Well Imaging and Fault Detection In Anisotropic Reservoirs

Abstract

Flow-confining image well location in both isotropic and anisotropic reservoirs can be reduced to one general equation. The equation is presented here, along with analyses of fault-location errors that result presented here, along with analyses of fault-location errors that result from conventional pressure transient studies applied to anisotropic systems. Introduction Engineers and geologists have long been aware of the directional flow properties of natural rocks. Permeabilities measured across the bedding are usually much Permeabilities measured across the bedding are usually much less than those along the bedding. Moreover, a change in flow direction along the plane of bedding will often show a significant change in horizontal permeability. This anisotropic behavior of natural permeability. This anisotropic behavior of natural rocks can pose some major problems for efficient reservoir exploitation. Some degree of horizontal permeability anisotropy is believed to be present, with few exceptions, in bedded rocks and limestones. In general, sandstones do not exhibit permeability ratios, k /k, in excess of 2.0. Homogeneous fractured reservoirs, such as the Spraberry trend field, however, may exhibit ratios as high as 144. In about half of some 10 limestone cores analyzed, Hutchinson reported significant permeability ratios averaging about 16. Arnold et al. and Elkins and Skov developed steady-state and transient pressure methods to evaluate horizontal, anisotropic media. Both the orthogonal permeability ratio and the orientation of the principal permeability ratio and the orientation of the principal axes can be determined by suitable well tests. The effect of anisotropy upon secondary recovery flood patterns has been studied by a number of patterns has been studied by a number of investigators. They conclude that the ease of flow or transmissibility of the fluid in one preferential direction causes premature water breakthrough and poor sweep efficiency with improper choice of pattern geometry and orientation. The combined effects of inhomogeneity and horizontal anisotropy have not been studied extensively. Greenkorn et al. extended the tensor theory of permeability to include heterogeneity for the case of flow permeability to include heterogeneity for the case of flow parallel to the plane of bedding. Merrick considered parallel to the plane of bedding. Merrick considered both anisotropy and stratification in evaluating recovery from gas cycling projects in bounded systems, using steady-state streamline flow solutions. Transient pressure buildup and drawdown tests used to define linear discontinuities within a reservoir should also be affected by anisotropy. Conventional methods of analysis us in their mathematical development the principles of superposition and direct mirror imaging of wells - principles applicable to isotropic systems only. The purpose of this study, therefore, was to:develop a valid technique for well imaging in anisotropic media,verify its use by a study of steady-state streamline patterns about a single well in the vicinity of a linear sealing boundary,devise methods for fault detection and location in anisotropic media, andinvestigate the error incurred by use of isotropic methods of fault location when applied to anisotropic reservoirs. JPT P. 1317