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Abstract

Diagnostic techniques are presented for detecting and quantifying poorly drained compartments in volumetric gas reservoirs. It is shown that p/z versus G data can be used to assess unrecovered gas reserves and assist in targeting infield development.

Introduction

The impact of reservoir heterogeneity as a factor limiting recovery in oil and gas reservoirs has become well recognized. Consequently, improved reservoir characterization is now a major concern in both oil and gas reservoir development. Recent estimates' indicate that significant quantities of unrecovered gas will remain in presently identified reservoirs under historic development practices. Thus, new techniques are required to obtain maximum recovery.

In the following discussions, we introduce a method for analyzing production data for gas wells to detect poorly drained compartments and estimate reserves in those compartments. When combined with detailed geological studies and transient pressure well tests, particularly interference tests, this method can be used to target sites for infill wells to maximize recovery at minimum risk. The technique is based upon simple material balance relationships which provide significant insight into production characteristics of volumetric, compartmented gas reservoirs.

MATERIAL BALANCE IN A COMPARTMENTED RESERVOIR

We consider an idealized "tank" model for a volumetric gas reservoir having two compartments separated by a low permeability barrier as in Figure 1.

The material balance, or conservation of mass, for gas is then described approximately by the pair of equations

......(1)

......(2)

Here V1,p1 and V2,p2 are the pore volume and gas density in the respective compartments, psc and qsc are the gas density and flow rate, at standard conditions, Tor a well draining compartment 1, and the "barred" quantity is the mass flow rate of gas from compartment 2 into compartment 1 across the barrier. This last term comes directly from Darcy's law with k and A the permeability and area of the "thin" barrier. Note that no pore volume is assigned to the barrier domain and the "bar" indicates averaging over the domain of the barrier.

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