Stressed Shale Drilling Strategy-Water Activity Design Improves Drilling Performance

摘要

Non-aqueous drilling fluids are often chosen to drilltroublesome shale formations in an effort to minimizewellbore instability problems. However, Gulf of Mexico(GoM) experience has indicated that when drilling in highlyfaulted areas, oil- and synthetic-based fluids do not alwaysprevent wellbore destabilization. This is evidenced bywellbore collapse, and the resulting difficulty with holecleaning,tripping, logging and casing running.It is known that the chemical, physical, and mechanicaleffects resulting from the interaction between the drilling fluidand the formation may degrade the stability of the borehole inthe already weakened and stressed fault interval. Commonly,the practice has been to increase the drilling fluid salt contentto enhance the borehole stability. The perception that lowdrilling fluid water activity is beneficial to wellbore stability isone that lends itself to a needed revision.A detailed laboratory investigation using preserved shalecore and drilling information have confirmed that the wateractivity of drilling fluids is often much lower than necessary.This study has shown that when drilling faulted or fracturedshale, the correct, not higher salt content in drilling fluids willreduce wellbore collapse problems and improve drillingperformance.A laboratory method, which allows the quantitativemeasurement of water and ion movement during shale/mudinteractions, combined with geological information, optimizesthe salinity design of drilling fluid, which controls water andion movement. Laboratory data and field cases from GoMdrilling support the concept of optimum salinity to enhanceborehole stability in naturally fractured formations as part ofthe stressed shale drilling strategy to improve drillingperformance.