Research On Microscopic Characteristics Of Carboniferous Shale Reservoir In Eastern Qaidamu Basin

Shale gas reservoir was mainly composed of pore and throat in nanometer to micrometer scale,the complexity of the microstructure determined its extremely low porosity and permeability,the microscopic characteristics of the shale reservoir controlled the migration patterns,occurrence state,flow mechanism and enrichment regularity and many other aspects of the shale gas.Based on the study of geological characteristics of the carboniferous mud stone and shales,this article emphasized on the research of the microscopic pore structure of the shale and its influence and control on the gas migration and accumulation as well as its impact on the occurrence of macro fracture in shale.Shales in eastern part of the Qaidam basin carboniferous developed well.The lithology charcateristic of shales revealed that the organic-rich shale and carbonaceous shales were the main component.More and more oil&gas showing was found,which indicated that there had been the process of hydrocarbon generation there.The borecore samples of the shale showed a relative well gas potential.The content of organic carbon is about 2?,the maturity is about1.2%;the content of the brittle mineral is nearly 55% overall,which showed that fracability is well to some extent.Porosity of the shale reaches up to 10.75%.Three types of pore are found in the Carboniferous shale: the organic pore,intergranular pore and intragranular pore.The intergranular pore and intragranular pore are better developed than the organic pore.The scale of the pore diameter was variable,the pore in nanometer and micrometer all developed.The micropores have small volumes but high specific surface area and are good for the adsorbed gas.However,small and large pores have small specific surface area but a high pore volumeand suitable for the free gas.The micro-pore structure of shale can significantly affect the shale gas adsorption.Theoretical studies have shown that in supercritical condition the adsorption potential microporous filling model was agreeable with the experimental data.According to the analysis,there is a relative good correlation between the micropore volume,specifice area and adsorption quantity.From the correlation we find that the correlation index are respectively0.94 and 0.954 for the micropore the diameter of which is less than 0.821 nm and 0.5nm,which shows that shale gas mainly adsorps or fills in the micropores,especially the pore diameter of which is less than 1nm.The micro-pore structure of shale can significantly affect the shale gas migration.The migration of gas molecules in the nanoscale pore of the shale is restricted,what's more,gas is of high density at high pressure,so the volume of the gas molucule can not be omitted.Through introducing the theory of dense gas,we found that when the pressure is greater than1.135 MPa(350K),the Knudsen diffusion will not occur in the pore the diameter of which is larger than 2nm.Meantime,Knudsen correction factors of permeability decrease with pressure increased,along with the decrease with the pore radius increased,and the increase with the rise of temperature.As the increasing of effective pressure,permeability redues gradually,shale permeability is of high sensitivity to the effective pressure;Permeability is highly anisotropy;Shale permeability increases with the increase of porosity,shale pore structure is effective for evaluation of shale permeability by nitrogen adsorption method.The developing extent of the pores the diameter of which is 3-20 nm play an important role in gas migration.There is a close relationship between the microscopic pore structure and macro fracture of the shale.The development of the micro fractures can be effected by fabric characteristics of shale.Macro fracture develops from the propagation,connection and link-up of inner microcracks.Macroscopic failure modes includes shear fracture,tensile fracture and slip rupture,then the micro failure can be divided into the shear slip type(type II),tensiletype(type I).In essence,the shear rupture and slip fracture of shale are all caused by shear slip rupture of microcracks.They are just different failure modes under the influence of micro structure of shale;Tensile fracture is caused by the tensile extension of microcracks.In addition,shear and tensile microcracks co-existed during the process of the macro fracture generation,but with different contribution to macro rupture.The establishment of a shale fracture pattern,namely shear,tensile rupture and slip bursts,and explains the microscopic mechanism of the macro fracture from the microscopic.