Synergistic CH4 hydrate recovery and CO2 storage by coupling depressurization with CO2/N2 injection: A pilot-scale investigation

To ensure sustainable energy supply while mitigating climate change, combining CH4 recovery with CO2 storage in marine natural gas hydrate (NGH) reservoirs is a promising carbon-neutral technology. In this study, a pilot scale reactor (V = 22 L) was used to study the combined processes of CH4 recovery from the methane hydrate bearing sediment by using depressurization followed by in-situ CO2/N2 injection to form mixed CH4/CO2/N2 hydrate (Mix-H) at marine NGH reservoir conditions (T = 283.8 K and P = 11.0 MPa). The bottom hole pressure (BHP = 3.2 MPa, 5.3 MPa, and 6.8 MPa) during depressurization and the setting of horizontal wellbore (HW), were systematically investigated to optimize the synergistic CH4 recovery and CO2 storage. The CH4 recovery ratio (55.4-80.6%) and the CH4 production rate (5.15-134.98 mol/d) increased as BHP decreased. After CO2/N2 injection, Mix-H formed first in the CH4-rich region in the upper section of the reactor and propagated downward to the lower. The Mix-H formation kinetics were significantly enhanced with an increase in BHP, leading to a decrease in CO2 composition in the Mix-H. The use of a horizontal wellbore (HW) facilitates a more even distribution of the CO2/N2 within sediments, resulting in a 13.5% increase in CO2 composition of Mix-H compared with no HW setting. The CO2 conversion ratio and hydrate restoration ratio reaches 52.8% and 69.4% at BHP = 5.3 MPa with HW, respectively. The technical feasibility of CO2 storage in exploited marine NGH reservoirs was first proved at pilot-scale. The results provided insights into the synergistic method of NGH exploitation and CO2 storage at the marine NGH reservoirs.

Automated summary

Combining CH4 recovery with CO2 storage in marine natural gas hydrate (NGH) reservoirs is a promising carbon-neutral technology. The study found that reducing bottom hole pressure (BHP) can increase CH4 recovery ratio and CH4 production rate. Mix-H formation starts in the upper section and propagates downward after CO2/N2 injection. The use of a horizontal wellbore (HW) facilitates a more even distribution of CO2/N2, resulting in a higher CO2 composition in Mix-H.