Temperature Dependence Of Evolution Of Pore Structure And Permeability Characteristics Of Oil Shale

During the in-situ exploitation of oil shale,the evolution of the pore structure and permeability characteristics of oil shale subjected to high temperature fluid play a decisive role in its effective in-situ heating and the efficient production of pyrolysis oil and gas.Therefore,the research on the evolution law of pore structure and permeability characteristics of oil shale with temperature is of great theoretical significance and engineering application values.In this study,the Fushun East Open-pit oil shale and Xinjiang Jimsar oil shale were chose as the research object.Firstly,evolution of the physical properties,meso-scale full size pore structure characteristics and connectivity of oil shale with temperature were investigated using X-ray diffraction?XRD?,Thermogravimetry-Fourier Transform Infrared Spectroscopy-Mass spectrometry?TG-FTIR-MS?,low temperature nitrogen adsorption?LTNA?,high pressure mercury injection?MIP?and low-field nuclear magnetic resonance?NMR?technology.Then,the relationship between permeability of oil shale and temperature and pore pressure were studied using self-developed high-temperature triaxial permeability testing apparatus.Finally,combined with the previous conclusions and the thermo-hydro-mechanical?THM?coupling mathematical model of in-situ mining oil shale originally established by the predecessors,the temperature propagation law,the rock mass deformation law and the permeability distribution law during the in-situ pyrolysis of oil shale were analyzed.The main research contents and conclusions are as follows:?1?The analyses of thermal response of oil shale inorganic minerals,the pyrolysis characteristics of organic matter and the gas products formation law show that the XRD diffraction peak of oil shale changes slightly before 500°C.However,after 500°C,the Fushun oil shale exhibits quartz?/?phase change and dehydration of clay minerals,and the Xinjiang oil shale is characterized by quartz phase change and decomposition of carbonate minerals.The TG curves show three-stage weight loss characteristics.The organic matter pyrolysis stage of Fushun oil shale is 350-523°C,and pyrolysis of kerogen and secondary reaction of intermediate products is completed in the same temperature range.The organic matter pyrolysis stage of Xinjiang oil shale is 390-527°C,and two obvious weight loss peaks appear in the stage.The began and end temperature of each stage of weight loss is mainly related to the composition of oil shale.The pyrolysis gas products of oil shale mainly contain H₂,H₂O,CH₄,CO₂ and light hydrocarbon C_nH_m,which are mainly produced in the pyrolysis stage of organic matter.Due to the metamorphism and structure difference of kerogen,the production of H₂,CO₂and hydrocarbon gases at the third stage of Xinjiang oil shale?560-690°C?indicates that some organic matter is degraded at this stage.The pyrolysis reaction characteristics of oil shale and the formation characteristics of its products are closely related to its composition and content,which show regional characteristics.?2?The combined analyses of mesoscopic pore structure of oil shale by LTNA and MIP show that the pore structure parameters change little before 300°C,which mainly due to the removal of pore water inside oil shale and the fine adjustment of inorganic minerals under thermal stress.The pore volume,specific surface area,average pore diameter and porosity increased greatly in the stage of300-650°C.During 350-550°C,it was mainly caused by pyrolysis of organic matter,pore pressure induced by oil and gas products and thermal cracking of inorganic minerals,and the effect of this temperature range on the pore structure is the most significant.During the temperature range of 550-650°C,it was mainly caused by the decomposition of carbonate minerals,the rupture and collapse of mineral skeletons.Among them,Fushun oil shale mainly exhibits the increase of transitional pores and mesopores,and the pore morphology changes from cylindrical pores to slit-shaped pores begin at 500°C.The Xinjiang oil shale mainly exhibits the increase of mesopores and macropores,and the growth rate is not synchronized with temperature increasing.Moreover,the LTNA results show that the specific surface area of Xinjiang oil shale is greatly reduced after 400°C.The pore morphology shows cylindrical pores in the low temperature and high temperature stages,while slit-shaped pores at 300-400°C.At room temperature,the porosity of the oil shale is extremely low.However,when the temperature rises to 650°C,the porosity of the Fushun oil shale increases by 12.4 times,and the Xinjiang oil shale increases by 15.6 times.Fractals of pore structure and surface show that inflection point of the fractal dimension of Fushun and Xinjiang oil shale were 300°C and 400°C,respectively,indicating that the internal structure of oil shale is strongly adjusted at this temperature.The pore structure and morphology of oil shale change intrinsically under the temperature effect.Moreover,due to the difference in mineral composition and metamorphism,there are regional differences in the evolution of pore structure.?3?On the basis of the analysis of pore structure evolution,the relationship between pore connectivity of oil shale with temperature was analyzed by NMR.When the temperature is>300°C,the connectivity of the pores of same level is enhanced,and the connectivity between the pores of different sizes is obviously improved.The effect of temperature on the porosity of the movable fluid is greater than on the porosity of the bound fluid.The porosity of the movable fluid plays a major role in promoting the increase of the total porosity,and thus the permeability is improved.The relationship between the predicted permeability based on the NMR model and temperature is in accordance with the PP model.?4?The analyses of the oil shale permeability versus temperature and pore pressure show that under the simulated in-situ heat-injection mining conditions,the permeability of oil shale increases regularly with temperature after 200°C.To be specific,during 200-350°C,permeability shows a slight variation,in which Fushun oil shale has a temperature threshold of 350°C.During 350-550°C,the permeability increases greatly.And during 550-650°C,the increase rate slows down.Variation of the permeability with the increasing pore pressure generally show a trend of decreases rapidly first and then slows down.When the pore pressure is lower than 2.5MPa,its inhibiting effect on the permeability is obvious.However,when it is higher than 2.5MPa,the permeability is not sensitive to the pore pressure.Permeability of oil shale is a binary function of temperature and pore pressure.?5?Based on the measured parameters,the distributions of temperature field,displacement field and seepage field of oil shale reservoir under in-situ heat-injection mining conditions were simulated by THM coupled modeling.The results show that the temperature would quickly propagate along the fracture due to reservoir stimulation,which is beneficial to oil shale mining.Take 550°C as the temperature for complete pyrolysis of oil shale,the complete pyrolysis of Fushun oil shale in the range of 71.2 m takes 1000 days,while Xinjiang Oil shale takes720 days.During the heat-injection exploitation process,the Fushun oil shale reservoir sinks,while the Xinjiang oil shale undergoes expansion deformation,which can be ascribed to the differences of the thermal expansive coefficient.The distribution of the seepage field shows that the heat-injection exploitation process is conducive to the migration of oil and gas products,while not beneficial to the heat transfer of the heat-carrying fluid.Therefore,measures should be taken to improve the permeability of oil shale.