Effect of mud penetration on borehole skin properties

Abstract

Clay particles in drilling muds can form a filter cake (skin) on the walls of the well and migrate into the adjacent formation. In general, filter cakes are formed when a liquid phase containing solid particles is forced through a pervious surface which allows liquid transport, while retaining solid particles. Following a literature survey, the governing equations for the concentration of suspended solid particles in the pore fluid, cake permeability, and solid velocity are obtained by considering the mass balance equations for the fluid phase and suspended and captured solid particles. The capture mechanism is represented by a kinetic equation with rate constants for particle trapping and cake erosion. Numerical solutions for the cake permeability, cake resistance, solid particle velocity (cake compression rate), and concentration of suspended particles are obtained by utilizing experimentally available porosity and pressure variations along the dimensionless cake thickness. The results compared for a simplified experimental case, show excellent match. A sensitivity analysis shows that cake permeability and cake resistance are more sensitive to the rate constant of cake erosion than they are to the rate constant of particle capture. It was also determined that the permeability has larger values along the entire cake thickness when the rate constant of particle capture is zero in comparison to values obtained with a nonzero rate constant. Cake permeability is small when the rate constant of cake erosion is large.