Interference Testing In a Two-region Composite Reservoir With a Frontal Skin
Authors: Ambastha A.K. 
Published: March 2010

Abstract
A two-region composite reservoir model is used to analyze well-test data from a variety of enhanced oil recovery projects, geothermal reservoir, and acidization projects. Dynamic phenomena, such as phase changes and multi-phase flow effects in a region near the front, can cause a sharp pressure drop at the front. Such a sharp pressure drop can be modeled as a thin frontal skin.
This study considers interference testing in a two-region composite reservoir with a thin frontal skin. Wellbore storage and skin at the active well are also included in the analytical solution to the problem. Correlating parameters are established in the presence of a frontal skin for type-curve marching analysis of interference data from such reservoirs.
Our results indicate that for an observation well in the inner region surrounding the wellbore, the effects of storativity ratio (i.e. Φc, ratio) and frontal skin on interference tests are similar. Thus, frontal skin cannot be estimated independently of storativity ratio from an interference test on an observation well in the inner region. However, interference test data collected at an observation well located in the outer region beyond the front helps estimate storarivity ratio and frontal skin independently of each other. The information presented in this paper should be useful in obtaining a consistent analysis of well tests conducted in composite reservoir situations.

Introduction
This study considers interference pressure and pressure derivative responses in a two-region composite reservoir with an infinitesimally thin skin at the discontinuity. The observation well may be located in any region. Figure 1 shows a schematic diagram of a radial, two-region composite reservoir with an observation well located in the outer region (region 2) at a distance a from the active well. Region 1 (inner region) extends to a radius R, and a thin skin sr may occur at the discontinuity. Ambastha and Ramey(1) presented an analytical solution for transient pressure behavior at any point in a reservoir depicted by Figure 1. Their analytical solution as presented in the Appendix of this paper has been used to investigate interference test analysis in a radial, two-region composite reservoir with a frontal skin. All results presented in this study are for an infinite reservoir.
FIGURE 1: Two-region composite reservoir. (Available in full paper)

Correlating Parameters
Satman(2) presented an study of interference responses in radial, two region infinite composite reservoir. He did not consider the effects of a thin skin at the discontinuity. He correlated transient interference pressure responses using CD/aD, CDe2s, a/R, M and F as correlating parameters. He defined dimensionless pressure and time using the properties in the outer region, and considered the interference pressure responses for the observation wells located in the outer region only.
This study also considers the same correlating parameters as used by Satman(2). A thin skin at the discontinuity, Sr is an additional correlating parameter, Figure 2 through 7 verify the selected correlating parameters under different conditions. Figure 2 through 7 are for M = 10 and Fs = 100.
TABLE 1: Parameters used to generate responses shown on Figures 3, 6 and 7. (Available in full paper)