The Study Of Development Mechanism And Technologies Of Coalbed Methane In Southern Qinshui Basin, China

As the main development block of high rank coal in China, the development technologies’ promotion of coalbed methane (CBM) in southern Qinshui Basin will enormously accelerate the industrialization of CBM development. Based on the recent development practice, some key development issues were systematically studied in this paper, such as porosity and permeability characteristics, the law of desorption and seepage, well performance, productivity evaluation, well pattern and de-watering control.It was realized that, using intrusive mercury and nuclear magnetic resonance tests, the characters of coal construction in the southern Qinshui basin were poor-developed matrix pores and weak connectedness. The permeability was determined by cleat or exogenic fractures, which are mainly influenced by the coal structure, buried depth of coal seam and in-situ stress. The exponential relation between permeability and in-situ stress (or depth) was established according to the field measured data.It was found that CBM’s adsorption speed in 3# coal seam can accelerate or delay under the fracturing fluid. These results were used to evaluate the damage of fracturing fluid to the adsorption and optimize fracturing fluid prescription.Flow experiments revealed that CBM percolation flow showed obvious non-Darcy characters affected by multiple factors. It was observed that water saturation played an important role during seepage flow. The higher the water saturation was, the higher the starting pressure gradient was. Relative permeability experiments showed that the lower the permeability is, the higher the irreducible water saturation is. When the gas begins to breakthrough, gas relative permeability rises quickly. This results increase the difficulty of water-discharge, limiting the extensions of the drainage radius. So it is very necessary to control the gas growth speed in the initial stage for high de-watering efficiency.Through more than 400 wells’dynamic analysis of the Fanzhuang block,it was realized that gas content, permeability, postfracture response, de-watering control and coal fines are the main productivity influencing faators. Also their’s action mechanism and the influencing degree were evaluated. According to the above research results, well pattern and well spacing were optimized under different permeability using the analogy analysis, numerical simulation, drainage radius analysis and economic evaluation.Reasonable de-watering control is the key to obtain high yield, which should be emphasied on stable bottom hole pressure, rational casing pressure, fluid level and degasing rate in order to extend drainage radius as far as possible.