GENERATING THE DYNAMIC CHARACTERISTICS AND PREDICTING TECHNIQUES FOR COAL BED METHANE (CBM) PRODUCTION USING FIELD PERFORMANCE DATA

摘要

In an era of declining production and increasing demand for oil, producing gas economically from CBM sources will be the next level of the fossil-fuel recovery challenge. The vast volume and long-term potential of CBM is driving technical progress. Attractive gas prices around the world and unprecedented interest in world markets promise to bring CBM gas into the forefront of our energy future.CBM is notably different from a typical conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, and lines the inside of pores within the coal (called the matrix). The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water. The system is then called a "dual-porosity reservoir", one characterized by a complex interaction of the coal matrix and cleat system coupled through the desorption process. The mechanism for gas flow in the coal involves three steps: (1) desorption of the gas from the coal surface inside the micropores, (2) diffusion of the gas through the micropores of the coal matrix, and (3) Darcy flow through the fracture (cleat) network to the wellbore.The production profiles of CBM wells are typically characterized by a negative decline in the gas rate, as water is pumped away and gas begins to desorp and flow - and will naturally decline as reservoir pressure is depleted. During the flow through the fractures, a dynamic property, such as relative permeability, is the primary parameter for determining the reservoir production behavior of the CBM system. At times, however, laboratory-derived relative permeability data is unavailable.The purpose of this research was to develop a simple and reliable tool to predict the future production performance as well as to generate relative permeability for CBM systems utilizing past production performance data. The proposed method will extend the use of the Cut-Cum plot method which is firstly proposed by Ershaghi and Omoregie (1978) and continued by Ershaghi dan Abdassah (1984) based on the Frontal Advance Concept by Buckley-Leverett, and a modified Corey equation. The Cumulative Plot technique (Irawan and Abdassah, 2007) will be extended for prediction of CBM performance. Results are compared with laboratory data and this method was successfully used to examine the performance of over 40 CBM production performance data.