Mechanical properties and damage constitutive model of mud shale under different drilling fluids immersion

In the process of shale oil and gas drilling, the long-term erosion and weakening of drilling fluids promotes the extension and expansion of micro-fractures on mud shale, which leads to the reduction of rock mass strength and seriously threatens the stability of borehole wall. In order to explore the mechanical properties and damage evolution rules of mud shale under different drilling fluids immersion, Firstly, the mineral composition and structure of shale were analyzed by X-ray diffraction and environmental scanning electron microscopy. Then, uniaxial compression tests were carried out under different immersion systems and different immersion times, and the variation mechanism of mechanical parameters such as peak strength, elastic modulus and Poisson's ratio of shale under the weakening of drilling fluids were studied. Finally, based on the strain equivalence principle and Weibull statistical distribution theory, the damage constitutive model of rock under drilling fluids immersion and uniaxial loading was established. The results showed that the main body of mud shale has high content of clay minerals, abundant brittle minerals and dense distribution of micro-fractures. the elastic modulus of shale varied with the immersion environment and the increase of immersion time. Compared with the rock samples immersed in oil-based drilling fluid, the weakening effect of water-based drilling fluid soaking on its strength and elastic modulus is more obvious; by fitting the change trend of the mechanical parameters of shale under the weakening effect of different drilling fluids, the prediction model of elastic modulus of rock samples soaked in water-based drilling fluid and oil-based drilling fluid with respect to soaking time was proposed; For the mud shale under the weakening of drilling fluid, a uniaxial loading rock damage constitutive model considering the effect of pore compression stage is introduced. The evolution rules revealed the weakening damage mechanism of shale under different drilling fluids immersion.