The Impact Of Drilling Fluid Plugging Performance On Well Bore Stability Of Hard Brittle Shale

The shale is widely distributed in the world,and 70% of the total strata are mudstone,of which 2/3 are hard brittle shale.At the same time,shale gas as an important unconventional energy is attracting people's attention.During the hard brittle shale drilling progress,however,the hydration of the rock is obvious due to its fabric characteristics,which always causes borehole instability.The problem of the borehole instability in the layer of hard brittle shale is the bottleneck in the process of oil and gas exploration and development.Based on the unique mineral composition and microstructure of the hard brittle shale,this paper studies the hydration mechanism of the hard brittle shale and the expansion of the fractures under hydration.It is proved that the microcrack existence and crack propagation are the main cause of rock damage.Based on that mechanism,a new evaluation method of drilling fluid sealing characteristics is established,which is adapted to the microcrack in hard brittle shale.Using that evaluation method,this paper studies the sealing characteristics of different plugging materials type and dosage in different width cracks.Sealing mechanism of plugging agent in cracks.Experiments show that the drilling fluid with 4% CaCO3+2%DF-1+2% NFA-25 can achieve a good plugging effect.Then using ABAQUS,paper studies the change of the wellbore stability with different stress anisotropy and different bridging location in the crack.The results show that,the stronger the stress anisotropy is,the better the intensify of plugging on wellbore;the closer the bridging is to the crack entrance,the larger the circumferential stress is,and the better the well stability is.