An Environmentally Friendly Wbm System Can Prevent Hard Brittle Shale Instability

Abstract

In the Weizhou Southwest oilfields, drilling delays and suspension of wells prior to reaching the targets due to wellbore instability had occurred frequently. The hard brittle shale played a problematic role. Conventional water-based drilling fluids didn’t conquer the problematic formation due to intrinsically performance deficiencies. While Oil based drilling fluids are routinely preferred in the more technically demanding applications, they are cause for increasing concern due to offshore environmental restrictions and expensive disposal costs. An environmentally acceptable water-based drilling fluid was developed to challenge the problematic formation based on the combination of methylglucoside-silicate concept. It stabilized the reactive shale by the same mechanism as did oil-based drilling fluid in preventing shale hydration, pore pressure increase and weakening of shale by effectively developing sufficient osmotic force to offset hydraulic and chemical forces acting to cause filtration flux into the hard brittle shale. A field trial was initiated on the CNOOC 931 platform in Weizhou oilfield. The data from the pilot well showed that the novel drilling fluid exhibited excellent inhibition and lubricity which approached or even exceeded oil-based fluids.