Drilling Fluids Filtration and Permeability Impairment: Performance Evaluation of Various Mud Formulations

Abstract

Abstract This paper describes the results of a three-year programme achieved in a Joint Industry Project aiming at evaluating formation damage arising from overbalanced drilling and completion operations. Eight typical drilling fluid formulations, including water-based and invert synthetic oil-based muds have been used to perform static and dynamic filtration tests on outcrop sandstone core samples. The performance of each mud formulation has been quantified from continuous monitoring of fluid losses during mud invasion and from return permeability measurements after mud exposure. The impact of various parameters on permeability impairment, such as initial core permeability, fluid saturations (oil or gas), temperature and shear rate has been evaluated. Results show that fluid losses and permeability impairments obtained with water-based muds are significantly greater than those observed with invert oil- based muds. Return permeabilities measured on six sections of long core samples. at increasing distances from the inlet face, showed a dramatic near-wellbore permeability reduction due to the invasion of filtrate generated from water-based muds. On the contrary, a stimulating effect, i.e. oil return permeability greater than initial oil permeability, was observed after invasion of filtrate from invert synthetic oil-based muds. This was attributed to a favorable relative permeability effect due to a reduction of connate water saturation during mud filtrate invasion. The use of a heavy brine as completion fluid has generated a significant additional permeability impairment due to the trapping of the wetting phase. inducing an adverse relative permeability effect. P. 237