Coupling amino acid injection and slow depressurization with hydrate swapping exploitation: An effective strategy to enhance in-situ CO2 storage in hydrate-bearing sediment

Natural gas (mainly methane, CH 4 ) hydrates are a prospective energy resource. CH 4 -CO 2 hydrate swapping has the advantages of producing CH 4 gas for clean energy recovery and simultaneously storing CO 2 in -situ for carbon emission reduction. Depressurization is the most feasible strategy to recover CH 4 from hydrate -bearing sediments. The combination of these two methods has been proved feasible to produce CH 4 -rich gas and store CO 2 - rich hydrate. However, the efficiencies of CH 4 recovery and CO 2 storage decrease significantly when the reservoir is depressurized to the points below CH 4 /CO 2 mixed hydrate equilibrium pressure causing unwanted CO 2 -rich hydrate dissociation. In this work, amino acid injection (AAI) (L-tryptophan or L-methionine) was employed for CH 4 /CO 2 mixed hydrate formation with enhanced CO 2 storage, followed by slow depressurization in various controlled manners, i.e., multistep depressurization, constant -pressure depressurization or multistep constant -pressure depressurization (MCPD) to recover CH 4 from the mixed hydrates. In -situ Raman and gas chromatograph (GC) were employed to confirm the compositions of mixed hydrates formed and mixed gases produced. The results showed that AAI decreased CO 2 fraction from 30 mol% to 3.1 -8.4 mol% in the residual gas, indicating the technique helped regaining reservoir pressure and kinetically promoting further hydrate formation. The pressure recovered by AAI compensated the performance discrepancy caused by different solutions injected. Raman spectra and GC results confirmed CO 2 -rich hydrate formation, and up to 91.3% CO 2 was stored in the sediments. During mixed hydrate dissociation, slow depressurization maintained high CH 4 -rich gas production (CH 4 fraction over 87.5 mol%), with 9.7 -30.6% CH 4 -rich gas recovered and 5.8 -17.9% water produced among various trials. The best performance of CO 2 storage and CH 4 recovery was achieved by AAI of 1000 ppm L-tryptophan at 33.2 bar and MCPD to 20.9 bar, showing a final CO 2 storage efficiency of 82.3%, gas recovery percentage of 30.6% and gas/water production ratio of 37.9 STP m 3 /m 3 . These findings provided guidance to enhance CO 2 -rich hydrate storage and CH 4 -rich gas production by multiple AAI and slow depressurization at controlled depletion pressures after hydrate swapping.