Comparison of skin factors for perforated completions calculated with computational-fluid-dynamics software and the karakas-tariq semianalytical model

Many factors affect flow performance of perforated completions, including perforation-tunnel geometry, drilling and perforation damage, and formation-permeability anisotropy. The combined effect of these factors is usually accounted for by means of a single parameter: total-skin factor, which is an important input parameter for inflow-performance-relationship prediction and reservoir simulation. The Karakas and Tariq (1991) semianalytical skin-factor model is the most commonly used in the industry (Bell et al. 1995; Kabir and Salmachi 2009; Zhan et al. 2012). It assumes that the total-skin factor can be expressed as a linear combination of horizontal skin, vertical skin considering permeability anisotropy, perforation-damaged-zone skin, and other skins. The purpose of this study is to investigate the validity of the Karakas-Tariq semianalytical model in a realistic operational range of perforating-parameter values. For this purpose, we use computational-fluid-dynamics (CFD) software to simulate the production flow of a vertical cased-and-perforated well in a representative 3D geometric formation. We consider three effects: drilling damage, perforation damage (crushed zone around the perforation tunnel), and permeability anisotropy, assuming no pressure drop along the interior of perforation tunnels. All combinations of the three effects are considered. Computed skins are compared with the semianalytical skin model of Karakas and Tariq (1991). Computed results show good comparisons between skin factors calculated by use of CFD software and the Karakas and Tariq model (1991) for most cases. However, significant deviations in skin-factor comparisons are observed when both perforation damage and formation anisotropy exist if considering permeability anisotropy in the crushed zone. We also conclude that an additional skin-factor term, related to the ratio of the modified wellbore radius to the original wellbore radius, should be explicitly listed in the Karakas and Tariq model (1991) for perforation tunnels extending beyond the drilling-damage zone. Calculated CFD skin factors can be used as a database for predictive prejob analysis. Deviation between skin factors calculated by use of CFD and the Karakas and Tariq (1991) model highlights the need for improving industry methods to estimate skin factor in vertical perforated completions if considering crushed-zone anisotropy.