Study On Thermal Damage Evolution Properties And Damage Model Of Oil Shale

Oil shale reserves are rich enough to be converted into shale oil of more than400billion tons all around the word, which are about3times of the natural oil provenrecoverable reserves. And in China, the oil shale reserves are predicted as about719.9billion tons containing47.6billion tons of oil shale, fourth place in the world. Due tothe rapid growth of economy and population, energy demand is rising sharply while atthe same time the conventional oil and gas resources are dwindling. Therefore, at thebackground of conventional energy crisis, the oil shale resources are getting more andmore attention because of its huge reserves and bright development prospects.Among the oil shale mining methods, the in situ mining technology started latebut developed rapidly, and now, there are dozens of methods about the in situ miningtechnology, whose core is conducted through the underground shale reservoir heatingto make the oil shale kerogen pyrolysis to produce oil. To heat the oil shale layer,there will inevitably lead to the phenomenon of thermal damage, and the degradationof the mechanical properties of oil shale will be produced finally. Oil shale layers aregenerally buried in a certain underground depth under complex crust stress, when thethermal damage appears, a larger shale layer deformation and even destruction willappear, which is harmful to the safety production. Therefore, it is necessary to studythe process of thermal damage of oil shale and then understand the thermal damagedegradation characteristics of oil shale. The results can be utilized to predict and avoidthe ground surface settlement and some other accidents that may occur in the in situmining process, which has some theoretical significance and practical applications toa certain degree. Oil shale samples from Nong’an City were taken as the research object. Firstly,the thermogravimetric analyzer was used to measure the thermal decompositionweightlessness curve of the samples to determine the study temperatures raging fromroom temperature up to700℃, and finally8level test temperatures of25℃(roomtemperature),100℃,200℃,300℃,400℃,500℃,600℃and700℃were chosen.Secondly, the CT scanning and SEM technology were used to obtain themesoscopic structure images of samples under different level test temperatures. Andsome qualitative analysis and quantitative calculation of the pores, fractures andmorphological characteristics of the mineral grains were conducted through themeasured images, and then the damage evolution characteristics of samples underdifferent level test temperatures were analyzed. In this process, some conclusionswere obtained as following: as the temperatures increased, larger changes happened tothe minerals including quantities reduction, shape changes and size decreasing. And atthe same time, the number of fractures with larger size increased, whose shape wasconverted from rectangle to oval with directional property gradually obvious,resulting into the inner damage aggravation and mechanical properties reduction.Through the digital image box dimension algorithm, fractal analysis on the CT andSEM images at different level text temperatures was conducted and it turned out that,between25and700℃, the trend of fractal dimension D first increased and thendecreased and reached maximum at400℃, which meant the fracture shapes were themost complicated at the very400℃.Thirdly, some physical and mechanical performance tests of samples at differentlevel test temperatures were conducted. Combined with the compressional wavevelocity, elastic modulus, peak strength, the macroscopic damage characteristicsunder temperature action were analyzed, and some conclusions were obtained asfollowing: the longitudinal wave velocity and the elastic modulus declinedcontinuously while the peak strength first increased and then decreased and reachedmaximum at400℃, which shown that400℃was a special temperature node reflecting the macroscopic mechanical changes. Respectively according to thelongitudinal wave velocity and elastic modulus, the damage variables were defined,and the study showed that the damage variables increased with the temperatures, bothwere correlated. And at700℃,the damage variable reached maximum, and the valueswere0.936and0.785irrespectively through two different methods, which couldreflect the real damage situation of shale to some degree.Finally, on the basis of the measured test data, combined with the rock damagetheory, the thermal damage constitutive equation of oil shale was deduced. And theFlac3D was used for secondary development, in which, an oil shale constitutivemodel named OsTd considering the thermal damage was self-defined. And at last,compared with the results of uniaxial compression test, the correctness of the OsTdmodel was verified.