| At present,the increasing of carbon dioxide content is recognized as the main cause of temperature rises worldwide.Controlling changes in the Earth's temperature and reducing the amount of carbon dioxide in the atmosphere has a long way to go.The demonstration and practice of capturing atmospheric carbon dioxide and transporting it to deep underground for storage has been considered an effective way to reduce atmospheric carbon dioxide levels.In actual engineering,carbon dioxide is generally stored at a depth of 1500-3000 meters,and carbon dioxide will be in a supercritical state,and its properties will change greatly.A large amount of carbon dioxide is injected into the ground in a short time,causing a rapid increase in the pressure of the pore fluid.In order to ensure a stable injection of carbon dioxide and a long-term stable underground storage,the cap rock is required to have good trapping properties.On the one hand,carbon dioxide migrates upward due to buoyancy,accumulates in the bottom of the cap rock,and forms a high-pressure difference with the salt water in the pores of the cap rock,which causes the carbon dioxide to overcome the capillary resistance of the mudstone to break into the cap rock and cause leakage.On the other hand,due to the accumulation of pore fluid pressure,the rock is deformed,resulting in changes in the pore structure of the rock and the expansion of the micro-fracture,which in turn leads to changes in porosity and permeability.The reservoir rock deeply buried in the ground is affected by its carbon dioxide injection disturbance,and its own property parameters will also change.The repeated intrusion of supercritical carbon dioxide will affect the pore structure and sealing of the rock.Many scholars have studied it,and the influence of the mechanics on the pore structure and the sealing ability can not be ignored.However,the currentresearch on the mechanical aspects of geological storage of carbon dioxide is mainly focused on the macroscopic level.The study of the mechanical effects of the fine-to-porosity is rarely involved.In addition,the heterogeneity of the rocks and its influence on caprock trapping are often neglected.Starting from the experiment of cap rock,this paper makes a preliminary judgment on the sealing ability of rock by breaking the stress test and the porosity and permeability test,and then conducts the porosity and permeability test and the wave velocity test again after the high pressure carbon dioxide breaks through the rock sample.The parameter changes reflect the damage of the internal pores of the rock;In view of the cumbersome and time-consuming drawbacks of the method for measuring the breakthrough pressure of the cap rock,the relationship between the rock breakthrough pressure parameters and many other parameters such as rock porosity,permeability,density,specific surface area,pore radius,and total organic carbon is explored.Through the establishment of a neural network model,a large-scale and multi-quantity rock breakthrough pressure parameter can be efficiently predicted by establishing a neural network model.Micro-CT scans of different visual field and different resolution conditions of the caprock mudstone samples are performed.The heterogeneous distribution characteristics of rock pores under different scale conditions are studied.According to the heterogeneous distribution of pores,the spatial heterogeneity of rock porosity is calculated.Then the numerical model is used to study the heterogeneous distribution of porosity and permeability parameters for rocks.The influence of the pressure breaking parameters and the migration characteristics of supercritical carbon dioxide in the rock pores.Finally,the site model was established basing on porosity and permeability homogeneity and heterogeneity model,respectively.Through the stress distribution,carbon dioxide migration and the damage of the formation rocks after carbon dioxide injection,the trapping efficient of the injected carbon dioxide by caprock was analyzed.The breakthrough stress test results for the cap rock show that the breakthrough pressure values ? ? of the four target reservoirs in the Ordos carbon sequestration site increase with the increase of depth,which is closely related to the porosity andpermeability of the rock.And the comparison results before and after the gas breakthrough show that the gas will have certain transformation and damage to the pores of the rock during the breakthrough process,resulting in increased permeability,reduced sealing capacity,and rock internal pore structure for rocks with higher brittleness.It is more susceptible to damage from excessive pore fluid pressure.Further,through the method of data collection,a large number of parameters related to rock sealing ability are collected,and the breakthrough pressure of rock and other related parameters are deeply explored.The results show that the breakthrough pressure of rock is the most related to the rock radius and rock permeability determined by mercury intrusion method.Tight,secondly related to the porosity and specific surface area of ? ? the rock,and poorly related to the density and depth of the rock itself and the TOC content.Considering the parameters such as the radius,permeability,porosity and specific surface area of ? ? the rock,a mathematical model can be used to predict the breakthrough pressure of the cap rock.In order to further explore the closed characteristics of the microscopic pore structure of rocks,we used micro-CT and nano-CT to observe and compare the pores of rock at different scales.The results show that the smaller the observation scale,the smaller the pores of the rock.The homogeneity is stronger,and the pore size distribution of the rock is bimodal in different scales: in the range of 1~5 microns,the rock pore size is concentrated around 1.5 microns,in the range of 0.05~0.5 microns.The pore size of the rock is relatively concentrated around 0.15 microns.In terms of connectivity,the coordination number of macropores under micro-CT observation is 2,and the coordination number of small pores under nano-CT observation is only 0.12,indicating that in connectivity,large pores are much better than the small pores.Due to the heterogeneous distribution of rock pores,the porosity and permeability of rock also vary spatially with the change of spatial position.The porosity and permeability heterogeneity of rock will also be covered.The sealing of the layer rock has a certain impact.Numerical simulation technology provides a convenience for exploring the influence of various properties of rock on its capping ability.Based on the above observations,we assign the porosity and permeability heterogeneity of rock to thegrid of simulated carbon dioxide breakthrough cap rock.The simulation results show that the heterogeneity of porosity and permeability of caprock has many adverse effects on the carbon dioxide injected into caprock.1)In the case of long injection time,the low permeability zone can form a certain shielding effect on the carbon dioxide migration path and inhibit the carbon dioxide migration to a distant place;2)Because of the migration hindrance,the accumulation of pore pressure is higher causing more carbon dioxide to invade the caprock;3)The accumulation of high pore pressure also leads to the decrease of effective pressure,the increase of formation deformation and the possibility of damage.Therefore,the study of the influence of the heterogeneity of the pore structure of the cap rock on its trapping ability in the mechanical point of view has important reference significance for the evaluation of caprock sealing in the actual carbon dioxide geological storage project. |
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