Gas Drilling Wellbore Stability Analysis in Igneous Rock Carboniferous System of Dzungaria Basin

Abstract

Abstract During drilling operation in carboniferous system of Dzungaria basin, igneous rock was met, which leads to a low penetration rate and long drilling cycle, because of large thickness, high hardness and poor drillability of the igneous rock. Quasi-three axis confining pressure experiment was carried out with the igneous rock core of WUCAN 1 well. Experimental results showed that the igneous rock keeps a brittle fracture even in the confining compression pressure of 40MPa. Based on Mohr-Coulomb failure criterion, one analysis method about the applicability of gas drilling was put forward through comparing maximum principal stress while wellbore instability happened with upper-cover pressure. It can be concluded that gas drilling technology can be applied in carboniferous igneous strata of Dzungaria basin, when embedding depth is less than 6110 meters, which provides theoretical basis on the application of gas drilling technology in deep exploratory wells whose target zone is in igneous strata of carboniferous system of Dzungaria basin. Introduction After the discovery of SHIXI oil field and DIXI oil field in recent years, in the western new area of SLOF, SINOPEC, strata of carboniferous system are considered to be high potential. So several deep exploratory wells are mapped out by SINOPEC, including WUCAN 1 well. However, from current data, stratum of carboniferous system is mainly igneous rock, which has large thickness, high hardness and poor drillability. It leads a low mechanical drilling speed and a long drilling cycle. For WUCAN 1 well in the northern part of Dzungaria basin, conventional slurry drilling method is applied from well depth 4286 meters to 6323.6 meters. Many new technology solutions were successively used to enhance the drilling speed, such as ‘high efficiency roller bit + screw rod’ composite drilling, ‘Diamond head pregnant bit + high speed screw rod’ composite drilling, high frequency torsion impact drilling tools, waterpower impulse injector + high efficiency drilling bit of Baker Hughes. The results aren't pleased: average drilling speed is only 0.72m/h, and the drilling cycle is 239 days, which is more than half of the whole well cycle. In the same stratum, compared to conventional slurry drilling technology, drilling speed and bit life can be highly increased by gas drilling technology [1–4]. So the application of gas drilling technology in igneous rock of carboniferous system in Dzungaria basin can be a possible way to overcome the bottle neck of drilling speed. Field engineering application shows that borehole stability is the main problem in gas drilling technology[5–7]. In this paper, mechanical properties of the rock nearby well wall are experimental investigated by quasi-three axis compression test, to evaluate gas drilling technology applicability in the igneous stratum of carboniferous system in northern Dzungaria basin.