Storage Characteristics And Production Mechanisms Of Shale Gas Reservoirs

Shale gas is an important successor of conventional oil and gas resources.Since the 21 st century,shale gas resource development has attracted much attention.Especially in the United States,the development of shale blocks has made remarkable achievements and formed a relatively perfect commercial development model.However,due to the different geological characteristics and development features of various shale gas blocks,the exploitation of shale gas in China is mainly based on the experience of American shale gas development.Therefore,it still shows wide differences in the development effect among most blocks,and contradictions between production and gas-bearing.This phenomenon is likely to be attributed to the lack of deep understanding of the characteristics of gas and water distribution and the relationship between gas and solid in the shale reservoirs in China.In consequence,it is necessary to objectively evaluate the shale gas exploitation potential of shale blocks in China through the key factors influencing the shale gas production and storage capacity.Based on the experimental demonstration and the theoretical analysis,this paper focuses on the formation mechanism and evolution characteristics of shale gas,combined with geochemistry,kerogen geochemistry,adsorption,interfacial chemistry,mass transfer,micro seepage mechanics etc.,emphasizing on the characteristics and distribution of water in shale reservoir and its influence on shale gas adsorption and flow characteristics,and so forth.Key conclusions and awareness are as follows:(1)The variability of the gas-water distribution at different maturation stages and pore types of shale are revealed.The results show that with the formation of shale gas,organic matter metamorphic pores(such as gas pore)are mainly fulfiled single phase gas,with a small amount of water molecules adsorbed near the functional groups;inorganic pores evolved from "saturated water" to "water film gas core",and even part of the inorganic micro-pores will be abounded with capillary water.Further,based on the experimental method,the distribution characteristics of water saturation in different types of porosity were quantified at the nanometer scale,which provided the basis for the study of shale occurrence and production mechanism under water-existed condition of reservoirs.(2)The complicated characteristics of gas-liquid-solid interaction in shale reservoirs are quantified.For the organic matter pores,the adsorption of water particles on the surface of the pores is manifested as "gas-liquid interface" adsorption;when the pores are adsorbed with capillary water,it will lose its ability to absorb.Further,based on the experimental method,the influence regularity of water on the methane adsorption capacity was quantified.Taking clay minerals as an example,the adsorption capacity of methane was reduced by about 80 ~ 90% under high water content.Therefore,the current evaluation of the adsorbed gas content based on the isothermal adsorption experiment of dry samples will lead to overestimation of shale gas resources.(3)The transport characteristics of shale gas nanoscale pore-micro fracture under the influence of reservoir water are evaluated.Based on the theory of nano-pore singlephase gas mass transfer,combined with the actual shale pore-fracture profile,considering the influence of water saturation distribution on effective pore,the gas transmission model of tube hole and slit hole in nanometer scale is established.In the case of inorganic(clay,quartz)slit pores and micro-fractures,the gas permeability is reduced by 33% ~ 51% under the condition of water saturation Sw = 30%.Thus,although the original water content of the shale reservoir is usualy in the state of bound water,the effect of the bound water on the gas flow capacity cannot be ignored.In this paper,the influence mechanism of reservoir water on the occurrence and output of shale gas is quantified by studying the characteristics and distribution regularity of reservoir water,laying a theoretical foundation for rational evaluation of gas content and capacity of shale reservoir under actual water content,so as to improve shale gas development theory,and further promote reasonable and effective development of shale gas resources in China.