Study On Spatiotemporal Evolution Of Surface Deformation During In-situ Exploitation Of Oil Shale

Oil shale is a kind of unconventional oil and gas resources.By in-situ production,the efficiency of oil shale production can be greatly improved to capture more oil and gas resources.This approach not only improves oil recovery,but also ensures product quality with less environmental impact and environmentally friendly development.However,in-situ mining is carried out underground,and in-situ heating will have a certain impact on the structure and physical and mechanical properties of underground rock strata.At present,research on how to realize in-situ mining economically and effectively for oil shale is mostly carried out,while research on the impact of in-situ mining on surface deformation is very few.In this paper,Fuyu oil shale underground in-situ cracking pilot test base is taken as the research object,and the change law of physical and mechanical properties of oil shale in the study area with temperature is deeply studied.Based on the method of convective heating,the whole formation numerical model of the study area is established,and the whole process of in-situ mining of oil shale is simulated and analyzed by heat-fluid-structure coupling.Combined with SBAS-InSAR technology,the surface deformation of oil shale in-situ cracking test base in the study area was analyzed.The research results obtained in this paper are as follows:1)The in-situ production layer oil shale in the study area has the highest oil content of 10.55%,which is high quality oil shale.According to the thermogrgration curve of oil shale,it is found that oil shale starts to intensively pyrolysis at 350℃.Based on the pyrolysis mechanism of oil shale,the change mechanism of physical and mechanical properties of oil shale during the heating process is analyzed,and parameters of different bedding directions of oil shale are compared.The anisotropy characteristics of oil shale are revealed,which provides a basis for the modeling of the following paper.2)The thermal fluid-solid coupling simulation analysis is carried out on the whole process of in-situ oil shale mining.The research shows that,during the heating stage,the temperature of rock mass near the heating well of the oil shale layer increases rapidly in a short term,and the temperature rises slowly further away from the heating well.Finally,the temperature reaches 350℃ at 80 cm away from the heating well,and the oil shale near the heating well produces great thermal expansion under high temperature conditions.A certain concentration of stress occurs.In the cooling stage,the temperature of oil shale near the heating well drops rapidly in a short period and then tends to be stable,while the temperature drops more slowly than that far away.After the simulation,the stress concentration phenomenon is greatly weakened and the stress state gradually tends to be balanced.According to the temperature and stress changes,the deformation and surface displacement of oil shale are studied.3)In the whole process of in-situ oil shale mining simulation,the surface displacement fluctuates during heating and becomes stable after rapid settlement during cooling.During the heating process,due to the large thermal expansion of oil shale production layer,the surface uplift occurs,which is relatively fast in the early stage.After heating for a period of time,the deformation of rock mass tends to be stable,and the surface uplift rate slows down.When the heat source heats up,the thermal expansion increases again,and the surface uplift accelerates again.In the cooling process,the mechanical properties of oil shale decrease continuously in the heating stage.After the thermal expansion effect disappears,the oil shale appears large compression under the action of in-situ stress,which makes the surface settlement rapidly.After the deformation of oil shale becomes stable,the surface settlement becomes gentle.4)In the simulation process,the surface displacement is affected by the deformation of the oil shale layer.After heating,the overall surface displacement rises by 2.93 cm,and after cooling,the overall settlement is 3.52 cm.The final surface displacement shows settlement,and the settlement amount is 0.59 cm.In the whole process,the displacement of different positions of the surface is not different.This is because the oil shale is buried in a large depth,the local stress generated in the heating and cooling stage is gradually eliminated by the ground stress of the overlying rock mass as the depth decreases,so that the deformation of the overlying strata in different positions on the same plane tends to be consistent,so the displacement of the surface in different positions is basically the same.5)The SBAS-InSAR technology was used to process 95 satellite data from October2016 to December 2022 in the study area,and the surface deformation of the study area in 17 months from the construction of the base to the end of in-situ mining was obtained.It was found that the surface deformation rate of the study area was between-10.84-5.95mm/a.The overall displacement is represented by settlement.According to the cumulative deformation map of the surface,it is found that in the pre-heating stage,during the base construction in the study area,the surface displacement has a certain fluctuation,and the overall is settlement.During the heating stage,the surface near the heating well was uplifted.In the cooling stage,the surface subsidence appears obviously in the short term,and the subsidence rate slows down in the later period.6)By comparing the processing results of SBAS-InSAR technique with the numerical simulation results,it is found that the surface uplift in the heating stage is between 2.05-2.69 cm,and the surface subsidence in the cooling stage is between2.97-3.30 cm.The maximum surface uplift in the heating stage is 2.93 cm,and the maximum surface subsidence in the cooling stage is 3.52 cm.The difference between the two methods is small,and the rule of change over time is basically the same,and the numerical simulation results have high reliability.It is found from the deformation curve that the surface displacement of oil shale gradually tends to be stable after a period of time after in-situ mining of oil shale,indicating that in-situ mining of oil shale has little influence on surface deformation in practical engineering.