Performance of Slanted and Horizontal Wells on an Anisotropic Medium

Abstract

Abstract Although the technology of highly inclined and horizontal wells has recently progressed significantly, a reliable and practical formula to estimate the productivity of the well with respect to any angle of slant and anisotropy of permeability is still missing. This paper provides a method of estimation which fills the gap. The well pressure decline curves are generated by a semi-analytical in-house simulator. They lead to a geometrical pseudo skin factor which is matched for slanted wells with an analytically-derived equation. For horizontal wells, comparison is made with several existing formulae. An unrestrictive approach of anisotropy Is possible through a spatial transformation from possible through a spatial transformation from real medium into equivalent isotropic medium this is achieved by modifying the length, azimuth, inclination and radius of the well. Equations of the pseudo skin factor are modified according to this transformation. Calculations reveal that a slanted well Is less affected by anisotropy than a horizontal well : with a low vertical permeability, it can be more advantageous than a horizontal well of the same length. Graphic charts are displayed to easy the choice between horizontal and highly inclined wells. Introduction The recent development of horizontal wells technology can be associated to a long list of advantages and fields of application. For low permeability reservoirs, the major interest in drilling horizontally is to increase productivity. But in predicting horizontal well performance, one should not forget a possible unfortunate effect : a low vertical-to-horizontal permeability ratio. The uncertainties linked to the difficulties to quantify this ratio (local heterogeneities, core sampling, etc..) may lead to abandon a horizontal well project. In order to minimize the risks, an appropriate answer is to drill almost horizontally, but not parallel to the stratigraphy or system of deposition. A wide range of trajectories is available, from the more complex "snake well" to the simple linear, highly Inclined well. Such a well has a double purpose : increase the producing length, and dispatch the production upon the whole thickness of the reservoir. production upon the whole thickness of the reservoir. Several authors have proposed formulae to predict the performance of a horizontal well, taking Into account permeability anisotropy and various shapes of the drainage volume. CINCO et al. have Introduced the definition of a pseudo skin factor to estimate the long-time performance of a conventional slanted well (less performance of a conventional slanted well (less than 75D) Change has built a gridded model suitable to non vertical wells, and compared performance of horizontal and slanted wells performance of horizontal and slanted wells under Isotropic permeability conditions. But a reliable and practical formula to estimate the productivity of the well with respect to any productivity of the well with respect to any angle of slant and degree of anisotropy is still missing. The proceeding of this work is divided into four steps–a semi-analytical simulator generates pressure decline curves for a set of slanted and horizontal wells producing in a homogeneous, isotropic, isopach, infinite reservoir. Comparison with a fully penetrating vertical well leads to a geometrical pseudo skin factor