Mechanisms of and Technical Measures for Solving Borehole Wall Instability in Ten-Thousand-Meter Scientific Exploration Wells in Tarim Basin

In Tarim Basin, borehole wall instability has been an issue encountered in deep and ultra-deep well drilling in the Taipen block, the studies of which are insufficient and the understanding of the mechanisms of which is not clear. In laboratory studies, cores taken from the outcrop rocks of the Silurian, Ordovician and Cambrian strata buried at depths between 5,595 m and 10,900 m in the Taipen block were analyzed, and it was found that fractures with opening sizes between 300 nm and 800 nm are good passages for the invasion of a drilling fluid; borehole pressure is easy to transfer across these fractures and the collapse pressure of the formation is hence increased. The contact angles of water and oil on the surfaces of the mudstone are 5.8° and 5.3° respectively, the contact angles of water and oil on the surfaces of the limestone are 42.5° and 15.6°, respectively. The percent water adsorption and the percent oil adsorption of the mudstones are 2.74% and 3.63%, respectively, and those of the limestones are 1.42% and 2.14%, respectively. The percentages of the recovery of the Silurian system rocks, the Sangtamu mudstone and the Awatage argillaceous dolomite on hot rolling test are 86.3%, 92.9% and 98.2%, respectively, and those of the limestone and the dolomite are both basically 100%. Part of the formations, because of the hydration effect and dispersion in water, have the rock cohesion reduced. These are the main mechanisms that cause borehole wall instability. Based on the mechanisms of borehole wall instability understood, a water based drilling fluid was formulated with a high efficiency nanometer plugging agent, a salt-resistant polymer filter loss reducer and other additives for ultra-deep well drilling. The nanometer plugging agent and the filter loss reducer are all ultra-high temperature resistant. By enhancing the plugging capacity, reducing the filtration rate of the drilling fluid and minimizing the amount of the drilling fluid lost into the formations, the collapse pressures of the formations are reduced. By improving the inhibitive capacity of the drilling fluid, the hydration and dispersion of the formations are inhibited, and the cohesion of the rocks is increased. With these measures, the borehole wall instability issue encountered in drilling the deep complex formations in the Tarim Basin is successfully solved.