A Numerical Investigation of Pressure Transient Analysis For Horizontal Wells In Composite Reservoirs Mimicking Thermal Recovery Situations
Authors: Ambastha A.K. , Ghaffari K. 
Published: March 2010

Abstract
Horizontal wells have become quite popular for primary and enhanced oil recovery operations due to their well-documented advantages over vertical wells. Steam injection through horizontal wells has also been attempted at several places to improve heavy oil recovery. For horizontal wells undergoing steam injection, a steam chamber containing high mobility steam is established. This steam chamber, which can be of a complex shape, is surrounded by low mobility reservoir fluids. Such reservoir situations are referred to as composite reservoirs. An analytical solution for pressure transient tests for horizontal wells under composite reservoir situations with complex swept region shapes is not available yet.
This study attempts a numerical investigation of pressure transient analysis for horizontal wells in two-region, composite reservoirs mimicking thermal recovery situations. A specialized three-dimensional, single-phase simulator was developed for this purpose. A closed, box-shaped reservoir was considered with a horizontal well.
A detailed sensitivity study of transient pressure responses is presented with respect to grid size, well location in different directions, swept region shape, mobility ratio and storativity ratio. This study emphasizes the effects of the aforementioned factors on the swept volume estimation using the pseudosteadystate method. The pseudosteady-state method is based on an analysis of a Cartesian graph of pressure response versus time. This study establishes that for horizontal wells, the swept (or inner) region volume in a two-region system can be accurately estimated by the pseudosteady-state method for large mobility and storativity contrasts between the two regions.

Introduction
Horizontal wells have become quite popular in recent years due to their well-documented advantages over vertical wells(1). Steam injection through horizontal wells has also been attempted at several places to improve heavy oil recovery. Steam injection through horizontal wells includes scenarios such as steam-assisted gravity drainage process pioneered by Butler(2). Under such situations, a steam chamber containing high mobility steam is established around a horizontal well. A fall off test conducted on a horizontal injection well can be analysed using the pseudosteady-state (PSS) method. The PSS method was first proposed by Eggenschwiler et al.(3) to analyse thermal well tests conducted on vertical injection wells to estimate the steam chamber volume. Though a number of papers(4–9) have evaluated the applicability of the pseudosteadystate method for vertical injection wells, to the best of our knowledge, Reference (10) is the only study concentrating on the applicability of the pseudosteady-state method for horizontal injection wells. Issaka and Ambastha(10) used a numerical thermal simulator to generate fall off test data for horizontal, steam-injection wells. In this study, a specialized three-dimensional, single-phase, nonthermal simulator is used to study the applicability of the pseudosteady- state method for horizontal injection wells.
In the next section, a brief description of the numerical model is presented followed by a section reporting our validation efforts for the numerical model. Thereafter, a detailed sensitivity study of transient pressure responses is presented with respect to various parameters, followed by concluding remarks.