The damaged zone around the well is considered as the internal region of a composite system comprising two regions of different permeabilities. An analytical solution is obtained in Laplace space and is inverted using Stehfest Algorithm. The skin factor is obtained as the difference in dimensionless pressure at the well of the composite system and that of the homogeneous system at the same dimensionless time. The ratio of the skin factor to the steady state skin factor is estimated and correlated with the permeability ratio and the dimensionless depth of invasion.
The developed model and the correlation charts can be used to evaluate the formation damage from well test analysis around the well by estimating both the permability and depth of invasion in the damaged zone. This is an improvement in the conventional method that gives the steady state skin factor which is a combination of both permeability ratio and invasion radius.
Additional pressure drop due to damage is found to be time dependent and varies from zero at the start and approaches that obtained from steady state skin factor at very large dimensionless time. The unsteady state skin factor and skin factor ratio depends on both the permeability ratio and the dimensionless invasion radius and not uniquely on their combination.