Alteration of permeability by drilling fluid invasion and flow reversal

Permeability impairment caused by drilling fluids and subsequent cleaning and permeability enhancement by backflow are investigated by means of experimental and simulation studies. Damage caused by two different drilling fluids is measured experimentally by core tests as a function of the filtration pressure and analyzed using a simulator describing the fines migration and retention in porous media. Simulations were run both with experimental and synthetic data in forward and backward directions along the core samples. Permeability was correlated with respect to drilling filtration pressure in terms of the deposited particle volume fraction. The clean-up time was determined after back-flush with fresh water and improvement was observed both in porosity and permeability. Simulation results accurately match the experimental data, indicating that this simulator can be used for the estimation of permeability reduction and the permeability and porosity variation along the core samples at various filtration pressures. It was also determined that a polymer-added drilling fluid characterized with 65% permeability damage ratio may be the optimum drilling fluid causing less formation damage than the water-based bentonite mud. (c) 2007 Elsevier BY. All rights reserved