Invasion of drilling fluid into laminated shale model

The wellbore instability problem has always been a global technical challenge in drilling engineering. The development of weak planes and the high clay mineral content in laminated shale increases the risk of borehole collapse. In the process of drilling construction, the invasion of a large number of drilling fluids gradually deteriorates the strength of shale oil and causes wellbore instability, leading to several engineering problems (e.g., diameter reduction, stuck drilling, and hole expansion) and greatly restricting the efficient development of shale horizontal wells. Therefore, to study the invasion characteristics of drilling fluid into laminated shale, displacement experiments and nuclear magnetic scanning were conducted to determine the depth of drilling fluid invasion into laminated shale under different displacement times. This method directly and accurately studies the process of drilling fluid invading laminated shale oil. With the increase in intrusion time, the pore sizes of small and large pores increase continuously, and the increase in various pore sizes causes the separation between the grain sizes of the rock skeleton to produce micro-cracks and eventually leads to rock failure. The influence of striation is not considered in the mathematical model established by most scholars due to the development of striation in shale oil and the fact that striation is the main advantageous channel for fluid intrusion. In this study, a new mathematical model of shale oil liquid intrusion is established by considering the joint action of end-face intrusive and striated intrusive fluids. Under constant total water absorption, the model can determine the depth of drilling fluid invasion into laminated layers and total intrusion based on the invasion time. The accuracy of the model was verified based on experimental results. The numerical model established using the finite-discrete element method is used to simulate the influence of different displacement times, different numbers of layers, and different layer openings on the invasion depth under certain displacement pressure. The results show that the depth of drilling fluid invasion into laminated shale increases logarithmically with invasion time under a fixed volume of drilling fluid. As the invasion time and the opening of the laminated layers increase, the invasion depth gradually increases, but the growth slows down in the later stages and eventually stabilizes. Moreover, the invasion depth gradually decreases as the number of laminated layers increases. The primary aim of this study is to reveal the invasion mechanism of drilling fluid into laminated shale under invasion and provide a theoretical basis for selecting safe drilling fluid density and wellbore stability.