Pore Structure And Its Control On Shale Gas Occurrence Of Continental Shale

As an important unconventional natural gas resource,shale gas has attracted great attention all over the world.Shale reservoir research is one of the important tasks in the geological evaluation of shale gas.The pore structure characteristics of shale reservoir have important controls on the occurrence state and content of shale gas,while previous studies mainly focused on marine shale.Based on the above description,this paper takes the continental shale from the Jurassic Ziliujing Da'anzhai Member in the northeastern Sichuan Basin as an example,and utilizes conventional geological and geochemical analyses and unconventional experimental test methods such as field emission scanning electron microscopy,low-pressure gas?CO₂ and N₂?adsorption,mercury intrusion porosimetry and high-pressure methane adsorption.First,the pore structure and methane adsorption characteristics of the Jurassic Ziliujing Da'anzhai Member shale were investigated.Then the control mechanism of continental shale pore structure on the occurrence of shale gas was discussed.The following are the main conclusions in this paper:The Da'anzhai Member shale mainly develops organic matter-hosted pores,clay minerals-associated pores and framework minerals-associated pores.The kerogen pores are generally not developed or only a small number of isolated pores are developed,and the connectivity is poor.The solid bitumen pores are generally developed and have good connectivity.The controlling factors of shale pore structure mainly include organic matter abundance,occurrence form of organic matter,maturity,soluble organic matter and inorganic mineral components.The organic matter abundance mainly affects the development of mesopores and macropores in shale;solid bitumen develops more pores than kerogen;maturity promotes the development of organic matter-hosted pores and intraparticle dissolution pores;soluble organic matters occupy a part of mesopores space with pore diameter less than 10nm,which reduces the specific surface area and pore volume of shale;clay minerals mainly affect the development of micropores and mesopores in shale;framework minerals can form rigid frameworks to prevent collapse or compaction of original pores,and can also be dissolved by organic acids to form intraparticle pores.The Da'anzhai Member shale with complex small pores?0?7.5nm?and moderately uniform large pores?7.5?80nm?has a strong capacity to store both adsorbed gas and free gas,and it also facilitates the exploitation and production of shale gas.The average isosteric heat of adsorption of methane on shale is 30.22k J/mol,indicating that the dominant adsorption process of methane on shale may be physical adsorption.The isosteric heat of adsorption increases with increasing absolute methane adsorption amount,indicating that the adsorption heat is mainly affected by the interaction between the adsorbed methane molecules.The main influencing factors on adsorption capacity of the Da'anzhai Member shale mainly include organic matter characteristics?abundance,type and maturity?,temperature,pressure,clay minerals content and relative humidity?water content?.Among them,the higher the organic matter abundance,the stronger the gas adsorption capacity;type II or type III kerogen has stronger gas adsorption capacity than type I kerogen;within a certain maturity range,with the increase of maturity,the gas adsorption capacity of shale increases with the increase of maturity;the higher the temperature,the lower the gas adsorption capacity;the influence of pressure on the gas adsorption capacity is mainly reflected in the low pressure section;there is a negative correlation between the maximum gas adsorption capacity of shale and clay minerals content,which may be closely related to the adsorption of gas mainly on the surface of organic-matter pores.The higher the relative humidity?the more water content?,the lower the gas adsorption capacity of shale.The Langmuir equation was extended by using the main factors affecting the gas adsorption capacity of the Da'anzhai Member shale.The adsorbed gas content of shale under different geological conditions?different temperature and pressure?was calculated,and the methane adsorption capacity of shale with different TOC content or different relative humidity?water content?as function of depth in the study area was established.The prediction results indicate that when the depth is shallow,the methane adsorption capacity increases gradually due to the influence of pressure.After reaching the maximum value,the methane adsorption capacity begins to decrease gradually due to the influence of temperature,and reaches the maximum value at 500?800m.The higher the TOC content,the higher the maximum methane adsorption capacity,and the greater the depth to reach the maximum methane adsorption capacity;the lower the relative humidity?water content?,the greater the maximum methane adsorption capacity,and the greater the depth to reach the maximum methane adsorption capacity.In organic-matter pores having a radius less than 2 nm,all the methane molecules are affected by the pore wall and are in the adsorbed state;in organic-matter pores having a radius greater than 2 nm,when the distance between the methane molecules and the pore wall is less than 2 nm,the methane molecules are in the adsorbed state due to the influence of the pore wall;when the molecules are farther than 2 nm away from the pore wall,the interaction force between the pore wall and the methane molecules is close to 0,and the methane molecules are in the free state.With the increase of pore diameter,the adsorption-zone volume of shale generally increases first and then decreases,while the free-zone volume gradually increases until it is equal to the adsorption-zone volume when the pore diameter reaches 19 nm.The adsorption mode of shale gas under supercritical geological conditions should be both micropore filling and monolayer adsorption.One of the fundamental reasons for the lower gas content of the Da'anzhai Member shale than the Longmaxi shale is that organic-matter pores occupy a lower percentage,and organic-matter pores are the main space for the occurrence of shale gas.The adsorbed gas content of the Da'anzhai Member shale is affected by the specific surface areas of micropores,mesopores and macropores.The free gas content is mainly affected by the volumes of mesopores and macropores,while the total gas content is closely related to total porosity.