A semi-analytical model for drainage and desorption area expansion during coal-bed methane production. (15th September 2017)

• Record Type: Journal Article
• Authors: Sun, Zheng; Li, Xiangfang; Shi, Juntai; Yu, Pengliang; Huang, Liang; Xia, Jun; Sun, Fengrui; Zhang, Tao; Feng, Dong
• Is Part Of: Fuel. Volume 204(2017)
• Page Start: 214
• Page End: 226
• Publication Date: 2017-09-15
• Subjects: Coal-bed methane -- Semi-analytical model -- Drainage and desorption area -- Continuous succession of steady states -- Production system optimization; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2017.05.047
• Languages: English
• ISSNs: 0016-2361

Abstract

Highlights: A practical model for the expansion characteristics of drainage and desorption area is proposed. Variable production rate, BHP and saturation distribution are considered in this method. The model was verified by numerical simulation with excellent agreements. A set of quantitative criteria for the optimization of production system is firstly proposed. The optimization criterion was applied to Qinshui CBM region and achieved good results. Abstract: In coal-bed methane (CBM) field development plan, more attention is paid to production forecasting, well spacing optimization, pressure transient analysis and adsorption/desorption mechanism within coal matrix. Few studies were carried out to investigate the expansion of drainage and desorption area based on the characteristics of pressure propagation in CBM reservoir. Moreover, the production rate and bottom-hole pressure (BHP) are usually variable, while the production system in current analytical models are either at constant production rate or constant BHP. When optimizing the production system of CBM well, the knowledge of expansion principle of drainage and desorption area can help obtain the critical time of interference and its intensity, which are both key factors for efficient development of CBM reservoirs. In this paper, a comprehensive method for drainage and desorption area expansion during coal-bed methane production is proposed, in which the variable rate and BHP in different production stages and saturation distribution are considered. The coal seam is divided into two areas, namely desorption and without-desorption area, and the production life is divided into five stages. The pressure profiles at different production stages are detailed investigated utilizing diffusivity equation which is solved by the continuous succession of steady states. By assuming the desorption area with low water-gas ratio and without-desorption area with high water-gas ratio, the pressure-squared approach is applied in desorption area and pressure approach is employed in without-desorption area. Finally, a model for predicting drainage and desorption area is developed by generating pressure profile and material balance equation. The proposed semi-analytical model is verified by numerical simulation with excellent agreement with the simulation data. Results show that the drainage area expands much faster at early production stage than that at late stage when desorption area starts expanding in the coal-bed reservoir. In addition, the production system of CBM well is found to play a significant role in the expansion characteristics of desorption area. Considering these characteristics of CBM, we shed light on the production system optimization in different production stages and apply the developed production system to a CBM well in Qinshui field, China. In summary, this method only requires BHP, gas and water production, and turns out to be simple, reliable and powerful in application.