TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCoalbed Methane (CBM) currently accounts for nearly 8 percent of U.S. annual gas production and approximately 12 percent of estimated total U.S. natural gas reserves. Coalbed methane proven reserves in the United States have increased from 3.7 Tcf in 1989 to 18.5 Tcf in 2002. This number is expected to increase even further as more resources are discovered and a better understanding of the existing resources is achieved. Appalachian Basin accounts about 10 percent of U.S. CBM resources. However, CBM production is very limited in the Appalachian Basin. The contribution of CBM to overall mix of natural gas sources in U.S. is expected to increase for next two decades. However, this cannot be achieved without substantial increase in CBM production in the Appalachian Basin. The problems causing the lag in development of CBM in the Appalachian Basin need to be overcome for CBM to reach its true potential in the U.S. energy equation.Gas production from CBM reservoirs is governed by complex interaction of single-phase gas diffusion through micro-pore system (primary porosity) and two-phase gas and water flow through cleat system (secondary porosity) that are coupled through desorption process. In order to effectively evaluate CBM resources, it necessary to utilize reservoir models that incorporate the unique flow and storage characteristics of CBM reservoirs. These models are often complicated to use, expensive, and time consuming. The typical gas producers in the Appalachian Basin suffer from the lack of scientific, userfriendly tools that can assist them in development of CBM resources. Therefore, it is necessary to develop tools that make it possible for typical (small to medium size) producers to seriously consider this important resource.This study presents a set of production type curves that would help the producers to predict the production from their CBM wells. As a consequence, the producers would be able to make better, more informed decisions regarding the CBM resources in the region. A reservoir model that incorporates the unique flow and storage characteristics of Coalbed Methane reservoirs was employed in this study to develop the type curves. The type curves provide a reliable tool to predict the production performance of CBM reservoirs both during dewatering and stable gas production phases. The application and issues concerning the production performance of CBM reservoirs are also discussed.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPrediction and analysis of the coalbed methane (CBM) production is challenging especially at the early stages of the recovery when production is subject to two-phase flow conditions. CBM reservoirs are dual-porosity systems that are characterized by a complex interaction of coal matrix and cleat system that are coupled through desorption process. As a result, the conventional methods cannot be utilized to predict CBM production. Currently, only numerical models (simulators) can be used to predict (CBM) production behavior since they incorporate the unique flow and storage characteristics of CBM reservoirs. Often, the number of variables needed for evaluation of a given prospect is greater than that typically measured. In such cases, parametric studies are conducted to evaluate the impacts of reservoir properties on recovery factor, well performance, and future revenues. However, parametric studies with a reservoir simulator is cumbersome, time consuming and expensive. Therefore, there is a need for scientific, user-friendly tools that can assist the typical gas producers in evaluation of CBM prospects.This study presents a set of production type curves for CBM reservoirs. A reservoir model that incorporates the unique flow and storage characteristics of CBM reservoirs was employed to generate the type curves. Our previous study (SPE 91482) introduced a new set of dimensionless groups that led to development of unique set of type curves. The impact of basic reservoir characteristics was also investigated to confirm the uniqueness of the type curves. This study investigates the impact of key parameters including isotherm constants and relative permeability data on the type curves. The type curves can be used to predict and/or analyze gas and water the production from CBM wells both during de-watering and stable gas production phases. The issues concerning the application and limitation of the type curves are also presented. Finally, a type curves-based simple yet reliable tool with graphical user interface (GUI) was developed. This tool can be used for predicting and analyzing production performance of CBM reservoirs with simple and straight forward input requirements. This tool is particularly useful for parametric studies to evaluate CBM prospects.
The complex nature of gas production from CBM reservoirs requires significantly more reservoir properties than conventional reservoirs. Parametric studies are often conducted to evaluate the impact of the reservoir parameters on recovery factor, well performance, and future revenues. To conduct parametric studies, the production behavior must be predicted for ranges of various reservoir parameters. In order to incorporate the unique flow and storage characteristics of CBM reservoirs, it necessary to utilize complicated and expensive reservoir models to predict production performance of the CBM reservoirs. Therefore, a simple yet reliable tool is needed for predicting production behavior of CBM reservoir. The CBM Production Predication Tool developed in this study is a quick and reliable tool that can assist the typical gas producers in evaluation of CBM prospects. The tool utilizes the previously developed CBM Production Type Curves as a foundation to predict the production performance. The results can be used to evaluate the economic feasibility and to maximize potential recovery. To simplify the application of type curves for parametric studies a software package with a graphical user interface using Visual Basic programming language has been developed. The input requirements are simple and straight forward. The output is provided both in graphical and tabulated forms which can be exported into spreadsheet for further analysis by the user. Introduction CBM has grown from an unconventional gas play in the 1980's into a commercially important, mainstream natural gas source. CBM currently account for over 10 percent of the estimated total U.S. natural gas reserves and 8 percent of U.S. annual gas production. These numbers are expected to increase even further as more resources are discovered and a better understanding of the existing resources is achieved. However, for CBM to reach its true potential in the U.S. energy equation, substantial increase in CBM production in less developed basins, such as the Northern Appalachian Basin, must be achieved. Therefore, it is necessary to develop tools that make it possible for producers to seriously consider this important resource. There is a need for scientific, user-friendly tools that can assist the typical gas producers in evaluation of CBM prospects. The complex nature of CBM production behavior precludes use of conventional techniques such as decline curve methods to forecast the recovery factor, future revenues, and well performance. Production from CBM reservoir is controlled by the complex interaction of gas desorption from coal matrix and two-phase flow of gas and water through cleat system. Numerical reservoir simulators that account for various mechanisms that control CBM production are by far the best tools for predicting the performance CBM reservoirs. Often, the number of variables needed for evaluation of a given prospect is more than those typically measured. Therefore, parametric studies must be conducted to evaluate the impacts of reservoir properties on recovery factor, well performance, and future revenue evaluation. When one or more of the key parameters are not available, it is necessary to perform Monte Carlo simulation to establish a reliable estimate of production potential and to evaluate the risk However, it is time consuming and cumbersome to conduct Monte Carlo simulation or parametric studies with a reservoir simulator. Many of the producers, particularly in Northern Appalachian Basin, cannot afford the personnel and economical requirements involved in use of the reservoir simulator to evaluate CBM prospects. This paper introduces a simple yet reliable tool that can be used for parametric studies to evaluate CBM prospects.
Coalbed methane is an unconventional gas resource that consists of methane production from the coal seams. CBM reservoirs are dual-porosity systems that are characterized by a complex interaction of coal matrix and cleat system that are coupled through desorption process. In order to effectively evaluate CBM resources, it necessary to utilize reservoir models that incorporate the unique flow and storage characteristics of CBM reservoirs. These models are often complicated to use, expensive, and time consuming. The typical gas producers in the Appalachian Basin suffer from the lack of scientific, user-friendly tools that can assist them in development of CBM resources. Therefore, it is necessary to develop tools that make it possible for typical (small to medium size) producers to seriously consider this important resource. This study presents a set of production type curves for CBM reservoirs that would help the producers to predict the production from their CBM wells. As a consequence, the producers would be able to make better, more informed decisions regarding the CBM resources in the region. A reservoir model that incorporates the unique flow and storage characteristics of CBM reservoirs was employed in this study to develop the type curves. The type curves provide a reliable tool to predict the production performance of CBM reservoirs during dewatering phase. The application and issues concerning the production performance of CBM reservoirs are also discussed In order to achieve the objective of this study, four steps were performed: (a) Development of a base model for Coalbed methane production in Northern Appalachian, (b) development and verification of the dimensionless groups for water production type curves (c) Generation of the CBM water production type curve, and (d) validation of the CBM water production type curve. A modified correlation for peak gas rate estimation was also proposed as an alternative to forecast gas production along with a correlation for computing initial (maximum) water production rate.
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