Effects Of A Thin Skin At The Front On Composite Reservoir Well Tests

Abstract

A radial composite reservoir model is used to analyze well-tests from a variety of enhanced oil recovery projects, geothermal reservoirs, and acidization projects. A composite reservoir is made up of two or more regions. Each region has its own rock and fluid properties. Also, dynamic phenomena, such as phase changes and multi-phase 1ow effects in a region near the front, can cause a sharp pressure drop at the front. Such a sharp pressure drop is modelled as a thin skin at the front in this study. An analytical solution for the transient pressure behaviour of a well in a two-region composite reservoir with a skin at the front is obtained using the Laplace transformation.

Analysis shows that a thin skin at the front can explain a short duration pseudo-steady state corresponding to the inner swept volume for small mobility and storativity contrasts. The effects of is skin at the front are similar, to the effects of storativity ratio. Thus, neglecting a thin skin at the font can cause large errors in parameter estimation using a type-curve matching method. Also, graphical correlations are presented for the time to rile end of pseudo-steady state behaviour corresponding to the inner swept volume with a skin at the front. Such correlations should help in selecting a proper pseudo-steady Cartesian straight line for data analysis.

Introduction
Figure 1 shows a schematic diagram of a two-region, radial composite reservoir. The inner and outer regions of a composite reservoir have different, but uniform rock and fluid properties, and are separated by a discontinuity. The distance R is the front (or discontinuity) radius, which is an important parameter sought from well tests in composite reservoirs. Eggenschwiler et al. ^(I) presented an analytical solution in Laplace space for the transient pressure behaviour of a well producing (or injecting) at a constant rate from (or into) a two-region, radial infinite composite reservoir. Horne et a ⁽²) extended them Eggenschwiler et a, solution to finite composite reservoirs.

This study considers transient pressure derivative behaviour of a well in a two-region, composite reservoir with an infinitesimally thin skin at the front. The effects of a thin skin at the front on the transient pressure and pressure derivative behaviour of a well in a composite reservoir is considered important because a thin skin at the front may be a practical approach to model the following physical situations:

1. vapourization at the discontinuity while injecting cold water in a hot geothermal reservoir;


2. condensation at the steam front such as in steam injection projects;


3. cases where a transition region is apparent. For in situ combustion cases, Onyekonwu⁽³⁾ observed a transition region.


4. Pressure profiles presented in Figures 6.8 and 6.11 of Reference 3 suggest that the system may be modeled as a two-region reservoir with a thin skin at the discontinuity; and


5. simulated CO₂ flooding results show that from 600;0 or the over-all pressure- drop occurs in a small region around the front⁽⁴⁾.