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 to analyze 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 pseudosteady-state 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 ccurately estimated by the pseudosteady-state method for large mobility and storativity contrasts between the two regions.
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 falloff test conducted on a horizontal injection well can be analyzed using the pseudosteady-state (PSS) method. The PSS method was first proposed by Eggenschwiler el al.3 (l) to analyze 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 pseudosteady-state 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 falloff test data for horizontal, steam-injection wells. In this study, a specialized three-dimensional, single-phase, non-thermal 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 espect to various parameters, followed by concluding remarks.
A three-dimensional, single-phase, numerical simulator has been developed to study the transient pressure responses for a horizontal well located in a box-shaped, composite reservoir as shown in Figu