Shale Stabilizers Improve Borehole Stability

The petroleum industry is increasingly drilling more technically challenging and difficult wells, inherently expensive. Shale composes roughly 75% of the formations drilled and shale instability is the main cause of over 90% of the borehole instability-related problems. The cause of shale instability can be mechanical and chemical. Mechanical-induced rock instability can occur because the in-situ stress state of equilibrium has been disturbed after drilling. Chemical-induced shale instability is caused by the drilling-fluid/shale interaction, which alters shale mechanical strength as well as the shale pore pressure in the vicinity of the borehole walls. The mechanisms that contribute to this problem include capillary pressure, osmotic pressure, pressure diffusion in the vicinity of the borehole walls, and borehole fluid invasion into the shale when drilling overbalanced. Total prevention of borehole instability is unrealistic but it is possible mitigate the problems of borehole instabilities by adhering to good field practices. These practices include proper mud-weight selection and maintenance, the use of proper hydraulics to control the ECD, proper hole-trajectory selection, and the use of drilling fluid compatible with the formation being drilled. Investigating the swelling characteristics of shale formation is vital in selecting a proper drilling fluid to give maximum inhibition and borehole stability. Swelling of selected shale and clay samples in different drilling fluids were determined in laboratory using Dynamic Linear Swell Meter as well as influence of shale stabilizers on filtration and rheological properties of drilling fluids. Results of the testing are presented in the paper.