二氧化碳置换法开采天然气水合物研究进展

Advances in research on CO_(2)replacement for natural gas hydrate exploitation

应用CO_(2)置换法开采天然气水合物被认为是一种极具潜力的提高CH_(4)采收率和CO_(2)埋存率的技术。论述了CO_(2)及其混合气置换法开采天然气水合物的机理,梳理了CO_(2)混N_(2)/H_(2)及地热辅助CO_(2)提高水合物中CH_(4)采收率的技术进展。研究表明:①应用纯CO_(2)置换开采天然气水合物时,CH_(4)的采收率较低,而将CO_(2)与N_(2)、H_(2)以不同比例混合后注入天然气水合物藏中进行CH_(4)开采,能够有效提高CH_(4)的采收率。②CO_(2)与N_(2)或H_(2)混合注入水合物层时,多种气体分子在竞争吸附作用下降低了CH_(4)分子与水合物分子笼之间的范德华力,同时降低了混合气中CO_(2)的分压,导致水合物相平衡曲线上移,抑制了置换过程中CO_(2)水合物的生成速率,减轻了包裹作用的不利影响,从而提高CH_(4)采收率。③CO_(2)混合N_(2)注入开采水合物时,N_(2)的混入虽然能够减轻包裹作用的影响,但新形成的N_(2)水合物会堵塞CO_(2)进入水合物分子笼的通道,因此提高CH_(4)采收率效果有限。④在水合物层条件下H_(2)并不会形成新的水合物,而且混入少量的H_(2)又会与N_(2)发生吸附竞争,从而抑制N_(2)水合物的形成,故将低浓度的H_(2)气混入CO_(2)与N_(2)的混合气中能够进一步提高对水合物中CH_(4)的置换率,从而提高CH_(4)的采收率。因此,混入H_(2)被认为是提高CO_(2)置换开发水合物效果的重要途径。⑤混合气周期注气方式可明显提高水合物中CH_(4)的采收率和CO_(2)水合物藏封存率。⑥应用地热辅助CO_(2)开采水合物的方法也能够降低新形成水合物的包裹作用,同时实现CO_(2)在地热层和水合物层的两次埋存,在提高CH_(4)采收率的同时,大大提高CO_(2)在地层中的埋存率。

The application of CO_(2)replacement method to develop natural gas hydrates(NGHs)is considered a highly promising technology for enhancing both CH_(4)recovery and CO_(2)sequestration.This study presents a review of the replacement mechanisms with CO_(2)and its mixed gas for NGH exploitation,as well as technological advances in the replacement with CO_(2)mixed with N_(2)/H_(2)and geothermal-assisted CO_(2)replacement for enhancing CH_(4)recovery from NGHs.Key findings are as follows:(1)To replace NGHs with pure CO_(2)yields a low CH_(4)recovery.In contrast,injecting CO_(2)mixed with N_(2)/H_(2)at varying ratios into NGH reservoirs proves effective in enhancing CH_(4)recovery.(2)Injecting CO_(2)mixed with N_(2)or H_(2)into NGH reservoirs can reduce the Van der Waals’forces between CH_(4)molecules and NGHs’molecular cages through the competitive adsorption among various gas molecules.Furthermore,it can decrease the partial pressure on the CO_(2)phase in the mixed gas,resulting in an upward shift in the phase equilibrium curve of NGHs.Such shift can inhibit the generation rate of CO_(2)hydrates during the replacement process and mitigate the adverse effects of hydrate encapsulation,thus enhancing CH_(4)recovery.(3)Injecting CO_(2)mixed with N_(2)for NGH exploitation can reduce the adverse effects of hydrate encapsulation.However,the newly formed N_(2)hydrates can block the pathways through which CO_(2)molecules enter the molecular cages of NGHs,thus leading to limited performance in enhancing CH_(4)recovery.(4)Unlike CO_(2)and N_(2),H_(2)does not form new hydrates under the conditions of hydrate reservoirs.Furthermore,competitive adsorption will occur between H_(2)and N_(2)when a minor amount of H_(2)is injected,further curbing the formation of N_(2)hydrates.Therefore,introducing H_(2)of low concentration to mixed CO_(2)-N_(2)gas can further increase the displacement rate of CH_(4)in NGHs,thus boosting the CH_(4)recovery,establishing it as a crucial method to enhance the performance of CO_(2)replacement for NGH exploitation.(5)Cyclic injection of mixed gas can significantly enhance both the CH_(4)recovery from NGHs and the sequestration rate of CO_(2)hydrates.(6)Geothermal-assisted CO_(2)replacement for NGH exploitation can not only reduce the encapsulation effect of newly formed CO_(2)hydrates but also facilitate CO_(2)sequestration in geothermal and hydrate reservoirs,thereby markedly increasing subsurface CO_(2)sequestration rate while simultaneously enhancing CH_(4)recovery.