A New Skin-Factor Model for Perforated Horizontal Wells
- Kenji Furui (U. of Texas at Austin) | Ding Zhu (U. of Texas at Austin) | A.D. Hill (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2008
- Document Type
- Journal Paper
- 205 - 215
- 2008. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.1.1 Exploration, Development, Structural Geology, 5.5 Reservoir Simulation, 5.6.8 Well Performance Monitoring, Inflow Performance, 3.3.1 Production Logging, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 2.2.2 Perforating, 2.3 Completion Monitoring Systems/Intelligent Wells, 3 Production and Well Operations
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Using a combination of analytical calculations and 3D finite-element simulation, we have developed a comprehensive skin-factor model for perforated horizontal wells. In this paper, we present the mathematical model development and validation by comparison with finite-element simulation results. With the new perforation skin model, we then show how to optimize horizontal well perforating to maximize well productivity.
A cased, perforated well may have lower productivity (as characterized by a positive skin factor) relative to the equivalent openhole completion because of two factors: the convergence of the flow to the perforations, and the blockage of the flow by the wellbore itself. Because of the orientation of a horizontal well relative to the anisotropic permeability field, perforation skin models for vertical wells that consider these effects, notably the Karakas and Tariq model (1991), are not directly applicable to perforated horizontal completions. Using appropriate variable transformations, we derived a skin-factor model for a horizontal perforated completion that is analogous to the Karakas and Tariq (1991) vertical-well model. The empirical parameters in the model were determined from an extensive 3D finite-element simulation study.
The results of the new model show that the azimuth of a perforation (the angle between the perforation tunnel and the maximum permeability direction, usually thought to be in the horizontal direction) affects the performance of perforated completions in anisotropic reservoirs. When perforations are normal to the maximum-permeability direction, perforations will enhance horizontal-well flow compared with an openhole completion (a negative skin factor). But if perforations are in the same direction as the maximum permeability, significant positive skin will result. The new skin-factor model provides a clear guide to the shot density, perforation orientation, and level of perforation damage that is tolerable to create high-productivity perforated completions in horizontal wells.
A skin factor can be used to mathematically account for any deviations of the flow and pressure field in the near-well vicinity from perfectly radial flow to a wellbore of radius rw . A perforated completion obviously has a flow and pressure field near the perforations that is not perfectly radial. As shown by Karakas and Tariq (1991), the altered flow characteristics near perforations can be conveniently divided into three parts: the flow in a plane perpendicular to the wellbore, the blockage of flow to the perforations by the wellbore itself, and the fully 3D flow resulting from the asymmetric distribution of perforations along the wellbore. These effects on the near-well flow field and the corresponding perforation skin factor components are illustrated in Fig. 1.
Perforation skin models for vertical wells (Karakas and Tariq 1991; Harris 1966; Locke 1981; Klotz et al. 1974) have already been presented in many papers. However, they are not directly applicable to a horizontal well because the reservoir anisotropy in a horizontal well creates complex plane-flow geometry normal to the well, which alters the flow efficiency of a perforated completion. In this work, we present a new skin-factor model developed for a cased, perforated horizontal well. From our observations, the 2D plane flow skin, s 2D , the wellbore blockage skin, swb , and the 3D convergence skin, s 3D , greatly depend on the magnitude of the permeability anisotropy and the perforation angle measured from a horizontal plane. Our model is based on the conventional perforation skin model for a vertical well presented by Karakas and Tariq (1991).
Our perforation skin model is a semianalytical solution that is correlated with numerical simulation results. The reliability of any empirical correlation for perforation skin factors will depend on the accuracy of the numerical simulations. The finite-element method (FEM), which is suitable for complex flow geometry problems, has been widely applied by many authors (Karakas and Tariq 1991; Klotz et al. 1974). In this study, we used the FEM to numerically model the performance of perforated horizontal wells. Our model uses an automatic and adaptive mesh generation program, GID (CIMNE 2006), to generate the finite-element grid.
One of the great advantages of introducing a skin model for a perforated well is that it can be easily incorporated into any existing model of reservoir inflow performance or into a reservoir simulator. The modified perforation skin model developed here gives optimized perforation conditions and helps us to understand complex flow geometry in a horizontal perforated well. Using an accurate finite-element simulator, we also show a verification of the model.
|File Size||1 MB||Number of Pages||11|
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