Mechanism of Strength Degradation of Stratified Shale under the Action of Drilling Fluids

The infiltration of drilling fluids into stratified shale and the subsequent degradation of its strength play a critical role in borehole instability, with the mechanisms in stratified shale differing significantly from those in conventional mudstone. This study aims to elucidate the strength degradation mechanisms of stratified shale under the influence of water-based and oil-based drilling fluids. Laboratory experiments were conducted to collect relevant parameters, and the driving forces of different drilling fluids during their penetration into stratified shale joints were compared and analyzed. Through drilling fluid intrusion experiments, the patterns of drilling fluid penetration into stratigraphic joints were examined. Furthermore, the degradation of shale strength following fluid intrusion was evaluated by measuring mechanical parameters after the intrusion process. Based on these measurements, the study analyzed the causes of shale strength deterioration due to the intrusion of various types of drilling fluids. the intrusion of drilling fluids along the stratified joints causes its strength deterioration, and the intrusion driving force of drilling fluids is mainly controlled by the capillary force when the size of the stratified joints is less than 0.007 μm, and vice versa by the bottomhole differential pressure; the driving force for fluid intrusion into smaller stratified joints is greater, but due to differences in permeability, drilling fluids are more likely to infiltrate larger stratified joints, and its strength degradation of shale is mainly due to the lubrication of filtration fluid on the formation surface and the hydrodynamic spiking effect, which is dominated by the physical effect; the depth of intrusion of water-based drilling fluid is slightly smaller than that of oil-based drilling fluid, but the hydration of illite surface caused by the filtration fluid will result in the serious dispersal of shale, and its strength degradation is controlled by the combination of physical and chemical effects.