Abstract
The geomechanics effects and seabed subsidence are critical issues that should be considered in the development of a hydrate reservoir. The purpose of this study is to couple the geomechanics, hydrate reaction, and multiphase fluid flow modules to investigate the feasibility of CO2 enhanced gas recovery (CO2-EGR) of a Class-1 hydrate deposit by observing the formation deformation, and the seabed subsidence. The production methods of depressurization and CO2-EGR are modeled, respectively. The production behaviors and seabed subsidence of different production methods are compared. The positive influence on the gas recovery for a Class-1 hydrate deposit via CO2-EGR is observed. The calculations of seabed subsidence showed a significant improvement can be achieved when CO2-EGR was used. The subsidence is only 6.8% of that from the pure depressurization in the case of a pressure drop of 30%. The effects of production pressure drop and production gas rate are investigated. The association between the gas production and the pressure drop of the well is different from the cases of pure depressurization and the CO2-EGR. The appropriate initial time for the CO2 injection is tested. Slighter seabed subsidence is observed when the CO2 injection is initiated earlier. The case of different injection pressure control showed that a lower injection pressure leads to a heavier seabed subsidence. A higher CO2 fraction allowed in the produced gas stream results in a higher cumulative gas production, but there is no significant impact on the seabed subsidence.
Highlights
Gas hydrates are currently recognized as an up-and-coming energy resource
The pure depressurization method was applied to the reservoir to produce natural gas from the free gas zone beneath the hydrate layer
The maximum pressure drop of the horizontal production well is 50% of the initial reservoir pressure, and the maximum gas production rate is 1 MMscm/day for the 1-km-long production well
Summary
Gas hydrates are currently recognized as an up-and-coming energy resource. The natural gas produced from hydrate deposits has the advantages of relatively lower carbon emissions and air pollution [1,2]. Three major kinds of gas hydrate deposits naturally exist that have production potentials based on their geological structures, hydrate distributions, and strategies for production. Class-1 hydrate deposits possess hydrate strata with the accumulation of free natural gas beneath the hydrate bearing layer [4]. The hydrate bearing layer can act as a cap rock for the gas layer. Class-1 hydrate deposits are recognized as the most profitable reservoir type because of the additional gas resources under the hydrate zone. Production of the free gas without complications and extra costs is almost guaranteed [5,6]
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