Study On Efficient Convection Heating Mechanism And Application Of In-situ Steam Injection For Oil Shale Exploitation

Oil shale is a sedimentary rock rich in organic matter.Organic matter in oil shale will be pyrolyzed under high temperature to produce oil and gas products.Oil shale resource reserves are huge in China,and it is an important complementary energy to traditional fossil fuels.However,due to its low energy density,traditional well mining or open-pit mining will not be able to obtain economic benefits for oil shale deposits with large burial depth.Therefore,in-situ exploitation of oil shale will be the main method for the development and utilization of medium and deep oil shale in the future.Based on the above situation,Zhao Yangsheng’s team invented the"In-situ steam injection technology"（i.e.MTI method）.In this method,superheated water vapor is used as the heat carrier to heat the ore bed.The MTI method first needs to arrange a group of wells in the target oil shale,then fracture the ore bed,connect the group of wells,and form the initial channel of superheated water vapor.Then superheated water vapor is injected along the heat injection well to heat the oil shale deposit.In this process,will the water vapor return to the ground directly from the production well only along the fractures formed by fracturing,forming a short circuit of the water vapor,thus affecting the heating range,heating rate and resource recovery?Does this method have high efficiency?In order to solve the above problems,the interaction of water vapor,thermal fracture and permeability on temperature conduction was studied by laboratory experiment and numerical simulation.The research in this paper reveals the high efficient heating mechanism of oil shale under the condition of water vapor heating,and proves the feasibility of in-situ water vapor injection for oil shale production.The specific research contents and main conclusions of this paper are as follows:（1）Based on CT experiments,this paper studied the distribution characteristics of organic matter,pores and fractures in oil shale after water vapor heating at different temperatures,and then analyzed the connectivity characteristics of pores and fractures in oil shale after water vapor heating at different temperatures without the interference of organic matter clusters.It is found that there are a large number of organic matter clusters in oil shale at room temperature（25℃）,with a volume percentage of 4.29%（the spatial resolution of CT image in this experiment is 3.6μm）.At this time,the oil shale is dense and complete.After heated by high temperature water vapor at 382℃,a small amount of holes appear in these organic matter clusters,which is caused by the pyrolysis of organic matter.At this time,a large number of fractures begin to appear in the oil shale.With the further increase of temperature,the pores in the organic matter mass gradually increase.At 511℃,only a small amount of organic matter remains in the oil shale,and the organic matter pyrolysis forms a large number of pores,and the fractures distributed along the bedding are further developed.At 555℃,most organic matter is pyrolyzed,and only a small amount（volume contribution of 0.71%）remains in isolated pores and fractures.At this time,pores and fractures in oil shale are fully developed and have good connectivity.Under the condition of water vapor heating,the pores and fractures of oil shale are constantly developing,and the newly generated pores and fractures provide new channels for the flow of water vapor,which is the microscopic basis for efficient water vapor heating of oil shale.（2）Based on the percolation theory,this paper studied the site percolation law considering the pore distribution characteristics and fracture anisotropy,and analyzed the anisotropic percolation law of oil shale based on the CT imagies.It is found that each newly generated pore in natural porous media such as oil shale has a certain size（big or small）,and the pore shape has a strong randomness,which is quite different from the process of generating new pores by randomly occupying a site each time in the classical site percolation model.This also leads to a stronger heterogeneity of pore distribution in natural porous media.Even if the porosity is the same,the pore connectivity of natural porous media such as oil shale is stronger than that of the classical site percolation model,which is the reason why the percolation threshold of natural porous media such as oil shale is lower than that of the classical site percolation model.Under the action of temperature,oil shale will form penetrating fractures in the direction parallel to the bedding,resulting in extremely low percolation threshold of oil shale in the direction parallel to the bedding.The percolation threshold of oil shale in the vertical bedding direction is about 15%.The percolation threshold of oil shale parallel to the bedding direction is low,which is conducive to the flow of water vapor in the ore bed and efficient heating of oil shale.The percolation threshold of oil shale perpendicular to the bedding direction is high,which effectively prevents the flow of water vapor outside the mining area and improves the heating efficiency.（3）In this paper,the self-developed high temperature triaxial permeability testing system was used to study the real-time high temperature permeability of fractured oil shale under different water vapor temperature.At the same time,the evolution mechanism of the permeability of fractured oil shale with the water vapor temperature was studied by combining CT and hot stage microscope real-time high temperature imaging technology.It is found that under water vapor heating,the magnitude of permeability of oil shale with single penetrating fracture decreases from 10^-15m²at 25℃to 10^-17m²at 350℃.When the temperature is higher than 350℃,the permeability of oil shale rises rapidly again,and recovers to the order of 10-¹⁵m²above 500℃.The permeability threshold temperature of fractured oil shale is around 350℃.From normal temperature to 300℃,some micro-fractures are formed in the oil shale under the action of pressure and temperature.The length and aperture of micro-fractures are small.Therefore,in this temperature range,the oil shale specimen still contains only one penetrating fracture,which belongs to single fracture seepage.The decrease of oil shale permeability in this temperature range is caused by fracture closure.From 300℃to 350℃,the micro-fractures formed in the oil shale are closed,resulting in the minimum permeability of fractured oil shale at 350℃.When the temperature reaches 400℃,the closed fractures in the oil shale re-open,and new penetrating fractures appear.The oil shale changes from single fracture seepage to multi fracture seepage,and the permeability increases rapidly.When the temperature reaches above 500℃,the pores and fractures in the oil shale are fully developed,which is the reason why the magnitude of permeability of fractured oil shale increases to 10^-15m².（4）The temperature field of oil shale heated by high temperature water vapor was studied,and the mechanism of high efficiency heating of oil shale by convection-conduction under water vapor heating was revealed.It is found that when high temperature water vapor is used to heat oil shale,high temperature water vapor first flows along the prefabricated fractures and heats the surrounding rocks on both sides of the fracture surface.The temperature of surrounding rock on both sides of the fracture surface gradually rises,and the heat source at this time is the high-temperature water vapor in the prefabricated fracture.The oil shale on both sides of the fracture surface breaks after being heated,forming some new water vapor flow channels.After entering these new channels,water vapor will bring heat energy into these fractures,and new high-temperature areas will appear in these new fracture locations under water vapor heating.The high temperature area continues to expand to both sides of the fracture surface,and new thermal fracture will occur.The above process repeats in the oil shale,realizing efficient heating of the oil shale.（5）In this paper,the pore connectivity of oil shale heated by water vapor at different temperatures,the change law of fracture permeability of oil shale with temperature,the temperature conduction law under water vapor heating and the distribution law of organic matter in oil shale heated by water vapor at different temperatures were brought into the Thermal-hydraulic-mechanical（THM）coupling mathematical model by combining the research results of indoor experiments and referring to the relevant research results of predecessors.Then,Comsol Multiphysics software was used to study the key economic and technical indicators such as oil and gas recovery rate and energy return rate of oil shale under different prefabricated fracture number and fracture evolution mode,and evaluate the efficiency of the in-situ steam injection technology.It is found that in the process of in-situ heat injection for shale production,the oil shale is efficiently heated when the influence of the water vapor caused by the weak surface of the ore bed evolving into a penetrating fracture under the effect of temperature is considered.In the first year of heating,the rate of return on energy fluctuates with the change of heating time and remains at a high level（higher than 5）.When the heating time is more than one year,the rate of return on energy starts to decline and no longer fluctuates.When there are two prefabricated fractures in the 40m thick ore bed and considering the influence of the weak surfaces of the ore bed evolving into penetrating fractures under the action of water vapor temperature,there is still a 5%energy return rate when heated to 794d,and more than 90%oil yield can be achieved when heated to 1000d.