CO₂ Containment Mechanism In Deep Anthracite Related To CO₂-ECBM And Assessment Methodology For CO₂ Storage Capacity

In 2018,China's CO₂ emissions reached 9,428.7 Mt,accounting for 27.8%of the total global CO₂ emissions and became the largest CO₂ emitter.The geological sequestration of CO₂ can achieve the carbon emission reduction target in a short time and is regarded as an effective emission reduction technology.The CO₂ geological sequestration in deep unminable coals,which integrated greenhouse gas emission reduction and high-efficient development of coal bed methane,is an important option of CCUS?carbon dioxide capture,storage and utilization?technology.In this study,we took deep coals in the Qinshui Basin as an example and carried out pore structure tests and various high pressure gas adsorption experiments on coals based on the comprehensive understanding of geological setting and coal reservoir characteristics in the Qinshui Basin.The effects of temperature,coal rank,coal quality and pore structure parameters on the adsorption of supercritical CO₂/CH4 were analyzed.The temperature and free phase density control mechanisms of supercritical CO₂adsorption on coals were discussed in term of molecular interaction,and the model of supercritical CO₂ adsorption on coals combined with micropore filling and multi-layer adsorption was established.The effect of CO₂ supercritical isochore on adsorption under reservoir conditions is also revealed.On the basis of summarizing the CO₂ adsorption containment,static containment,dissolution containment and mineralization containment in coals,the models of theoretical CO₂ storage capacity and effective storage capacity in coals were established.Finally,based on the optimization of evaluation parameters of the geological models of the Qinshui Basin and Zhengzhuang Block,the theoretical and effective storage capacity of CO₂ in 3#coal in the Qinshui Basin and Zhengzhuang Block are evaluated.The main understandings and results of this study are as follows:?1?The control mechanism of temperature and free phase density on multi-layer adsorption in the supercritical CO₂ adsorption onto coals was revealed.Both temperature and free phase density changed the adsorption by changing the strength of the interaction between CO₂ molecules.However,the action objects are different.The increase in temperature expanded the distance between adsorbed CO₂ molecules,resulting in a decrease in the layer number and adsorption amount.However,the increase in the free phase density did not affect the adsorbed molecules,and only reduced the distance between the outermost adsorbed molecules and their adjacent free phase molecules,resulting in an increase in the layer number and adsorption amount.?2?The comprehensive model of micropore filling and multi-layer adorption for supercritical CO₂ adsorption onto coals was established.The calculation results shown that the average number of adsorbed CO₂ molecular layer ranged from 1.04 to 1.7under the experimental temperature.The increase in temperature resulted in a decrease in the layer number of adsorbed CO₂ molecular.The upper limit of diameter of micropore which was completely filled is 1.12nm,which is larger than the dominant micropore diameter range of the Qinshui anthracite?0.33nm-0.68nm?.The adsorption model of supercritical CO₂ onto coals is microporous filling combined with multi-layer surface covering.?3?The effect of supercritical CO₂ isochore on the adsorption behavior of supercritical CO₂ in coals under deep reservoir conditions was expounded.Under deep reservoir conditions,supercritical CO₂ adsorption behavior in coals presented two stages controlled by supercritical CO₂ isochore and the boundary depth is about920 m in the Qinshui Basin.In the gas-like supercritical stage,the increase in the free phase density contributed to an increase in the layer number of adsorbed CO₂,whereas,the negative effect of temperature was enhanced.In the liquid-like supercritical stage,temperature turned to be a primary factor,resulting in a decrease in CO₂ adsorption capacity.The maximum CO₂ adsorption capacity in coals occurred at the lower depth in the gas-like supercritical stage.?4?CO₂ geological storage related adsorption containment,static containment,dissolution containment and mineralization containment were discussed and the assessment models of the theoretical CO₂ storage capacity and effective CO₂ storage capacity in coals were established.The CO₂ adsorption storage capacity is dominated?>81%?in the total storage at the depth between 1000m and 2000m.The CO₂ static storage capacity presented a positive relationship with depth and it accounted for 20%at the total storage capacity in the depth of 2000m.The proportion of the CO₂dissolution storage capacity is less than 2%in the total CO₂ storage capacity in coals.The CO₂ mineral storage capacity can be negligible.The calculation method combined with excess adsorption capacity and free amount in void volume in coals was applied for reducing errors induced by free phase density and adsorbed phase density.?5?The theoretical CO₂ storage capacity and effective CO₂ storage capacity in 3#coal seam in the Qinshui Basin and Zhengzhuang Block were obtained using our calculation model of CO₂ storage capacity in coals.The theoretical CO₂ storage capacity was 9.72Gt and the effective CO₂ storage capacity was 2.53 Gt in 3#coal seam in the Qinshui Basin.The deep coals in the Qinshui Basin show a promising potential for CO₂-ECBM.The theoretical CO₂ storage capacity was 416.18 Mt and the effective CO₂ storage capacity was 108.2 Mt in 3#coal seam in the Zhengzhuang Block.The areas in which coals has a highest CO₂ storage capacity?0.6 Mt/km²?belonged to gas-like supercritical CO₂ area?depth distributed between 800m and1100m?.In combination with CO₂ injectivity and preservation,the gas-like supercritical CO₂ area was considered as a priority for implementation of CO₂-ECBM projects in the Zhengzhuang BlockThis dissertation contains 85 figures,25 tables and 279 references.