EXPERIMENTAL STUDY ON THE INFLUENCE OF DIFFERENT DRILLING FLUID SYSTEMS ON THE MACRO AND MESO CHARACTERISTICS OF DEEP SHALE

After the drilling fluid enters the wellbore, it will affect the distribution of the stress field around the wellbore, thereby degrading the strength of the rock, and induce wellbore instability, reservoir damage, well-bore collapse, tool jamming and even wellbore scrapping. In this paper, the impact of drilling fluid on the macro and meso characteristics of deep shale after entering the formation has been systematically studied. This study found that whether water-based or oil-based drilling fluid enters the shale micropores and fissures, it will degrade the shale microscopic characteristics over time. For the interval with developed fractures, oil-based drilling fluid is more likely to cause fractures to propagate under pressure in a relatively short period of time. However, with the prolongation of the drilling fluid soaking time, the water-based drilling fluid is easier to communicate between natural fractures and artificial fractures, which in turn leads to a significant decrease in rock strength and instability of the borehole wall. Both water-based and oil-based drilling fluids have the law of weakening the strength of shale. The compressive strength, Young's modulus and Poisson's ratio of the rock show a decreasing trend with the extension of the drilling fluid immersion time, and the rock mechanics parameters of immersed water-based drilling fluid have a greater attenuation than oil-based drilling fluid. This research can provide a theoretical basis for safe and fast drilling of deep shale.