Study On The Evolution Of Pore Structure And Permeability Characteristics Of Oil Shale After Superheated Steam

With the continuous depletion of conventional energy in China,unconventional energy has attracted more and more attention in China and even in the world,and oil shale is the typical representative of unconventional energy.Oil shale,also called "mother shale",is a kind of sedimentary rock.In this paper,the pore size,porosity,permeability and microscopic pore structure of oil shale under the action of superheated steam were studied.The conclusions are as follows:(1)The oil shale specimens were pyrolysed by superheated water steam pyrolysis system and weighed before and after pyrolysis to obtain the pyrolysis weight loss rule of oil shale specimens.Then,the pyrolysis characteristics of Balikun oil shale were studied,and the following results were obtained:When the temperature exceeds 350?,the reaction inside the oil shale begins to become complicated,and the reaction is dominated by organic matter.Some kerogen pyrolysis generates shale oil,gas and residue,while some kerogen begins to decompose to form asphaltene.When the temperature exceeds 450?,the asphaltene begins to decompose.The weight loss rate of oil shale at 350??450? is larger,and the weight loss rate is 19.06%,accounting for 73.39% of the total weight loss.(2)By using microscopic CT scanning to study the evolution law of microscopic pores and fractures in oil shale,it is found that:As the temperature increases,the tendency of increase with the porosity,pore diameter are present,in the 25? to 350? temperature range shale porosity change is small,because it hasn't reached this stage oil shale pyrolysis temperature threshold of the organic matter,therefore this stage the size of the porosity is mainly caused by the precipitation of moisture and heat burst.In the temperature range of350? to 600?,the porosity of oil shale changes greatly.At this stage,the organic matter is gradually pyrolyzed and affected by the thermal shock of high-temperature steam,and the pores are gradually formed and connected.(3)The pore size structure of oil shale after steam pyrolysis at different temperatures was characterized by mercury injection method.The results showed that at 25?,the oil shale was mainly composed of macropores and micropores,followed by small pores,and the mesopores did not develop.As the temperature increases,when the temperature reaches350?,the proportion of pore volume of micropores,small pores and macropores is roughly the same,indicating that the pyrolysis of organic matter and the influence of water precipitation make some micropores evolve into small pores,while mesopores do not develop.Within the temperature range of 400? to 550?,the pores are the most growth,followed by the big hole,micro hole,the hole is not development,this is because under the effect of superheated steam,the organic matter pyrolysis ceaselessly,make pore evolve after a series of physical and chemical reaction for the holes,it can be seen to 550?,the porosity and the hole volume accounted for 84.35%;At 600?,the pore volume ratio of pore and mesopore reaches 87.73%.(4)By means of electron microscope scanning,based on Visual C++ and Open CV machine vision library,the surface morphology of oil shale after pyrolysis at different temperatures was studied,and the following results can be obtained:Electron microscope scanning can clearly see the shale surface looks pore fissure structure,normal temperature condition,shale surface topography is smooth,but with the rising of temperature according to the image can be clearly observed that the surface looks to form the more "honeycomb structure,it is because the organic matter pyrolysis and the influence of high temperature steam thermal shock,Makes a lot of small pore structure by the inflation pressure,in turn,make pore size bigger,some holes in the process of organic matter pyrolysis connected together,formed the pore connectivity,arrives at 600? can see a big crack,and the pore connectivity with larger area,fully shows that under the influence of high temperature steam shale pore fissure structure,great changes have taken place.(5)Using KDQY-001 corrosion-resistant gas-liquid two-phase penetrator to study the evolution law of oil shale permeability characteristics,it can be found that the variation law of permeability with temperature can be divided into four stages: 25??100?,oil shale permeability is very low;At 100??300?,the permeability increases due to the thermal stress caused by the uneven thermal expansion of mineral particles and the influence of water evaporation.At 300??350?,due to the influence of kerogen depolymerization,the permeability decreases.Generally speaking,the permeability of the first three stages is in the low permeability stage.350??600? is the main stage of pyrolysis.With the continuous deepening of pyrolysis,pyrolysis pores and fractures continue to form and rapidly connect,forming the flow channel of oil and gas products and rapidly increasing permeability.