An Alternate Mud Proposal to Minimise Borehole Instability

Abstract

There exists a strong relationship between mud and borehole stabilities. Mud instability is associated with flocculation that is primarily caused by high active solids, high electrolyte concentration, and high temperature. Drilling in hot environments coupled with active or reactive solid contamination results in severe operational problems which get worse with the presence of salt intrusion. Therefore, drilling engineers search for alternate drilling fluids to eliminate and/or minimize instability problems such as stuck pipes, circulation loss, excessive torque and drag, inadequate cuttings transport, and sloughing borehole. Such mud should tolerate for accumulation of high amount of reactive solids and mitigate diffetential pressure sticking to avoid stuck pipe. This study is an experimental work to investigate and remediate the rheological and fluid loss properties of both unweighted and barite-weighted sepiolite muds heavily contaminated with active solid. Sepiolite muds were formulated in a certain mixing order of additives. Bentonite clay with API specifications (OCMA) at a rate of 80-lbm/bbl was used to simulate active solid. Rheological properties of mud samples were tested at varying temperatures ranging from ambient to 400oF along with a sodium chloride content of 260,000-ppm (120-lbm/bbl). The filtration loss properties were determined at 300oF. Methylene blue test was also performed to determine the tolerance of sepiolite mud for reactive solid contamination. Sepiolite mud was also investigated for tendency to cause wall sticking to consider incidence of differential pressure sticking of drill pipe. Bulk Sticking Coefficient (Ksc) of sepiolite based mud was determined as indicator parameters under high pressure (500 psi) and high temperature (up to 400oF). Sepiolite mud samples resulted in appropriate yield point, plastic viscosity, and water loss values. More importantly, Methylene Blue values lower than 13-lb/bbl and appropriate sticking coefficient were the concrete usability indicator of sepiolite muds against heavily reactive-clay contamination. In other words, sepiolite muds might be a good alternative to drill wells experiencing instability problems resulting from active solid contamination and differential sticking.