Numerical Simulation Study On Large-Scale In-Situ Exploitation Of Oil Shale

In order to study the differences of temperature field and hydrocarbon generation of well groups of different sizes during in-situ cracking process,the in-situ production models of oil shale under four conditions,including 7,19,37 and 61 hexagonal well groups,were modeled respectively.The oil shale production test area in Fuyu County,Jilin Province was taken as the pilot area.Based on the Mesh module of ANSYS software,the fluid field and temperature field were simulated by Fluent.The process of hydrocarbon generation of oil shale was simulated by solving the equations of chemical reaction kinetics.According to 61 well group models,the 3D in-situ mining models of oil shale with 10 different engineering parameters(including well spacing,gas injection rate and the spacing between hydraulic fracturing and double fractures)were established,and the temperature field simulation and hydrocarbon generation were analyzed for these 10 parameter models.The influence of various engineering parameters on large-scale in-situ mining of oil shale is explored to find the best parameter scheme when reaching the maximum energy rate of return,in an attempt to provide a scientific basis for the formulation of large-scale in-situ mining of oil shale.Through data simulation,various schemes for in-situ cracking development of oil shale are analyzed and discussed in depth in order to obtain more accurate results.(1)Hydrocarbon generation in the process of large-scale in-situ exploitation of oil shale is mainly divided into three stages,namely rapid hydrocarbon generation stage(500 days before heat injection),slow hydrocarbon generation stage(500 days to 1500 days of heat injection)and stable stage(1500 days to 3000 days of heat injection).When heat injection time reaches 3000 days,7,19,The effective heating volume ratios of 37 and 61 well models were 73.5%,76.9%,79.0% and 80.6%,respectively.(2)When heat injection reaches 3000 days,the ratio of the volume with temperature greater than 823 K to the total volume of oil shale in 7,19,37 and 61 well groups reaches 28.1%,32.5%,34.9% and 35.4%,respectively.Increasing the number of well groups can improve the heat utilization efficiency of heat gas to the central heating area.However,increasing the number of well groups gradually reduces the increase rate of hydrocarbon generation and energy return rate of a single well group.When the number of well groups increases from 37 to 61,the increase rate of hydrocarbon generation and energy return rate of a single well group heated for 3000 days is less than 3.1% and 8.7%,respectively.Therefore,it is cost-effective to select 37 or more well groups for in-situ exploitation of oil shale.(3)In the model of changing production parameters,reducing well spacing can improve the heating efficiency of oil shale during the rapid hydrocarbon generation stage.Increasing gas injection rate can make the oil shale formation enter the stable stage earlier.Compared with the single fracture scheme,double fracture is more conducive to improving the heating effect of large-scale in-situ mining of oil shale.The smaller the space between double fractures,the faster the growth rate of hydrocarbon generation in the rapid hydrocarbon generation stage.When the heat injection time reaches 3000 days and the well spacing is less than 40 m,the hydrocarbon generation increases by 7.8% and 7.6% with every increase of 10 m.When the gas injection rate is less than 16m/s,hydrocarbon generation increases by 5.1% and 2.4% for every 4m/s increase.When the fracture spacing is less than 2mm,the hydrocarbon generation increases by 3.1%,3.5% and 3.2% with the increase of 0.5mm.(4)In 61 well groups,the mining parameters that achieve the maximum energy return in different mining parameter schemes are 30 m well spacing,12m/s gas injection rate and 1mm fracture spacing.A continuous heat injection period of 600 to 750 days is required to obtain the maximum energy return from the different production parameters.