Abstract

The determination of rock mechanics properties and drillability are significant for the prediction of the rate of penetration (ROP) in drilling into complex formations. Previous studies indicate that these properties can be strongly correlated with formation conditions, e. g. temperature, and in-situ stress. However, these two parameters are hard to be considered simultaneously due to the limitation of the experimental apparatus. To address such an issue, rock samples are collected from the eastern Sichuan Basin, mainly including the sandstone located in the formations of Shaximiao Bottom, Ziliujing. and Xujiahe. A series of tests are conducted to investigate the evolution of rock mechanical properties subjected to the joint action of temperature and in-situ stress based on a self-developed experimental setup. The results indicate that the rock strength under in-situ conditions is 2~3 times higher than that determined under atmospheric conditions. The plasticity and volumetric strain of rocks increases as well, which is accompanied by the transition of rock failure modes from volumetric failure to abrasive rock-breaking. Meanwhile, the variation of drillability under different formation conditions shows that rock failure characteristics change significantly. The average ROP of rocks in the Shaximiao Bottom Formation, Ziliujing Formation, and Xujiahe Formation under in-situ conditions decrease by 45.12%, 61.19%, and 43.82% respectively, leading to an increase in the drillability of rocks. Compared with in-situ stress, the increase in temperature has few effects on rock strength and drillability. This study provides an in-depth understanding of the evolution of rock properties under different formation conditions, which is of great significance for optimizing drilling parameters and improving drilling efficiency in hard-to-drill formations.

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