Research on the mixed-mode fracture damage characteristics of shale soaked in different drilling fluids

Abstract

Drilling fluid is a vital component in drilling engineering and can be considered as the lifeblood of the drilling. It serves multiple functions such as cooling the drill bit, cleaning the wellbore, and maintaining wellbore stability. However, when the drilling fluid penetrate into the formation, it significantly reduces the mechanical properties of the rocks thus making the wellbore prone to instability. This paper focuses on the studying the fracture characteristics of shale samples with different pre-crack angles (15°, 30°, 45°) when immersion in different types of drilling fluids (water-based drilling fluid, alcohol-based drilling fluid, oil-based drilling fluid) using the SCB (Semi-circular bending) test. During the experiment, digital image correlation and acoustic emission techniques were employed to monitor and analyze the fracture characteristics of shale. The research findings showed that as the pre-crack angle increases, the fracture toughness and effective fracture toughness of Mode I fractures decrease continuously, while the fracture toughness of Mode II fractures gradually increases. The impact of drilling fluids on the fracture toughness of shale samples is in the following order: water-based drilling fluid > alcohol-based drilling fluid > oil-based drilling fluid. The length of the fracture process zone (FPZ) decreased with the increase of the pre-crack angle, and the FPZ length under different drilling fluid immersion shows varying degrees of enlargement, with the following effect: water-based drilling fluid > alcohol-based drilling fluid > oil-based drilling fluid. The horizontal strain and horizontal displacement generated before the rock sample fractures increase with the increasing angle of the pre-crack. After immersion in drilling fluid, the layering of horizontal displacement field becomes more evident, with the degree of straightness of horizontal displacement affecting it in the following order: oil-based drilling fluid > alcohol-based drilling fluid > water-based drilling fluid. As the pre-crack angle of the SCB sample gradually increases, the relative decrease in acoustic emission energy becomes more pronounced, and the decrease in b-value becomes more significant. After immersion in drilling fluid, the acoustic emission becomes more pronounced, and the b-value becomes more disorderly and decreases further. Finally, by analyzing the fracture patterns of SCB samples immersed in different drilling fluids, relevant fracture mode diagrams were established. The findings could be useful in developing strategies for optimizing drilling fluid selection and fracture stimulation design in shale formations.