Research On The Influence Factors Of CO₂ Sequestration In Coal Seams And Numerical Simulation Of CO₂ Sequestration Process

Global warming is one of the most serious environmental issues that we face today.Countries are actively taking measures to reduce the carbon dioxide?CO₂?amount in the atmosphere to mitigate the environmental problems caused by greenhouse effect.CO₂ sequestration in coal seam is considered to be one of the effective methods to reduce CO₂ concentration.Because coal has a stronger adsorption capacity for CO₂ than CH₄,thus injecting CO₂ into coal seam could achieve CO₂ sequestration and meanwhile coalbed methane recovery enhancement.Injecting CO₂ into coal seam led to gas competitive adsorption between CO₂ and CH₄,which caused the coal matrix swelling and then changed the coal properties including coal porosity and permeability.These properties directly determined the injectivity of the CO₂ and the following fluid transportation in the coal system.However,this process is too complicated to be monitored in the field.Therefore,several methods have been employed to simulate the CO₂ sequestration process,which allows in-situ observations of the adsorption of gases in the coal and how it changed the coal properties afterwards.Moreover,the lab based experiments are also limited by a lot of factors and cannot fully represent the real conditions in the field.Therefore,the numerical simulation method should also been carried out to the further understanding of the process of CO₂ injection into the coal seam.Most existing research focused on the effect of CO₂ injection on CH₄ recovery and influencing factors on CH₄ recovery.Rare attentions have been paid to study the CO₂ sequestration situaton in coal.However,in order to safely and effectively carry out CO₂ sequestration project,it is of great significance to carry out a comprehensive study on the factors that affect the CO₂ sequestration before the project.Therefore,the effect of various parameters,including the geological conditions,engineering specifications,and injections conditions,on CO₂ sequestration should be taken into consideration.These information will be essentially important to benefit the scale up of the CO₂sequestration in coal seam.Synthesis of coalbed methane geology,coal petrology,coalbed methane development geology,surface chemistry and adsorption thermodynamics as theoretical basis,We used the lab-based experimental simulation and numerical simulation methods to investigate the multiphysics coupling factors in the process of CO₂ injection into coalbed.This research aimed at understanding what are the key factors influencing the process of CO₂ injection in to the coalbed,and how they altered this process.Specially,we started from the lab-based experimental simulation,which investigated the CO₂adsorption in the coal and how it changed the coal properties under different confining pressures,hydrostatic pressures,and injection pressures.In this experiment,the injection pressures are tuned from low pressure to high pressure crossing the CO₂liquefaction point.Basing on the experimental results,the model descripting the evolution of coal permeability during CO₂ injections process under triaxial stress conditions has been built.Basing on this model,several parameter that are hard to investigate during the lab-based experimental simulation were discussed,including the maximum adsorption strain,coal compressibility,and Langmuir pressure.Combining the experimental simulation and numerical simulation,the effect of various parameters,including the geological conditions,engineering specifications,and injections terms,to the CO₂ sequestration in coal has been investigated separately.Moreover,this dissertation point out the current limits on evaluating the CO₂ storage capacity in the coal seams.Specifically,this dissertation can be divided into four parts.?1?This research employed to No.3 coal seam in Tunliu mine in Qinshui basin as coal sample to perform to CO₂ injection experiment at 25?.Different confining pressures were designed to replicate the conditions of coalbed reservoir under different depths.Under certain constant confining pressure,different hydrostatic pressures were applied to represent the CH₄ pressure,while a 0.5MPa higher pressure was employed as the CO₂ injection pressure.From this work,we realized the understanding of first,the deformation behavior of coal and induced stress-dependent permeability evolution during methane displacement by CO₂;second,the ad-/de-sorption characteristics of coal under stressed conditions;third,the permeability variation with respect to different buried depths so as to better mimic the geological and stress situation.The results indicated that the stress of the coalbed reservoir directly influenced the gas adsorption in the coalbed.The total gas adsorption amount increased when increasing the injection pressure while keeping the confining pressure constant.Moreover,the total gas adsorption amount increased when increasing the confining pressure while keeping the pore pressure constant.The CO₂/CH₄ adsorption ratio increased when increasing the confining pressure under a constant pore pressure,indicating the replacement of CH₄ by CO₂ was suppressed under high confining pressure.Thus,it is well established that the gas adsorption was suppressed in the deep buried depth.When injecting liquid CO₂,further increase in injection pressure causes the increase of CO₂ adsorption and decrease of CH₄ adsorption.Additional,liquid phase CO₂ has a better effect in CH₄ displacement than gas phase CO₂,which benefits the CO₂sequestration.The coalbed volumetric deformation is determined by confining both confining stress and sorption-induced strain.Among then,the sorption-induced swelling is more significant than that of stress-induced mechanical compression.And the coal swelling was observed when increasing the confining pressure while keeping the pore pressure unchanged.Also,the radial strain is not equal to the axial strain at these conditions,indicating the existence of the anisotropy characteristics of coal shrinkage/swelling.When injecting gas phase CO₂,the coal volumetric deformation was enhanced by increasing the pore pressure.And the adsorption induced coal swelling is the main reason for the coal deformation.When injecting liquid phase CO₂,coal deformation was suppressed by increasing the pore pressure.As a summary,injecting liquid phase CO₂caused less coal swelling compared to injection gas phase CO₂.The coal permeability showed a U shape varitaiton trend under one particular confining pressure when increasing the injection pressure.Specially,the permeability decreased first and then increased during the gas depletion,and the critical injection pressure is 3.5MPa.When further increasing the injection pressure higher than the liquefaction point,the storage pressure increased continuously.Under a particular pore pressure,the permeability decreased when increasing the confining pressure,indicating that deep buried depth with a low permeability is not beneficial to the CO₂ injection.?2?Basing on the experimental simulation results,the relationship between the permeability change and the gas adsorption behavior and confining stress changes was established.Basing on the simulated model and the experimental results,several initial parameters were obtained,including the maximum adsorption strain,coalbed compressibility,and Langmuir pressure.By further investigating how these parameters changed as a function of confining pressure,we can find that the increasing the confining pressure decreased the mix gas adsorption induced coal swelling,increased the coalbed compressibility,and further decreased the Langmuir pressure.Moreover,the effect of the gas adsorption on the coal swell can also be considered as how the confining stress changed the coal swell.Basing on the relationship between the coal volumetric deformation and the gas adsorption amount,the relation between the confining stress and the adsorption amount can be obtained.Furthermore,the relationship between the permeability and the gas adsorption amount can be established.It can be easily observed,this relationship can fit in the S&D numerical model well.This proves that the S&D model can be used to simulate the changes of coal permeability when injecting CO₂ into the coal.Thus,in the following section,we try to analyze how the coal permeability changed as a function of confining stress and the gas adsorption/desorption induced coal volumetric deformation.?3?On the basis of the geological environment of No.3 coal seam in Qinshui basin,we investigated the process of the CO₂ injection to the coal by employing single factor control variable method.We detailed the influence of the geological conditions,engineering specifications,and injections terms to the CO₂ sequestration in coal.For the simulation,we used the maximum hole bottom pressure to inject the CO₂ in the inject well,while we employed the minimum hole bottom pressure to release the pressure in the product well.To get detail understanding on the influence of the geological conditions on the CO₂ sequestration,mainly focusing on the discussion of the coal temperature,confining pressure,coal thickness,coal permeability,gas adsorption-desorption properties,and gas component in the coal.Through this study,we can obtain the following conclusions.?1?The buried depth changed the coal temperature and also the confining pressure.Basically,coal with deeper bury depth shows properties of high temperature and high confining pressure,which are not beneficial to the injection and adsorption of CO₂.When supercritical CO₂ was injected,the closer the reservoir temperature is to the supercritical temperature,the higher amount of CO₂ can be stored in the coal.?2?The coal permeability directly determined the CO₂ injection and transportation.In this thesis,we compared the CO₂ sequestration process in coal with different permeabilities?0.01¹mD?.It can be easily found that,the storage capacity of CO₂ in the coal showed positive correlation with the coal permeability under a particular injection time.?3?Coal with higher CH₄ gas content causes higher reservoir pressure near the injection well,which is negative to the CO₂ injection.?4?When the coal shows higher CO₂/CH₄ adsorption ratio,corresponding to higher selectivity for CO₂ adsorption,more CO₂ storage capacity can be expected.As to the understanding of the effect of engineering specifications on the CO₂ sequestration,several parameter,mainly focusing on the inject-product well distance and the fracturing effect,were investigated.According to the simulation results,under certain injection pressure,reducing the inject-product well distance caused higher CO₂ injection rate,shorter breakthrough time,and smaller CO₂ storage capacity.The effect of the fracturing effect on the CO₂ sequestration can be understood by monitoring how the fracture half-length and fracture conductivity changed the CO₂ injection process.For the fracture half-length effect,I found that longer fracture half-length gave rise to higher sequestration efficiency.However,this promotion effect was suppressed when increasing the fracture half-length higher than 100m.For the fracture conductivity effect,I found that better fracture conductivity gave rise to higher CO₂ sequestration efficiency.To understand the effect of the injection terms on the CO₂ injection,I decided to detail the influence of the following parameters,including the injection pressure,injection method and injection timing.We obtained the following conclusions.?1?Controlling the maximum bottom hole pressure lower than the breakthrough pressure of coal,improving the injection pressure helped to increase the CO₂ storage capacity and injection efficiency.?2?To inject the same amount of CO₂,keeping the injection pressure constant is better than keeping the injection flow constant.?3?Injecting CO₂ to the coal before the methane product rate reaching the peak position gave rise to the highest storage capacity before the breakthrough time.It is highly recommended not to inject the CO₂ at the later stage of methane production.?4?As long as the coal structure is not damaged,increasing the drainage rate?to reduce the reservoir pressure?shows advantages on improving the CO₂sequestration efficiency.Moreover,it is pointed out that adsorption ratio,temperature,coal thickness,well spacing and injection pressure are the main factors that affect the CO2 sequestration of coal seam.?4?In this section,SIMED simulation software was employed to simulate the CO₂injection process.Also,the SX006 well group was selected to be the research object.The simulation results indicated that the breakthrough happened at the production well,which is 412 m away from the injection well,after injecting CO₂ for 23 years.At the breakthrough moment,the CO₂ storage capacity is 2.051×10⁷ m³.Within the maximum simulation time,the maximum CO₂ storage capacity is 2.511×10⁷m³.On the other side,by employing the gas content calculation method,the CO₂ capacity was expected to be between 1.046×10⁷².530×10⁷m³.From our studies,the CO₂ storage capacity is 81%of the highest storage capacity obtained from the gas content calculation method.And the CO₂ storage capacity after injecting CO₂ for 10 years is 10%less than the amount obtained from the gas content calculation method.During the simulation process,the various factors than can influence the CO₂ injection process has been considered,which can give rise to more accurate results than those obtained from the gas content calculation method.Especially,this research has considered the influence of the dynamic responses of coal permeability during the CO₂ injection and migration.Thus,the results obtained from the simulation are more accurate.Applying this method to investigate the CO₂ sequestration in coal seam is highly important to gain deep insight into the coal responses to CO₂ injection and CO₂ sequestation potential that occur in field-scale injection and sequestration processes.