Study On Permeability Characteristics Of Deep Coal-measure Gas Reservoir In Yushe-Wuxiang Block Of Qinshui Basin

In recent years,the development of coal measures gas resources is mainly concentrated in the area near the depth of 800 m.With the maturity of shallow mining technology and the continuous deepening of the development process,the focus of the development has also begun to shift to the deeper part with more abundant reserves.Taking Shanxi Province as an example,the amount of coalbed methane resources buried in 1000m-2000 m reaches 18.71 trillion cubic meters,accounting for 63% of the national resources.In addition,the structural complexity of deep strata in Qinshui Basin of Shanxi Province has formed a typical combination of sandstone+coal,mudstone+coal and sandstone+coal+mudstone,and the reserves of shale gas and sandstone gas in coal-bearing gas resources are also very considerable here.If these two resources can be exploited simultaneously with the exploitation of coalbed methane,it will play a significant role in promoting China`s energy transformation and social development.However the deep coal measures gas reservoir has different characteristics from the shallow ones,such as higer ground stress,higer temperature than normal temperature and low permeability,which hinder the development of deep resources.If the exploitation is carried out by hydraulic fracturing,it is necessary to understand the seepage characteristics of the reservoir.Therefore,the study on the seepage characteristics of deep reservoirs and their control factors has important guiding significance for the development of deep coal measures gas resources.In this paper,the coal,sandstone,mudstone and different combinations in Yushe-Wuxiang block of Qinshui Basin are taken as samples.Firstly,the gas generation capacity and reserves of three coal-bearing gas resources are estimated according to the characteristics of organic matter,lithologic and gas content,and the reservoir brittleness index is calculated to predict the development and extension of fracturing fractures.The influence mechanism of three-dimensional stress,pore pressure and water content on single and combined samples was analyzed.The permeability characteristics of deep reservoirs under long-term high geostress are studied by creep-seepage experiment.Prefabricated fractured samples are used to replace the samples after hydraulic fracturing.The permeability before and after hydraulic fracturing was compared,and the permeability enhancement efficiency of hydraulic fracturing was obtained.The mechanism of hydraulicr fracturing was analyzed;explaining the working process of fracturing equipment.The migration mechanism of fluid and the dual effect of high pressure water are analyzed.The main conclusion are as follows:(1)By calculating the brittleness index of sandstone and mudstone,it can be seen that the brittleness index of the two reservoirs is good,which is beneficial to the expansion of fracturing cracks in the reservoir.The volatilization of vitrinite with the highest content in coal reservoir is beneficial to the formation of internal fractures,promotes the extension of fractures,and contributes to the formation of fracture networks;Majurity and organic matter indicate that the gas production potential of the three coal-bearing gas reservoirs is great,and gas content also indicates that the resources reserves of deep coal-bearing gas are considerable,and they all have development potential.(2)The permeability of single and combined samples decreases with the increase of three-dimensional stress,and the decreasing trend is an exponential function;with the increase of pore pressure,permeability shows a trend of quadratic function that decreases first and then increases;the relative permeability of gas also decrease with the increase of relative water saturation;the permeability of combined samples depends on the mudstone with poor permeability;the surface equation of three-dimensional stress-pore pressure coupling effect on permeability is fitted by equation,and the strengthening effect of water on coupling effect is analyzed.(3)When the samples creeps under each stress level,the volume deformation characteristics first increase significantly and then gradually stabilize,corresponding to the permeability change is rapidly reduced and then tends to be stable.Through the analysis of the change trend,the change equation of the two are fitted.By analyzing the creep-seepage characteristics of the combined sample,it can be seen that the deformation characteristics and seepage characteristics of combined samples are determined by the coal with small strength and the mudstone with poor permeability,respectively.The creep-seepage process is divided into two stages: volum compression and volum expansion.The model equations of permeability and volumetric strain during creep are derived.(4)Comparing the complete and fractured samples,it is concluded that the reservoir permeability will increase by 2-3 orders of magnitude after hydraulic fracturing.The working process of hydraulic fracturing is briefly described.It is explained that the mechanism of hydraulic is to destroy reservoir cohesion;form fractures and connect fracture networks to improve permeability.Combined with the situation in the study area,the migration of gas and water is analyzed,and the characteristics of deep reservoirs have a great influence on gas desorption.After water enters the reservoir,it will not only fracture the reservoir,accrelate the gas flow and improve the permeability,but also hinder the gas seepage and form a high starting pressure gradient.However,the increase of permeability after reservoir fracturing will reduce the starting pressure gradient,and even disappear when the permeability reaches a certain threshold value.By comparison,the advantages are greater than the disadvantages,and it is feasible to exploit deep gas by hudraulic fracturing.