Characteristic Of Deep Coalbed Methane Reservoirs And Mechanism Of Gas-Water Production In Western Linxing Area

The distinctiveness of geological conditions of deep coalbed is characterized by higher temperature,pressure and in-situ stress.In order to realize the effective development of deep coalbed methane,it is urgent to carry out the research on deep coalbed methane reservoir characteristics and gas-water production mechanism.This paper takes the 8+9#coal reservoir in the Western Linxing area on the eastern edge of the Ordos Basin as the research object.The characteristics of gas-bearing,physical property,reservoir energy and gas-water distribution in deep coal reservoirs are analyzed.Based on the physical simulation of coalbed methane production process,the gas production model under the condition of high temperature and pressure is established,and the law of gas and water production of deep coal reservior is revealed.The results show that the No.8+9 coal in western Linxing is mainly medium rank coal,and the maceral are mainly vitrinite.The samples are characterized by low to high ash yield,low moisture content,high sulfur content and high volatile yield.The micropore and transition pore take a leading position in pore structure.The permeability of samples is relatively low.The adsorption capcity increases The adsorption capacity increases with the increase of pressure and decreases with the increase of temperature.The hydrophilicity of coal is relative weak.The geothermal gradient decreases gradually from the high value area near the zijinshan rock to the surrounding area in plane.The reservoir pressure is slightly underpressure-overpressure.The in-situ stress of the reservoir is transformed three times in vertical direction and there are three types of stress in the study area.Compared with the shallow reservoir,the component of deep coalbed methane in the study area is characterized by high methane concentration,low nitrogen concentration and high hydrocarbon concentration.Most of the gas is coal-type,but few samples are oil-type.The genesis of deep coalbed methane is mainly thermal origin.The gas content from deep coal seam is much higher than that from shallow reservoir,and most part of the reservoir is in the state of supersaturation.According to the prediction results of gas content,the areas rich in CBM are distributed in the north,middle west and southwest.The groundwater dynamic condition is very weak.The NMR T₂ spectrum of saturated samples showed that there are three peaks,and the micropore peak was the most developed in coal,followed by the mesoporous peak and macroscopic pore peak.The water content of different phase in the sample is affected by the temperature,and the free water saturation increases and the bound water saturation decreases with the temperature increasing.The change of water saturation with displacement time can be devided into two stages.In the low-pressure displacement stage?2MPa?,the declining rate of water saturation is large,while in the high-pressure driving stage?4MPa?8MPa?,it is smaller.After the displacement at high temperature,the decrease of water saturation of the core is greater than that at normal temperature.At the same time,the declining rate of water in higher temperature condition is obviously greater than that of room temperature condition,indicating that the high temperature condition of deep coal reservoir is conducive to the production of coal seam water.In the process of coalbed methane production,the coal reservoir pressure decreases exponentially with time.The change of cumulative gas production with time shows an increasing trend of logarithmic function.The shrinkage effect of coal matrix and effective stress effect are considered for the establishment of the CBM production model according to the material balance equation,and the released CBM and the proportion of adsorbed gas and free gas in the output gas were calculated.Taking the methane displacement process at high temperature as an example,under the reservoir temperature,the CBM production increases as a polynomial function with the decrease of reservoir pressure.In the early stage of CBM drainage,the CBM produced mainly comes from free gas in the crack.With the decrease of reservoir pressure,the coalbed methane produced at this time mainly comes from adsorbed gas.There are 101 figures,27 tables and 149 references in this paper.