Abstract

Sound development decisions for coalbed methane reservoirs, require accurate determination of gas in-place and natural fracture permeability. The purpose of this paper is to outline procedures to evaluate reservoir properties required for prediction of well deliverability. The steps are documented with data and analyses from San Juan Basin Fruitland Formation coalbed methane development projects. The procedures are applicable to other sedimentary basins.

The first step in the evaluation program is to review available well and geologic data to select well locations and to design data collection programs. These data include structure and stratigraphic maps, drilling records, logs, well tests, production records, gas content information, and fracture studies.

The next step in the evaluation is to collect gas content and permeability data from the first well or wells drilled. Gas content is quantified with conventional core, pressure core, sidewall core, and drill cutting samples. An example of the accuracy of gas content determinations from these sources is discussed. The permeability of the fracture system is quantified with drill stem tests. Testing procedures will be presented. Following testing, open hole logging is performed. Specifications for complete log suites are included.

Once the well site operations are concluded, it is necessary to determine the coal gas storage capacity and the properties of the coal and non-coal rocks from core analyses. Natural fracture geometry is further quantified. Well log analysis techniques are developed. The core analyses, log analysis techniques, and the accuracy thereof will be documented.

The final step in the evaluation is to combine all of the above data and analyses to predict gas and water deliverability. The methodology utilizes reservoir simulation which will be briefly discussed.

Introduction

The purpose of this paper is to present examples of the methodology required to evaluate the properties of coalbed methane reservoirs for deliverability prediction. In order to make informed decisions concerning the economic viability of coalbed methane reservoir development, it is necessary to make accurate projections of fluid production rates over the life of the project. As a simplified summary, accurate projections require estimates of the following reservoir properties which are components of Darcy's and Fide's Laws.

Reservoir Geometry

Fluid Volumes in Place

Fluid Properties Affecting Fluid Flow

Rock Properties Affecting Fluid Flow

The purpose of the first exploratory wells drilled into a reservoir is to collect as much of this information as possible. Reservoir geometry estimates are most accurately determined by structural and stratigraphic mapping. The fluid volumes in-place must be quantified with core and fluid samples with appropriate analyses. The vast majority of the gas volume in-place is adsorbed in coal matrices. Estimates of this volume require that coal samples be placed in desorption canisters immediately upon retrieval to surface, and that the volume of gas desorbed is measured as a function of time, temperature, and pressure. With the exception of pressure core samples, it is necessary to estimate the volume of gas lost while retrieving the samples. The "lost gas" volume is added to the measured gas volume to improve the accuracy of gas content estimates.

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