Abstract

Prudent coalbed methane (CBM) reservoir management and optimization requires knowledge of reservoir pressure. This paper presents a simple three-step method to predict reservoir pressure throughout the producing life of a coalbed methane well using only initial reservoir pressure, Langmuir pressure and cumulative gas produced. Use of the method has the potential to significantly reduce operating costs by minimizing the number of pressure surveys required to understand, operate and optimize a well or reservoir. The equations and process used to predict reservoir pressure are discussed, results of simulation studies validating the method are presented and actual examples from Fruitland Coal wells in the San Juan Basin are used to test the applicability of the method to real world conditions.

Introduction

Coalbed methane reservoir management and optimization requires knowledge of reservoir pressure. Typically, only a limited number of pressure surveys are run during the life of a well because they are costly in terms of equipment, manpower usage, and lost production. Calculations requiring reservoir pressure are often made months, sometimes years, after the most recent pressure survey in a given well. At high reservoir pressures a small uncertainty in reservoir pressure will make little difference in deliverability and remaining gas-in-place calculations. Conversely, a small uncertainty in reservoir pressure can make a large difference in such calculations at low pressures.

Several methods exist to obtain reservoir pressure in addition to running a pressure survey. Reservoir simulation, traditional material balance (p/z*) and "best-guess" methods could all be used to estimate reservoir pressure. At high reservoir pressures the "best-guess" method may give adequate results. At lower reservoir pressures, more accurate methods such as reservoir simulation or p/z* material balance are needed to give reliable results. Unfortunately, both of these methods are time consuming and require large amounts of information.

The method presented in this paper is a simple material balance application of the Langmuir isotherm to give accurate reservoir pressures over the entire life of a producing coalbed methane well. The method requires knowledge of the initial reservoir pressure and Langmuir pressure and depending on the application, either initial gas in place, cumulative gas produced as a function of time or a history of reservoir pressure obtained from pressure surveys. This data is easily obtainable for a producing well. Given some of the above data, the proposed method can be used in an iterative fashion to determine the other properties.

This paper discusses the equations and a simple three-step procedure to obtain reservoir pressure, reports the results of a simulation study to validate the method, and gives several examples of applications to Fruitland Coal wells in the San Juan Basin.

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