Wellbore instability mechanism of hard brittle rock under unloading condition in the ultra-deep wells

Abstract

Limestone, igneous rock and dolomite are widely distributed in ultra-deep wells in Tarim Basin. Hard brittle rocks show different mechanical properties under high stress conditions in ultra-deep wells. Traditional wellbore stability theory states that the greater the rock strength the more stable the wellbore. The actual drilling in Tarim oilfield (more than 7000 m true vertical depth) shows that in the same section of hard dolomite and mudstone formation, there are more dolomite enlargement and block falling during drilling, while the caliper of mudstone formation is regular. The unloading mechanical experiment of hard brittle rock shows that the deformation and failure characteristics of rock during unloading are different from those during loading. In the process of unloading confining pressure, the axial stress and confining pressure decrease linearly with the increase of strain, and the unloading modulus increases with the increase of the decreasing rate of confining pressure. The smaller the unloading rate of confining pressure, the more obvious the axial plastic flow becomes. The failure of unloading rock occurs mainly due to its brittleness. By comparing the failure degree under different stress paths, the unloading confining pressure before peak is greater than that after peak, and the unloading confining pressure after peak is greater than that of the loading test. Most of the unloading cracks are tensile cracks, but there are shear cracks in the sample, and tensile flakes on the surface of the sample after unloading confining pressure. Within the experimental range (unloading rate), there may be an unloading rate that causes the most severe damage to the rock sample, that is, with the increase of the rate of penetration (ROP), the degree of rock damage caused by stress unloading does not increase monotonously, and there may be a certain ROP that causes the greatest damage to the wellbore. Large unloading will aggravate the damage evolution and failure of tight brittle rock. For the safe drilling of such rock formation, the drilling rate and drilling fluid density should be optimized based on complete understanding of the geo-mechanical environment to minimize the damage and instability of surrounding rock caused by drilling unloading.