Formation damage around horizontal wells can result in poor performance and failure of these wells. The objective of this paper is to present a new solution for transient pressure behavior of horizontal well injectors and/or producers in injection processes. Equations for productivity and injectivity of horizontal wells during injection processes are introduced that are based on the effective horizontal well length contributing to unimpaired injection or production. Effective horizontal well length in steady state reservoirs is the well length with zero skin factor that will yield the same steady state productivity or injectivity index as the one obtained with actual drilled horizontal section length with skin factor. A relationship between mechanical skin factor and effective horizontal well length is derived and explained. Guidelines for analyzing transient testing of horizontal well injectors for determination of reservoir permeability, skin factor, and effective length are presented. It was found that flow efficiency is the ratio of steady state shape factor evaluated at actual length of the horizontal section and the one evaluated at effective length. Furthermore, flow efficiency is always greater than the fraction of horizontal well contributing to unimpaired production or injection, Xf2/Xf1. This indicates that a decrease in effective length is not equal to the loss in injectivity or productivity of the horizontal well, thus implying tolerance for formation damage around these wells to a certain degree.