ECBM 공법에서 흡착과 확산 특성을 고려한 치환유동 거동분석 실험연구

Abstract

석탄층에 메탄가스 회수증진 공법 적용시, 주입된 CO2는 확산유동에 의해 암체로 이동하여 입자표면에 흡착된다. 여기서 확산속도는 석탄 암체의 공극구조, 주입가스의 주입압력 및 가스 분자의 크기에 따라 달라진다. 또한, 석탄 암체에서 CO2의 흡착친화성의 크기는 농도구배를 다르게 하여 확산거동에 영향을 미친다. 주입되는 가스의 흡착특성은 암체 팽창이나 수축으로 인한 투과도 변화의 정도와 치환되는 메탄가스의 양을 결정하는 반면, 확산속도는 투과도가 감소되는 속도와 치환양상을 좌우한다. 따라서, 메탄가스의 치환유동 거동을 보다 정확히 해석하기 위해서는 CO2 주입가스의 흡착친화성은 물론이고 확산특성까지 고려해야만 한다. 뿐만 아니라, CO2 단일가스만을 주입할 경우, CO2의 높은 흡착친화성과 빠른 확산유동 효과로 인한 과도한 암체팽창에 의해 투과도가 빠르게 감소되어 CO2의 주입성이 상당히 저하될 수 있다. 이를 보완하고자 N2의 낮은 흡착성을 활용한 CO2-N2 혼합가스 주입공법이 제시되어왔다. 이에 본 연구에서는 석탄코어에 CO2 단일가스와 CO2-N2 혼합가스의 주입에 의한 메탄가스 치환효율성 실험결과로부터 메탄가스 회수증진 공법 적용시, 이전의 연구에서처럼 주입된 CO2나 N2의 흡착뿐만 아니라 확산특성까지 고려한 보다 정확한 치환유동 거동분석을 수행하고자 하였다. CO2를 주입하는 메탄가스 회수증진 공법에서, 먼저 확산유동의 중요성을 확인하기 위한 석탄코어 유동실험을 수행하였다. 괴탄시료를 이용한 확산계수 측정결과로부터, CO2의 확산계수는 작은 운동직경과 높은 흡착성에 의해 압력이 커짐에 따라 메탄보다 크게 증가하였다. 또한, 괴탄시료에서의 투과도 감소속도는 보존된 공극구조에 의한 낮은 확산속도로 인해 분탄시료에서보다 느리게 나타났다. 이상과 같이, 주입되는 CO2에 의해 투과도 감소가 빠르게 진행되어 CO2 주입성이 감소하는 문제점을 보완하고자, 이번에는 N2의 매우 낮은 흡착성을 활용한 CO2-N2 혼합가스 주입실험을 수행하였다. 실험결과, CO2 주입에 의한 투과도 감소는 N2에 비해 약 5배 정도 빠른 것으로 나타났다. 그러나, CO2-N2 혼합가스 주입시에는 CO2의 확산특성으로 야기되는 빠른 투과도 감소가 혼합가스에 포함된 N2 성분에 의해 효율적으로 개선되었다. CO2의 경우, 치환유동 실험에서 메탄과의 상호확산이 발생하는 반면에 N2는 낮은 흡착성 때문에 암체에 흡착되기보다는 탈착된 메탄가스와의 공동확산에 의해 메탄가스의 운반체 역할을 하는 것을 확인하였다. CO2 단일가스가 주입된 경우, 미세한 공극으로의 빠른 확산과 높은 흡착선호도로 인해 약 0.6 PVI의 주입시점까지 메탄가스의 생산지연을 야기하였다. 반면에, CO2-N2 혼합가스 주입시에는 N2의 확산특성에 의한 운반체로의 역할로 인해 즉각적으로 메탄가스가 생산되는 결과를 나타내었다. 따라서, 본 연구에서는 메탄가스 회수증진 공법을 적용한 석탄층에서 메탄가스의 치환효율성 분석시, 주입가스의 흡착특성뿐만 아니라 확산유동 특성 또한 고려함으로써 보다 정확한 메탄가스의 생산성 예측이 가능함을 확인할 수 있었다. |In application of enhanced coalbed methane (ECBM) process, the injected CO2 moves into matrix by diffusional flow and adsorbs on coal grain surface. The diffusion velocity varies depending on pore structure of coal matrix, injection pressure, and gas molecular size. Moreover, the magnitude of sorption affinity of CO2 generates concentration gradient differently in coal matrix and thus affects the diffusion behavior. The sorption characteristics of injected gas determine the degree of permeability change occurred by swelling or shrinkage of matrix and the amount of replaced methane gas, whereas diffusion velocity dominates permeability reduction rate and replacement behavior. Therefore, in order to interpret replacement flow behavior of methane gas more accurately, diffusion characteristic as well as sorption affinity of injected CO2 gas should be considered. Furthermore, when only CO2 gas is injected, CO2 injectivity can be reduced enormously because permeability is decreased rapidly by excessive matrix swelling due to effect of high sorption affinity and fast diffusional flow of CO2 gas. In order to overcome this problem, CO2-N2 mixed gas injection method has been suggested by use of N2 gas which has low sorption affinity. In this aspect, this study attempted to analyze the replacement flow behavior of methane more accurately by considering the sorption properties as previous works and also diffusion characteristics of injected CO2 or N2 gas during ECBM. The replacement efficiency of methane was analyzed from experiments through coal core by injecting CO2 and CO2-N2 mixed gases. In ECBM process of CO2 injection, coreflooding experiments were performed firstly to confirm the importance of diffusional flow in coal. From the measurement of diffusion coefficient for coal core, the increasing rate with pressure for CO2 diffusion coefficient appeared to be greater owing to its smaller kinetic diameter and higher sorption affinity than that of CH4. In addition, the permeability reduction rate for coal core was slow due to low diffusion velocity in the case of the preserved pore structure for coal core compared with crushed coal. As described above, in order to supplement the decrease in CO2 injectivity which is yielded from the rapid reduction of permeability of the injected CO2 gas, the experiments for CO2-N2 mixed gas injection were conducted by utilizing of low sorption affinity of N2. From the results of permeability change, permeability reduction by CO2 injection was about five times faster than that of N2. However, it was observed that the fast permeability reduction caused by diffusion characteristic of CO2 was improved efficiently by N2 component included in mixture during CO2-N2 injection. In the replacement flow experiment, while counter-diffusion with methane occurred in the case of CO2, N2 gas mostly acted as a replacing agent of methane gas by co-diffusion with desorbed methane rather than being adsorbed into matrix owing to its low sorption affinity. When 100% CO2 was injected, the delayed methane recovery until almost 0.6 PVI by fast diffusion into fine micropore and high sorption affinity occurred. On the other hand, methane gas was produced promptly by the diffusion characteristic of N2 as a role of replacing agent for CO2-N2 injection. Therefore, this study confirms that diffusion characteristics of gas as well as sorption properties should be considered in the analysis of replacement efficiency between methane and CO2 gases as ECBM process for predicting the methane gas production more accurately.; In application of enhanced coalbed methane (ECBM) process, the injected CO2 moves into matrix by diffusional flow and adsorbs on coal grain surface. The diffusion velocity varies depending on pore structure of coal matrix, injection pressure, and gas molecular size. Moreover, the magnitude of sorption affinity of CO2 generates concentration gradient differently in coal matrix and thus affects the diffusion behavior. The sorption characteristics of injected gas determine the degree of permeability change occurred by swelling or shrinkage of matrix and the amount of replaced methane gas, whereas diffusion velocity dominates permeability reduction rate and replacement behavior. Therefore, in order to interpret replacement flow behavior of methane gas more accurately, diffusion characteristic as well as sorption affinity of injected CO2 gas should be considered. Furthermore, when only CO2 gas is injected, CO2 injectivity can be reduced enormously because permeability is decreased rapidly by excessive matrix swelling due to effect of high sorption affinity and fast diffusional flow of CO2 gas. In order to overcome this problem, CO2-N2 mixed gas injection method has been suggested by use of N2 gas which has low sorption affinity. In this aspect, this study attempted to analyze the replacement flow behavior of methane more accurately by considering the sorption properties as previous works and also diffusion characteristics of injected CO2 or N2 gas during ECBM. The replacement efficiency of methane was analyzed from experiments through coal core by injecting CO2 and CO2-N2 mixed gases. In ECBM process of CO2 injection, coreflooding experiments were performed firstly to confirm the importance of diffusional flow in coal. From the measurement of diffusion coefficient for coal core, the increasing rate with pressure for CO2 diffusion coefficient appeared to be greater owing to its smaller kinetic diameter and higher sorption affinity than that of CH4. In addition, the permeability reduction rate for coal core was slow due to low diffusion velocity in the case of the preserved pore structure for coal core compared with crushed coal. As described above, in order to supplement the decrease in CO2 injectivity which is yielded from the rapid reduction of permeability of the injected CO2 gas, the experiments for CO2-N2 mixed gas injection were conducted by utilizing of low sorption affinity of N2. From the results of permeability change, permeability reduction by CO2 injection was about five times faster than that of N2. However, it was observed that the fast permeability reduction caused by diffusion characteristic of CO2 was improved efficiently by N2 component included in mixture during CO2-N2 injection. In the replacement flow experiment, while counter-diffusion with methane occurred in the case of CO2, N2 gas mostly acted as a replacing agent of methane gas by co-diffusion with desorbed methane rather than being adsorbed into matrix owing to its low sorption affinity. When 100% CO2 was injected, the delayed methane recovery until almost 0.6 PVI by fast diffusion into fine micropore and high sorption affinity occurred. On the other hand, methane gas was produced promptly by the diffusion characteristic of N2 as a role of replacing agent for CO2-N2 injection. Therefore, this study confirms that diffusion characteristics of gas as well as sorption properties should be considered in the analysis of replacement efficiency between methane and CO2 gases as ECBM process for predicting the methane gas production more accurately.