| Assured energy security is a major demand in China,and oil shale has attracted extensive attention as an unconventional energy resource with great potential.Nevertheless,the current conventional oil shale mining has low level,energy waste,environmental pollution and other problems,and the in-situ pyrolysis technology of oil shale has the advantages of less water consumption,high energy utilization efficiency and less pollution,so it is of great theoretical and practical significance to carry out the in-situ pyrolysis characteristics of oil shale.The physical fields involved in situ electric heating mining of oil shale were theoretically evaluated and selected,and then the COMSOL simulation software was used to pair the physical fields,so as to simulate the in-situ pyrolysis process of oil shale.Regarding thermal-hydro-chemical phase coupling and thermal-hydro-mechanical-chemical multi-field coupling pyrolysis processes,the THC model and THMC model were established to study mining characteristics.The main analysis results are as follows:In the THC multi-field coupling model,the heating distances of 25 days,50 days,75days and 100 days were 81cm,110cm,135cm and 154cm,respectively,under the condition of constant temperature of 673.15K.The peak pressures at each time point were 41.5MPa,39.36MPa,32.6MPa and 28.2MPa,respectively.The heating rate decreases over time,and the heating distance increases over time.Different heating conditions have a great impact on oil and gas production.In this paper,oil production and gas production are effectively controlled by selecting the appropriate heating temperature.Under the heating condition above 473.15K,oil production rate fluctuates greatly in the initial stage.After 5 days of heating,the gas production rate and oil production rate tend to be stable,and the gas production rate is much higher than the oil production rate,with a difference of three orders of magnitude.Under the condition of constant temperature heating at 673.15K,the growth rate and heating rate of heating area decrease with time,and the reaction area is 56.95m~2and the reaction radius is 8.52m on the 800th day.Therefore,controlling the heating well spacing within 10 meters can effectively reduce the production cycle.In the THMC multi-field coupling model,the distribution trend of temperature and pressure has a high synchronization with the heating time.Under the initial conditions of porosity of 0.05,constant temperature of 873.15K and saturation of 0.14,the temperature propagation distance and pressure propagation distance of the 90th day are 433.5cm and374.6cm respectively.The chemical reaction of the main body has an obvious impact on the distribution of the pressure field.Proper control of heating conditions and the distribution of the temperature field is helpful in controlling the chemical reaction process.The pressure peak was mainly concentrated at the intersection of cladding layers at the wellhead of the heated well and reduced with time,from 332.26MPa on day 300 to292.73MPa on day 600.This indicates that the pressure accumulation effect is less than the flow field diffusion effect in the in-situ mining process,and the gap between the two decreases with time.The stress strain and pressure distributions also have high synchronization.The pressure and strain transfer distances are 385cm and 401.8cm respectively at the 600th day.Strain and pressure mainly spread along the direction of parallel formation,so there is little risk of oil and gas leakage to the overburden direction.The different depth of formation has a significant effect on the pyrolysis and diffusion of oil and gas.With the increase of formation depth,the consumption rate of kerogen has little difference.The distribution range of gas phase products expands with time,and the peak concentration of gas phase products and liquid phase products increases first and then decreases with the increase of formation depth.The distribution range of liquid products in formation is earlier than that of gas products. |
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