Abstract A new model is proposed to characterize the variation in skin effect along a horizontal well. Typically, a cylindrical-shaped damaged region is assumed; however, this work describes the damaged region as a combination cylindrical-conical shape. The shape of the damaged region and the severity of the damage is governed by the contact time of the drilling fluid with the formation. This time is a function of the drilling rate penetration (ROP) and the mud filtrate invasion rate. Simple, empirical models are used to provide ROP and mud filtrate invasion rate. The effects of anisotropy ratio, penetration rates, and horizontal length are included in the analysis. Anisotropy and increasing penetration rate both will result in a decrease in the skin effect. Any horizontal well length greater than the equivalent horizontal length of the cone-shaped damage region will result in a constant cylindrical-shaped damage region, which can be evaluated using Hawkins’ formula. The cone-shaped damage region will exist at the furtherest end of the horizontal length. The time to transform the cone-shaped damage region to a cylinder is the circulation time after drilling to the total length. This circulation time is determined for the various anisotropy ratios and penetration rates.