Experimental Study On The Physical And Mechanical Properties Of Oil Shale During In-situ Fracturing And Pyrolysis

Oil shale,as an unconventional oil resource,is a fine-grained sedimentary rock containing a lot of solid organic kerogen,from which liquid shale oil and mixed hydrocarbon gas can be produced after high-temperature pyrolysis.It can be used as a substitute for petroleum.Since environmental pollutions and resource waste problems due to the traditional oil shale exploitation way are growing severe,the technology for in-situ?underground?production will become the new way for exploiting oil shale.In-situ steam injection for exploiting oil shale is a new in-situ exploitation technology firstly raised by the team of ZHAO Yangsheng from Taiyuan University of Technology in 2005.In terms of the core of this technology,multi-well fracturing is applied to connect the steam injection well with the producing oil well,and through the surface pipeline system,over-heated steam at a temperature over 500?is injected into the oil shale deposits via the steam injection well,where oil and gas can be produced by pyrolytic cracking.Based on this technology,this paper focus on the analysis and research of the multi-well fracturing technology and the kerogen conditions with different temperature and pressures during the exploitation,taking oil shale from east Fushun surface mine as the research object.Through theoretical study and experiments from microcosmic to macrocosmic perspectives,from normal to high temperatures,from low to high pressures,in-depth analysis and researches are conducted on the mechanical property among different beddings,fracture initiation and propagation,pyrolytic characteristics,pore-fracture structure and evolution law and permeability feature and others.Research contents and conclusions are shown as follows:?1?With the servo universal testing machine,oil shale test pieces that are parallel to and perpendicular to the bedding are tested separately to observe the anisotropic mechanical parameters and laws.According to the observation results,oil shale is a typical heterogeneous porous medium with distinct anisotropy.Weak plane has significant impacts upon the mechanical property in different bedding orientations.The tensile strength of oil shale parallel to the bedding is 2 times the strengths of that perpendicular to the bedding.In the orientation perpendicular to the bedding,the energy required for crack initiation is small,so the fracture initiation and propagation occur more easily.Most fractures are brittle and tensile,and the I-type fracture toughness of the piece perpendicular to the bedding is much larger than the one parallel to the bedding.?2?True-triaxial fracturing methods are taken on the large-size oil shale test pieces of different bedding orientations to observe the laws of fracture initiation and propagation with different fracturing fluid.The result shows that with the same stress,compared with the hydraulic fracturing method,the supercritical CO₂ fracturing method requires a lower initiation fracture pressure but more time.In terms of the same media,initiation fracture time and pressure that the oil shale parallel to the bedding are all smaller than the one perpendicular to the bedding.Fractures extending in parallel relation to the bedding are all single through crack.However,when the fracture extends along the orientation perpendicular to the bedding,supercritical CO₂ fracturing method will shift and veer,making a secondary fracture and even a more complicated fracture net compared with the hydraulic fracturing method.?3?After a series of high-temperature and high-pressure pyrolysis,the weight and composition of oil shale test pieces are significantly changed.According to the research results,with the increase of temperature,weight loss ratio is higher.When the temperature is reduced by 300?to 500?,the loss is the most obvious.After a pyrolysis at a temperature of 600?,the moisture and oil length of the oil shale reduce while the semi-coke weight increases.With an increase of 300?,the pyrolysis of kerogen constantly makes the ash contents increase,volatiles reduce,and carbon,hydrogen and oxygen contents reduce.?4?Mercury intrusion porosimetry is done to analyze the evolution law of pore structure of the oil shale test pieces with different temperatures and pressures.According to the test results,when the temperature rises during the pyrolysis,the pore diameter becomes longer,the volume larger,the total porosity higher,and the percentage of small and medium pores larger.Under the external pressure,the pore volume and total porosity firstly decrease and then rise,and 5-Mpa pressure is the turning point.With the coupling effect of temperature and pressure,the pore volume,total porosity,and peak pore diameter become larger and larger and reach the maximum at a temperature of600?and pressure of 15 MPa.?5?Micro-CT scanning is used to analyze the evolution law of fracture structure of the oil shale test pieces with different temperatures and pressures.According to the research results,the quantity,area,and density of fractures grow exponentially with the increase of the temperature,especially the flat-microfracture.When the pyrolysis becomes severe,the fracture aperture and length will reach the maximum.Multiple pores are connected in to a closed fracture and the fracture will extend along the bedding and become larger.Under a pressure of 5 MPa,the fracture quantity and density are smaller than that in an unrestraint condition.When the pressure rises,the fracture develops.Therefore,with coupling effect of temperature and pressure,the pyrolysis is severe and the fracture will become larger.?6?The gas permeability test machine is used to test the permeability of oil shale test pieces at different pyrolytic temperatures.The results show that the temperature plays a significant role in changing the permeability of oil shale.In300?,with the oil gas emission,the permeability rises sharply.After a pyrolysis at a temperature of 600?,the permeability reaches a maximum which is 719 time that at the room temperature?20??.When the volume stress grows,some pores close due to the squeezing action and the test piece is compressed more severely,which causes the permeability channel blockage.Therefore,the permeability reduces.Conversely,when the temperature reduces the stress upon pores increases and the permeability rises,while the impact of pyrolysis at a temperature of 400?to 600?become more and more weak.