Critical-Gas-Content Technology Provides Coalbed-Methane-Reservoir Data
Authors: Lamarre Robert A. , Pope John 
Published: June 2014

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

The two most significant reservoir parameters for evaluating coals for possible coalbed-methane (CBM) production are the gas content (measured in scf/ton of coal) and the gas saturation (amount of gas stored in coal divided by the maximum amount of gas that the coal can hold, in %). Historically, these parameters could be obtained only by cutting a core of the coal, placing the core sample in a desorption canister, and waiting a few months for the gas to desorb from the coal.

Once desorption was complete, a representative sample of the coal was sent to a laboratory for determination of the adsorption isotherm, which indicates the gas-storage capacity of the coal.

Introduction

The described technique has many inherent problems, including cost, mechanical risk of coring, amount of gas that is released from the coal during its trip up the hole, the long time required for gas to desorb from the coal, and the loss of gas caused by handling and transport. Because of the long time required for laboratory analyses of initial appraisal wells, development wells might not be drilled until the next drilling season. In addition, the cost of these data usually prohibits collecting enough information to ensure that the data set is representative of the prospect. Instead, it is common to assume that coal-core data from one well are representative of the coals covering one or more townships (36 sq miles). Reservoir data collected from more than 170 CBM wells in the Powder River basin (PRB) of northeastern Wyoming by use of new critical-gas-content technology demonstrated that coal variability is significant at a scale much smaller than a township.

This new technology uses a downhole chemical-sensing tool developed specifically for the CBM industry. The tool provides the critical desorption pressure (CDP) of methane gas dissolved in coal-reservoir fluid in the wellbore. The key instrument in this tool is a Raman spectrometer that has been downsized and ruggedized so that it can be lowered by wireline into a 5-in.-diameter wellbore. The Raman effect was discovered in 1928 and has been used in many industries for mineral identification. The Raman effect occurs when light scatters from a molecule with a slightly changed energy, or color, caused by excitation of the molecule's chemical bonds. As a result, observing the colors of light reflected back from a sample indicates which molecules make up the material. An example of the spectroscopic response from three different concentrations of methane dissolved in water is shown in Fig. 1.