Micro-cracks evolution process and borehole stability analysis in deep pebbly sandstone based on THM coupling method

Abstract

The heterogeneity of pebbly sandstone is strong due to the random distribution of gravel particles, resulting in unique mechanical behaviors and failure patterns under high temperature and high pressure. It poses significant challenges to exploring and developing oil and gas resources in deep pebbly sandstone reservoirs. Thus, to investigate the impact of high pressure, high temperature, and gravel distribution on the strength characteristics and failure patterns of pebbly sandstone, based on typical samples from the Tarim Basin, a numerical model that accurately represents the distribution of gravel particles in pebbly sandstone was built. Lastly, a Thermo-Hydro-Mechanical coupling model for pebbly sandstone boreholes was established to quantify the influence of the difference coefficient of in-situ stress and gravel volume fraction. The results show that during the rupturing process of pebbly sandstone, the micro-cracks initiation occurs and expands around the gravel, forming a shear-type fracture. At the same time, several secondary cracks are formed inside the rock. With increasing confining pressure, the number of secondary cracks decreases. The fracture morphology is complex in concentrated gravel distribution, whereas a single fracture dominates the failure pattern in uniform distribution. The micro-cracks are more likely to occur in the gravel areas around the borehole during drilling, resulting in the gravel accumulation areas prioritizing collapse rather than in the direction of minimum horizontal in-situ stress, significantly reducing the safe density window. The safety density window decreases with the increase of the gravel volume but increases when the gravel volume fraction is higher than 20%. The research results provide valuable guidance for efficient and safe drilling in pebbly sandstone formations.