maximum principle stress is 2.00-2.20 g/cm3.(3)Evaluated the wellbore stability in Permian formation from the part of mechanics, analyzed the stress on the borehole, established the collapse pressure and fracture pressure prediction model based on Mohr-Coulomb strength criterion, and finally calculated the collapse pressure and fracture pressure profile of Permian formation. In this formation, collapse pressure equivalent mud density is up to 1.40g/cm3, the fracture pressure equivalent mud density is nearly 1.80g/cm3. The results show that the contradiction between higher collapse pressure and relatively lower mud density caused the wellbore instability.(4)Made researches on countermeasures to these two problems. Firstly designed the optimization fluid displacement and analyzed the mud detergent using to deal with bit balling in Triassic formation. The fluid displacement in the 21.59cm hole should be more than 33L/s and in 24.13cm hole should be more than 42L/s. Detergent should be used in Triassic drilling. Secondly designed the appropriate mud density and evaluated inhibitors combination to deal with the collapse in Permian formation. The mud density should achieve 1.50g/cm3, and inhibitors combination 3%KCL+0.4%KPAM+2%MMH is the best.