Abstract

Prediction and analysis of the coalbed methane (CBM) production ischallenging especially at the early stages of the recovery when production issubject to two-phase flow conditions. CBM reservoirs are dual-porosity systemsthat are characterized by a complex interaction of coal matrix and cleat systemthat are coupled through desorption process. As a result, the conventionalmethods cannot be utilized to predict CBM production. Currently, only numericalmodels (simulators) can be used to predict (CBM) production behavior since theyincorporate the unique flow and storage characteristics of CBM reservoirs. Often, the number of variables needed for evaluation of a given prospect isgreater than that typically measured. In such cases, parametric studies areconducted to evaluate the impacts of reservoir properties on recovery factor, well performance, and future revenues. However, parametric studies with areservoir simulator is cumbersome, time consuming and expensive. Therefore, there is a need for scientific, user-friendly tools that can assist the typicalgas producers in evaluation of CBM prospects.

This study presents a set of production type curves for CBM reservoirs. Areservoir model that incorporates the unique flow and storage characteristicsof CBM reservoirs was employed to generate the type curves. Our previous study(SPE 91482) introduced a new set of dimensionless groups that led todevelopment of unique set of type curves. The impact of basic reservoircharacteristics was also investigated to confirm the uniqueness of the typecurves. This study investigates the impact of key parameters including isothermconstants and relative permeability data on the type curves. The type curvescan be used to predict and/or analyze gas and water the production from CBMwells both during de-watering and stable gas production phases. The issuesconcerning the application and limitation of the type curves are alsopresented. Finally, a type curves-based simple yet reliable tool with graphicaluser interface (GUI) was developed. This tool can be used for predictingand analyzing production performance of CBM reservoirs with simple and straightforward input requirements. This tool is particularly useful for parametricstudies to evaluate CBM prospects.

Introduction

CBM has grown from an unconventional gas play in the 1980's into acommercially important, mainstream natural gas source. CBM proven reserves inthe United States have increased from 3.7 Tcf in 1989 to 18.75 Tcf in 2003 andcurrently account for over 10 percent of the estimated total U.S. natural gasreserves. This number is expected to increase even further as more resourcesare discovered and a better understanding of the existing resources isachieved. The contribution of CBM to U.S. annual gas production has exceeded 8percent and is expected to increase for next two decades. However, for CBM toreach its true potential in the U.S. energy equation, substantial increase inCBM production in less developed basins, such as the Northern AppalachianBasin, must be achieved. Therefore, it is necessary to develop tools that makeit possible for producers to seriously consider this important resource. Thereis a need for scientific, user-friendly tools that can assist the typical gasproducers in evaluation of CBM prospects.

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