document

J.P. Spivey* and M.E. Semmelbeck*

Abstract

A procedure is presented which allows forecasting long range performance of dewatered coal and fractured gas shale reservoirs having nonlinear adsorption isotherms, using constant pressure solutions to the flow equation for slightly compressible liquids. A correlation is presented to show the range of applicability of this procedure.

Introduction

Production decline curves are routinely used by engineers to predict the future performance of oil and gas wells. Because the results of decline curve predictions are used for calculating asset value and estimating future revenue, they are one of the most important tools reservoir engineers use. There are numerous variations on the basic exponential or hyperbolic decline analysis method. Fetkovitch and others have extended the decline curve analysis method to handle gas wells properly and to be able to estimate reservoir properties from the analysis of these data. However, there has been considerable drilling activity in the last 10 years into unconventional reservoirs whose wells do not follow the traditional production decline characteristic shapes. Among these problem reservoirs are coalbed methane and fractured shale reservoirs.

Two factors complicate the prediction of future gas production rates in many coalbed methane and fractured shale reservoirs such as the Devonian Shale of the Appalachian Basin, the Antrim Shale of Michigan and the New Albany Shale in Indiana. The first factor common in Antrim and New Albany reservoirs is high initial water saturation and essentially zero gas flow rate at the beginning of production. The second factor common to all fractured gas shales is desorption of gas from organic material within the reservoir rock. Both of these factors can result in well behavior that is not properly predicted by conventional decline curve methods.

Because of the complex production behavior of coalbed methane and fractured gas shale wells, the best way to predict performance is to use a numerical reservoir simulator which accounts for all of the mechanisms occurring during production. However, use of reservoir simulation may not be practical for all situations, particularly when many wells must be analyzed rapidly or when reservoir simulation is not available. This paper presents a rapid analytical solution that can account for production from reservoirs undergoing desorption. One extension of this method over others presented in the literature is that it accounts for nonlinear Langmuir sorption isotherms.

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