Study On The Evolution Law Of Oil Shale Pores And Fracture Structure And Seepage Path Under Different Heating Modes

With the increasing world energy consumption,China's energy consumption is increasing day by day,and the dependence of oil on foreign countries is increasing year by year.The energy security problem will be increasingly prominent in the future.Oil shale,one of the unconventional energy sources,is an important substitute for petroleum,and it has great advantages in terms of reserves and development potential.One of the key technologies for oil shale in-situ mining is to establish a complete seepage paths in the oil shale layer so that the pyrolysis products can successfully mined to the surface.Therefore,it is of great significance to study the evolution of pores and seepage paths of oil shale during pyrolysis.In the background of oil shale in-situ mining technology,Xinjiang Jimsar oil shale was used as an experimental sample.A combination of high-precision micro-CT and digital core analysis was adopted,and this study explores the evolution characteristics of pores and fracture and the change of permeability of oil shale after pyrolysis based on conduction and convection heating mode.The main conclusions are as follows:(1)After superheated steam convection pyrolysis at high temperature and pressure,according to the different characteristics of the pore and fracture development process oil shale pores evolution can be divided into "expanded pore structure" and "newborn pore structure".Pore development makes the porosity connectivity of oil shale along the bedding direction increase,parallel bedding cracks develop remarkably,and interbedding pore develops obviously.(2)Conductive heating extraction oil shale pyrolysis to form semi-coke products is poor,which can not make the semi-coke products which is adhering to the pore wall flow well.However,in the convection heating process,the superheated steam can react with the semi-coke product to form a good fluidity pyrolysis product to bring out the oil shale along with the gas stream,which can improve shale oil production and resource recovery.(3)The characteristics of the seepage channels formed by the oil shale after different heating modes are different.The efficiency of heat transfer during oil shale pyrolysis is different,resulting in different processes for forming seepage channels.Conductive heating causes the original pores and fracture of the oil shale to develop in parallel direction,and the development of vertical bedding direction is not significant.The convection heating causes the pores and cracks in the inner layer of the oil shale to penetrate each other to form a complete seepage channel.(4)The anisotropy of oil shale is the main factor affecting its permeability.At room temperature,the permeability of parallel and vertical bedding directions is very low because of the undeveloped pore and fracture conditions.After pyrolysis,the fracture and pore expand and connect each other,which increases the permeability of oil shale in parallel and vertical bedding directions,but the increase of permeability in vertical bedding direction is small.Under different pyrolysis conditions,there are great differences in the transformation of pore and fracture in the vertical bedding direction of oil shale.The conclusions obtained from the experiments in this paper can provide some guidance for in-situ steam injection for oil shale recovery in the future to a certain extent.