Production Method under Surveillance: Laboratory Pilot-Scale Simulation of CH₄–CO₂ Exchange in a Natural Gas Hydrate Reservoir

The “guest exchange” of methane (CH 4 ) by carbon dioxide (CO 2 ) in naturally occurring gas hydrates is seen as a possibility to concurrently produce CH 4 and sequester CO 2 . Presently, process evaluation is based on CH 4 –CO 2 exchange yields of small- or medium-scale laboratory experiments, mostly neglecting mass and heat transfer processes. This work investigates process efficiencies in two large-scale experiments (210 L sample volume) using fully water-saturated, natural reservoir conditions and a gas hydrate saturation of 50%. After injecting 50 kg of heated CO 2 discontinuously (E1) and continuously (E2) and a subsequent soaking period, the reservoir was depressurized discontinuously. It was monitored using electrical resistivity, temperature and pressure sensors, and fluid flow and gas composition measurements. Phase and component inventories were analyzed based on mass and volume balances. The total CH 4 production during CO 2 injection was only 5% of the initial CH 4 inventory. Prior to CO 2 breakthrough, the produced CH 4 roughly equaled dissolved CH 4 in the produced pore water, which balanced the volume of the injected CO 2 . After CO 2 breakthrough, CH 4 ratios in the released CO 2 quickly dropped to 2.0–0.5 vol %. The total CO 2 retention was the highest just before the CO 2 breakthrough and higher in E1 where discontinuous injection improved the distribution of injected CO 2 and subsequent mixed hydrate formation. The processes were improved by the succession of CO 2 injection by controlled degassing at stability limits below that of the pure CH 4 hydrate, particularly in experiment E2. Here, a more heterogeneous distribution of liquid CO 2 and larger availability of free water led to smaller initial degassing of liquid CO 2 . This allowed for quick re-formation of mixed gas hydrates and CH 4 ratios of 50% in the produced gases. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, and secondary gas hydrate formation in water-saturated sediments.