| The potential of deep coalbed methane(CBM) resource in our country is huge,with good prospects for exploration and development. In order to enhance the understanding of the development geologic conditions, development dynamics and production characteristics of deep coalbed methane, the Hancheng area is taken as the research target zone in this paper. Based on a large amount of geological data of CBM exploration, the sample test data and CBM production data, the geological conditions of deep and shallow CBM in the area are compared, development dynamics differences between deep and shallow CBM are discussed as well. At last, the production characteristics and the main controlling factors are analyzed. Concrete results and recognitions are as follows:The in-situ stress increases with the increasing of the depth of the coalbed. When the coal reservoir with a burial depth sallower than 1400 m, the in-situ stress state principally shows σhmax﹥σv﹥σhmin, which still not reach to the stress transition belt. The coal reservoir is in the uncompacted state, with an average pressure gradient of 7.73kPa/m. The geothermal increases following a linear relationship as coal seam burial depth increases, with an average geothermal gradient of 2.79 ℃ /100m; Permeability decreases with the increasing of coalbed buried depth. When the burial depth is deeper than 1000 m, the permeability of the coal reservoir is always less than 0.2md; With the increasing of the burial depth, gas content of the coal reservoir first increase then decrease, and the buried depth of the maximum gas content is between 1000 and 1600 m.During the process of depressurization desorption, the desorption efficiency of different pressure range is obviously different. As the reservoir pressure decreases, the desorption efficiency increases gradually. The rise of temperature is conducive to desorption. With the increase of burial depth, the molecular free path of methane in coalbed decreases. As a result, more methane molecules in the micropore diffuse follow the Fick diffusion laws in the deep conditions. The diffusion experiment of the coal samples show that diffusion coefficient of micropore and macropore all increase with the increasing of pore pressure. The results of flow experiments demonstrate that, when the temperature is constant, the permeability of the coal samples increase with the increasing of the effective stress, and when the other conditions are the same, the permeability decreases with the increasing of temperature.The productivity of CBM wells in shallow buried areas has significantly zoned features, and the production characteristics in different areas are obviously different. There is a negative correlationship between the production of gas and water, and the wells that produce large amounts of water generally produce a small amount of gas. Compared with the wells in shallow buried areas, deep wells have the following characteristics: long drainage period, low average daily water production, slowly growth of gas production and long duration during the unstable gas production stage, a poor stability of gas production, strong output volatility and overall low gas production.Because of the low in-situ stress and high permeability of the coalbed in shallow slope zone, the gas production of CBM well there is high. The coal seams, deeply buried but with high structural curvature value, may also have high permeability, and thus contribute to high yield wells. On the whole, the high yield gas wells are mainly distributed along the dominant developmental direction of the fracture system in this area. The combined effects of faults and hydrological conditions are significant in local areas. Reservoir reconstruction effects and the production system have important influence on gas production. For deep CBM wells, the permeability of the coal seams get dramatically important, and the effect of Reservoir reconstruction is essential to the gas production. |
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