Comparison of Skin Factors for Perforated Completions Calculated with Computational Fluid Dynamics Software and a Semi-Analytical Model

Abstract

Abstract There are many factors affecting flow performance of perforated completions. These factors include perforation tunnel geometry, drilling and perforation damage, formation anisotropy, etc. The combined effect of these factors is usually accounted for via a single parameter – total skin factor, which is an important input parameter for Inflow Performance Relationship (IPR) prediction and reservoir simulation. The commonly used semi-analytical skin factor model assumes total skin factor can be expressed as a linear combination of horizontal skin, vertical skin considering anisotropy, perforation-damaged-zone skin, and other skins. The purpose of this study is to investigate the validity of the semi-analytical model in the realistic operational range of perforating parameter values. For this purpose, we use computational fluid dynamics (CFD) software to simulate the production flow of a 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 anisotropy, assuming no pressure drop along the interior of perforation tunnel. All combinations of the three effects are considered. Computed skins are compared with the semi-analytical skin model used in the industry. Computed results show good comparisons between skin factors calculated using CFD software and the model for most cases. However, significant deviations in skin factor comparisons are observed when both perforation damage and formation anisotropy exist. We also conclude that an additional skin factor term, related to the ratio of modified wellbore radius to the original wellbore radius, should be explicitly listed in the model for the case of perforations tunnel extending beyond the drilling damage zone. Calculated CFD skin factors can be used as a database for predictive pre-job analysis. Deviation between skin factors calculated using CFD and the model highlights the need for improving industry methods to estimate skin factor in perforated completions.