| With the increasing consumption of oil and natural gas,countries around the world have begun to develop and utilize unconventional oil and gas resources to make up for conventional oil and gas resources.Oil shale has attracted the continuous attention of governments and scholars due to its huge reserves.The in-situ pyrolysis technology of oil shale is the current international cutting-edge technology in the field of oil shale research.However,due to the low permeability characteristics of oil shale,it is necessary to increase the permeability of oil shale stratum through reservoir fracturing technology such as hydraulic fracturing,so as to provide a migration channel for heat carrier and the shale oil and gas produced by oil shale pyrolysis.Nevertheless,groundwater in the stratum will enter the heating reaction zone through the fractures generated by hydraulic fracturing,which will reduce the temperature of the heating zone and cause a large amount of heat loss.At the same time,the migration of oil and gas products will also pollute the underground environment.In order to solve this problem,Shell ICP used underground freezing wall technology,yet the energy consumption of this technology is huge and the freezing cycle is relatively long.Jilin University has proposed a technology to achieve system closure by grouting and applied it to Jilin Nong'an oil shale in-situ pyrolysis pilot project.However,the traditional grouting technology based on the slurry diffusion theory and the formula of the permeability radius of the slurry,which didn't take into account the special formation conditions after oil shale hydraulic fracturing.First of all,the traditional slurry penetration radius formula is aimed at porous media rock and soil formation or fractured rock formation,while oil shale reservoir reconstruction forms several porous media fractures containing proppant.Additionally,the groundwater in the porous media rock and soil formation has a volumetric effect,while it has a crevice effect in the fractured rock formation.However,the oil shale formation is homogeneous and does not contain water.After fracturing,the groundwater only affects the porous media in the fracture.Therefore,the traditional slurry diffusion theory of grouting is not fully applicable to the porous media fracture containing proppant after oil shale fracturing.At the same time,the influence of groundwater on porous media fracture under grouting condition also needs further study.In view of the above problems,this paper used numerical simulation and experimental research methods to study and analyze the influence of groundwater flow and pressure on the diffusion range,diffusion speed and other factors of the slurry under the special porous media condition after oil shale fracturing,and revealed the influence law of groundwater on the diffusion characteristics of slurry.This study was applied to carry out the optimal design of grouting in the in-situ pyrolysis project of Nong'an oil shale,and the optimization design schemes such as the grouting well layout principle has been proposed.First of all,the influence of groundwater on the diffusion of the slurry under the condition of fracture with proppant was simulated by COMSOL Multiphysics.Taking the hydrogeological data of the oil shale formation in Nong'an oil shale in-situ pyrolysis project as the basic simulation parameters,the effects of groundwater flow and pressure on the slurry diffusion characteristics was analyzed.According to the comprehensive analysis of the numerical simulation results,the key dimension of the slurry diffusion simulation experimental device was determined,and the slurry diffusion simulation experimental device was designed and manufactured.This device includes three parts: fracture stratum system,slurry configuration and injection system and parameter detection system,which can realize the visual observation and real-time record of slurry diffusion.Secondly,the slurry diffusion experimental device was used to conduct the diffusion experiment of the slurry in the fracture containing proppant under different groundwater flow and pressure.The experimental results illustrated that,at the same grouting time,the diffusion distance of the slurry increases linearly with the increase of groundwater flow.When the grouting time was 7 minutes,the average diffusion distances of the three groups of grouting experiments(120ml/min,92ml/min,and 70ml/min)were 32.625 cm,31.125 cm,and 27.95 cm.Moreover,at the same grouting time,as the groundwater pressure increased,the reduction factor of the slurry diffusion distance was distributed as a power function.When the grouting time reached 7min,the average diffusion distances of four sets of grouting experiments(0.2MPa,0.3MPa,0.5MPa,and 0.6MPa)were 32.625 cm,27.85 cm,19.925 cm,and 16.4 cm,which was proved by the numerical simulation results in this paper.The fitting formula of the slurry diffusion distance under different groundwater flow and groundwater pressure was obtained by synthesizing the fitting formulas of slurry diffusion distance obtained from two factors,and the accuracy and feasibility of the fitting formula were verified by comparing the fitting data with the experimental data.Finally,based on the grouting engineering for pilot project of in-situ pyrolysis of Nong'an oil shale,the diffusion law of the slurry in oil shale fracturing formation were studied by using COMSOL Multiphysics,and diffusion characteristics of open grouting method and closed grouting method were analyzed.Then,the grouting wells pattern of Nong'an grouting project was optimized.The numerical simulation results presented that if the closed grouting method is used in the production area,there will be a large number of local high velocity zones of groundwater between the grouting wells,which will seriously hinder the diffusion and intersection of the slurry;on the contrary,if the open grouting method is used in the production area,formation sealing system can be obtained faster and better.In addition,through the analysis of simulation results of different grouting well spacings,it can be known that the farther well distance is,the more production area will be filled with slurry,yet if the well distance is too close,a large amount of manpower and material resources will be wasted for drilling wells.By comparing the groundwater output of well NK-1 between the optimized design scheme and the field project,the feasibility of the optimized hexagonal well layout scheme was verified.This well layout scheme can meet the requirements of water stop,reduce the number of grouting wells,and reduce the cost of drilling engineering.In conclusion,this paper designed a slurry diffusion simulation experimental device based on the oil shale in-situ pyrolysis project.Using this device,a systematic grouting experiment was carried out.Combined with the COMSOL numerical simulation software,the influence law of groundwater on the slurry diffusion characteristics in the oil shale fracture containing proppant was explored,and the Nong'an oil shale grouting project was verified and optimized.It provides theoretical support and technical guidance for grouting of oil shale in-situ pyrolysis project in the future. |
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