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 POP 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.
It is well known mud filtrate invasion into a formation creates a near wellbore damage region, resulting in an additional pressure drop near the wellbore and loss in pressure drawdown. The concept of skin factor was developed to account for the loss in productivity due to the near wellbore formation damage.
The damaged region is typically idealized as a uniform, concentric cylinder about the wellbore Figure 1a illustrates the idealized system for both a vertical and horizontal well. However, it is more likely that the damaged zone is conical in shape, due to the time the mud has to circulate and invade a penetrated foot of formation (Fig 1b). Since the horizontal length is greater than the reservoir thickness, a longer contact time per foot of formation thickness will occur in a horizontal well. The additional time will develop a significant variation in the skin damage along the length of the horizontal well.
The objective is to quantify the changing skin factor along the horizontal well, and note any differences with the idealized system. Correctly identifying the skin region will impact completion efforts to remove the damage and mud formulation to minimize the invasion and reduce the damage. Further, this work provides evidence of an "effective horizontal length" which contributes to the productivity of the well and not the entire horizontal length.