Study On The Evolution Laws Of Structural And Mechanical Characteristics Of Oil Shale During Temperature Rise And The Application Of Numerical Simulation

In recent years,the rapid development of China’s economy has brought about the sustained increase in energy demand,which leads to the increasingly prominent problems in the supply and structural adjustment of energy.The huge reserves of oil shale resources in China are of great significance to alleviate the lack of oil and gas resources.In view of the balanced demand of environment and energy,in-situ mining technology of oil shale is being vigorously studied and promoted.During the heating and pyrolysis of oil shale,the physical and mechanical properties and pore structure characteristics of oil shale will change,which affect the heating rate of the ore bed,the migration of pollutants and oil and gas,as well as the stability of the wellbore,the ground deformation of the mining area and other engineering problems.It is of great significance for the safe and efficient in-situ mining of oil shale resources to clarify the change laws of the properties and characteristics of oil shale during temperature rise,and predict and analyze the distribution and evolution characteristics of the temperature field and displacement field of the ore bed during the mining process.In this paper,the oil shale in the exploration area of the north of Hu Lan Ba Si Tao of Balikun County,Xinjiang is taken as the research object.Through laboratory experiments,the basic physical properties of oil shale are clarified,the thermal weight loss,permeability and thermophysical property parameters of oil shale during temperature rise are obtained,and the evolution laws of structural characteristics,pore characteristics and mechanical characteristics of oil shale during temperature rise are explored.Based on the parameters and evolution laws obtained from the laboratory experiments,the evolution of stress and fracture initiation mechanism in the rock sample under the pyrolysis of organic matter are simulated and analyzed by the PFC3D software,and the distribution and evolution characteristics of the temperature field and displacement field of the ore bed in the process of in-situ mining by electric heating are predicted by the FLAC3D software.The main works are as follows:（1）Study on the evolution of thermal weight loss,permeability,thermophysical properties and their anisotropy of oil shale during temperature rise.The significant weight loss is caused by the pyrolysis of organic matter in the sample,whose temperature range is about 350～530℃.The total weight loss of the sample is about21%.In addition,the degree of pyrolysis concentration increases with the increase in the heating rate.The evolution of the permeability and thermophysical properties of the samples is controlled by the process of organic matter pyrolysis.The threshold temperature of the change of permeability of the samples is 400℃.When the temperature rises to 400℃,the thermal conductivity of the sample decreases greatly,and the change in the direction perpendicular to the bedding is more significant,which leads to a significant enhancement in the degree of anisotropy.When the temperature is higher than 400℃,the thermal expansion rate of the sample in the direction perpendicular to the bedding sharply increases first and then tends to be gentle,which leads to a significant enhancement in the degree of anisotropy.（2）Study on the evolution of structural characteristics and pore characteristics of oil shale during temperature rise.Through the scanning electron microscopy（SEM）,the microstructure of the planes parallel and perpendicular to bedding of the samples is tested,which reveals the anisotropy of the structure.Under the effect of high temperature,only a small amount of thermal fractures are developed in the bedding plane,while large-scale fractures are developed along the bedding direction.Based on the results of CT scanning and the reconstruction method in three-dimensional space,the meso and macro structure characteristics of the sample under different temperature conditions are obtained.When the temperature is 400～500℃,the volume of the largest void in the sample increases sharply and the percolation transition is occurred.Low temperature N₂ adsorption experiment（LTNA）and mercury intrusion porosimetry experiment（MIP）are used to study the multi-scale pore characteristics of the samples,and the control effect of temperature on pore structure evolution are quantitatively characterized by fractal dimension.The results show that the threshold temperature of the change of pore characteristics is 400℃,and the pore volume,the characteristics of pore volume of each level and fractal dimension change obviously.（3）Study on the evolution of mechanical characteristics of compression and shear along bedding plane of oil shale during temperature rise.The results show that the mechanical properties of compression and shear of the samples are slightly strengthened with the increase in temperature at low temperature stage due to the structure without obvious change,thermal expansion of minerals and water loss.Under high temperature conditions,due to the pyrolysis of organic matter,the pores and cracks are developed in the sample,which leads to a deterioration in mechanical properties of compression and shear,and yield characteristics and ductile failure are shown.At the same time,an enhancement in the degree of anisotropy of the compressive strength and elastic modulus of the sample is caused by the development of cracks along bedding under the high temperature conditions.（4）Based on the PFC3D software,a numerical model of rock mass considering the effect of organic matter pyrolysis is established.The fracture initiation mechanism of oil shale is simulated and analyzed,and the evolution of local stress,the development of micro fracture and the deformation characteristics of rock mass are studied during the heating process.The results show that a large increase in the local stress is caused by the pore expansion effect under the effect of the expansion of oil and gas volume,which leads to the development of micro fractures,and then the local stress drops and fluctuates.In the process of seepage of oil and gas,the pore expansion effect dissipates,which leads to a trend of reduction and gradual stabilization of local stress.Under the effect of ground stress,with the combined effects of pyrolysis of organic matter and thermal expansion of minerals,the compressive deformation is finally presented in the rock mass model.（5）Based on the FLAC3D software,a numerical model of in-situ mining engineering by the electric heating considering the evolution of the properties of oil shale during temperature rise is established.The distribution and evolution characteristics of the temperature field and displacement field of the ore bed are analyzed during the mining process.The results show that the temperature of the ore bed increases logarithmically with the heating time.A deformation process of uplift-settlement is presented in the ore bed,and the settlement deformation with"reverse arch bridge type"is finally shown.The deformation of the ore bed near the production well is more significant than that of other positions.