Numerical Simulation Of Combustion Process Of Oil Shale In-Situ Pyrolysis

Since the 21 st century,the huge consumption of global conventional energy has brought obvious shortage of supply and demand.However,the progress of technology has made some progress in the field of unconventional oil and gas,which is expected to effectively alleviate the shortage of conventional resources in the future.Among the unconventional resources,oil shale has attracted more and more attention in recent years.Oil shale can be heated to produce oil by the decomposition of kerogen at high temperature.The surface dry distillation method requires the extraction of oil shale,high-temperature dry distillation and further processing on the ground,but this method can only be used in shallow oil shale formations that are easy to mine,and the waste gas generated by dry distillation can greatly pollute the environment.However,the method of underground in-situ mining can directly heat the oil shale in deep layers,and promote the cracking of kerogen in the oil shale to generate hydrocarbons.At present,in-situ mining methods mainly include electric heating method,fluid heating method,and oil shale reforming method.The utilization technology of oil shale began to develop in the direction of low cost,low pollution and high profit.In order to optimize the in-situ pyrolysis process of oil shale,a series of experiments and studies are needed to understand the influence of various factors.On-site in-situ pyrolysis test is relatively expensive.Therefore,numerical simulation is a safe,fast and low-cost method to explore the influence of various factors on in-situ pyrolysis of oil shale.In the oil shale in-situ extraction technology,hydraulic fracturing is performed manually in the oil shale layer to form relatively regular fractures.Gas injection wells and production wells are set up,high-temperature gas is injected into the oil shale,and the gas flows along the gas injection well and fractures to bring heat to the oil shale layer as much as possible,and then the kerogen in the oil shale cracks to produce oil and gas.Considering that the kerogen cracking will produce a certain amount of coke,the combustion of coke can generate heat.This part of heat can be used to promote the cracking of kerogen,then reduce the heat injected,further save costs and improve economic benefits.The change of the temperature field during the in-situ mining of oil shale is affected by many factors,including the injection of high-temperature gas and the endothermic and exothermic process of chemical reactions such as kerogen decomposition and coke combustion.The change of the temperature field is closely related to the amount of hydrocarbon generation,which directly affects the investment-return ratio of the oil shale in-situ mining technology.Therefore,the numerical simulation study of the above process is of great significance.In order to further understanding the relationship and interaction of combustion process,temperature field and hydrocarbon generation in the process of in-situ pyrolysis of oil shale,a simple three-dimensional model and complex three-dimensional model were established by using the computer numerical simulation software.The whole processes of in-situ fracture extraction of oil shale under different plans were simulated and discussed:(1)This paper establishes a three-dimensional model of in-situ oil shale pyrolysis,uses FLUENT software to simulate the influence of gas injection wells injecting high-temperature gas on the temperature field of oil shale formations,and uses MATLAB software to simulate the effects of kerogen cracking and coke combustion on the temperature field of oil shale,and finally,the simulation of gas injection,chemical reaction process and temperature field under different schemes is realized.Different plans include: continuous injection of high-temperature gas plan and combustion plan with different cut-off time(that is,firstly,high-temperature gas is continuously injected into oil shale to promote kerogen decomposition and hydrocarbon generation,then stop heat injection after reaching the cut-off time,and then make coke combustion provide heat to continue to promote the kerogen decomposition and the hydrocarbon generation).The oil shale layer near the gas injection wells and artificial fracturing fractures is first rapidly heated,and the temperature rises sharply,then the heat gradually spreads along the artificial fractures and gas injection wells to other field of the oil shale layer.It is found that the decomposition of kerogen in the oil shale far away from the gas injection well consumes heat,while the heat transfer to the oil shale layer by means of heat conduction by injecting high-temperature gas is relatively slow,and the decomposition of kerogen consumes relatively so much heat.When the time is longer,the temperature of the field far away from the gas injection wells is lower.The direct injection of high-temperature gas will heat the oil shale layer faster and more efficiently.However,the combustion of coke generates a large amount of heat,and the heat directly generated by combustion is greater than the heat transfer into the oil shale after the injection of high-temperature gas,which lead to a slightly higher temperature of the oil shale layer around the fissure after combustion.In addition,the later the cut-off time of injecting high-temperature gas is,the better the heating effect of oil shale is when the simulation time is the same,that is,a larger range of oil shale is heated and the temperature is higher.(2)Thinking of the reaction heat of the chemical reaction,the total package primary reaction model is used to calculate the single-day hydrocarbon generation and cumulative hydrocarbon generation during the in-situ pyrolysis of oil shale.The cumulative hydrocarbon generation increased with time,and the overall trend was roughly the same.Comparing different plans,it is found that the earlier the cut-off time,the smaller the corresponding value when the cumulative hydrocarbon generation curve tends to be flat,that is,the lower the final kerogen conversion rate.The single-day hydrocarbon generation curve shows a sharp increase first and then a rapid decline trend.After the cut-off time is reached,due to the combustion process,the amount of single-day hydrocarbon generation will increase slightly,and then slowly decrease until it is close to zero.It can be seen that the combustion process can maintain the continuous decomposition of the kerogen,it can only last for a short time.(3)This paper calculates the net production value of oil shale oil for different plans to further measure the economics and feasibility of the plans.In the different plans of simple model combustion,the net production value of oil shale oil shows a trend of first decreasing to a negative value and then increasing;the earlier the cut-off time is,the earlier the lowest point of oil shale oil net production value will appear,but the lowest value is larger;The later the deadline,the greater the net value of the final oil shale oil production.In the simple model,the oil shale in-situ pyrolysis Plan A? has a better effect than the Plan A?,and the effect of the burning Plan A?-? is far better than the continuous heating plan(plan A).In the continuous heat injection plan(Plan B)in the complex model,the net production value of oil shale oil first increases and then decreases.The overall trend of the oil shale oil net production value curve of the combustion plans is that it first rises rapidly and then slowly rises and tends to be stable.Comparing Plan B?-VI,the later the deadline,the greater the net oil shale oil output value,and in the 800-day simulation,the oil shale oil net output value of other combustion Plans(Plan B?-VI)except for Plan B? is greater than the last heating injection Plan(Plan B).The oil shale in-situ pyrolysis Plan B? has the best effect,and the effect of plan B is better than plan B?-? in the first 280 days.