Study On Composite Accumulation Process Of Coal Measures Gas For The Eastern Yushe Area In Qinshui Basin

Taking the coal measures gas?coalbed methane,shale gas,tight sandstone gas?of eastern Yushe in Qinshui Basin as the research object,based on the core samples of 10 boreholes in the study area,using TOC testing,R_o,max testing,organic matter pyrolysis,X-ray diffraction,scanning electron microscope?SEM?,high pressure mercury injection,N₂ and CO₂ adsorption,permeability testing,diffusion coefficient testing,isothermal adsorption,etc.,study on reservoir characteristics of unconventional reservoirs occurring in coal-bearing series was carried out.And,simutaneously,numerical simulation was used to study the unconventional gas coupling accumulation process in the coal bearing series.The characteristics of unconventional gas reservoirs in the coal bearing series of the study area were revealed.Among them,the inorganic mineral composition of coal seam and shale is dominated by clay minerals,and the tight sandstone is dominated by brittle minerals.The organic material belongs to type III kerogen,with high organic matter abundance,and the thermal evolution degree is in high-over mature stage.In the three types of reservoirs,the development of micropores in coal is less,and macropores are more developed,which shows the characteristics of low surface area and high pore volume.The pore size distribution of shales and tight sandstones is mainly concentrated in micropores,providing a large amount of specific surface area.Based on SEM images and the PCAS software,it is found that shale pores tend to have a"circular"shape,the smoother the surface,the lower the structural complexity,and the pores tend to be narrower and longerer,and the rougher the surface,the higher the complexity of the structure.The permeability of coal reservoirs is the highest,followed by the tight sandstone,and finally the shale.The horizontal diffusion coefficient of tight sandstone is two orders of magnitude higher than the vertical diffusivity,and the vertical diffusivity of shale is lower than that of tight sandstone,indicating that under the same conditions,the diffusion rate of tight sandstone is higher;the coal seam with adsorption capacity is much larger than that of shale,and the gas content of shale and tight sandstone is considerable,but the vertical distribution rule is poor.Based on sedimentary,and sequence stratigraphic features and hydrogeological conditions,four major gas-bearing systems and five favorable strata in the coal-bearing strata were recognised,and the combination of different reservoirs was determined and the combinations of seven mixed reservoirs were classified.It is concluded that the three types of III and VI reservoirs are not developed in the favorable zone,and each section is the largest proportion of Type I?single-coal reservoirs?,followed by Type II?single mud reservoirs?and Type IV?coal-mud mixed reservoirs?,again VII?type coal-mud-sand?mixed reservoirs,most notably The distribution of V-type?mud-sand mixed reservoirs?is mostly undevelopmental.Based on the above work,the methane-forming geological history is constructed.The geological model and mathematical model of the processes of methane generation,migration,accumulation,and dissipation was computer compiled and a simulation software?UCGHS?espically for the simulation of unconventional gas accumulation history in coal-based systems was developed.The simulation results show that the accumulation process and accumulation model in the study area can be divided into five stages,namely the initial stage of weak hydrocarbon accumulation,the early stage of hydrocarbon accumulation?depth burial stage?,the stage of stagnation and hydrocarbon dissipation accumulation,the overall high-intensity accumulation and charging,and stage of dissiption and secondary charging.Among them,the second and fourth stages play important roles in the accumulation and enrichment of gas in various types of reservoirs in coal-bearing series,and the fifth phase plays a decisive role in the preservation of gas reservoirs.