Type Curves for Coalbed Methane Production Prediction

Abstract

Abstract Coalbed 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, 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 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 de-watering and stable gas production phases. The application and issues concerning the production performance of CBM reservoirs are also discussed. Introduction After a humble beginning in the 1980's, coalbed methane (CBM) has become a significant natural gas resource worldwide. Presently CBM accounts for 8 percent of total annual U.S. dry gas production and 12 percent of estimated total recoverable U.S. natural gas resource base. The contribution of coalbed methane to the overall mix of natural gas resources in the U.S. is expected to increase in the next two decades. Northern Appalachian Basin, which is comprised mainly of large areas of West Virginia, Pennsylvania, and Ohio, is one of the most potential basins in United States. The basin's CBM resources have been estimated to be 61 Tcf. To date, CBM production in the basin however has been virtually non-existent. The projected increase in CBM production in US cannot be realistically achieved without a substantial increase in CBM production in the Northern Appalachian Basin. The technical problems and uncertainties causing the lag in development of CBM in the Northern Appalachian Basin need to be overcome for CBM to reach its true potential in the U.S. energy equation. The typical gas producers in the Northern 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 essential tools that make it possible for the operators to realistically evaluate and characterize coalbed methane reservoirs.