Simulation Study on the Effect of Fracturing Technology on the Production Efficiency of Natural Gas Hydrate

Abstract

Natural gas hydrate (NGH) concentrations hold large reserves of relatively pure unconventional natural gases, consisting mainly of methane. Depressurization is emerging as the optimum conversion technology for converting NGH in its reservoir to its constituent water and natural gas. NGH concentrations commonly have a pore fill of over 80%, which means that NGH is a low-permeability reservoir, as NGH has displaced water in terms of porosity. Fracturing technology (fracking) is a technology employed for increasing permeability-dependent production, and has been proven in conventional and tight oil and gas reservoirs. In this work, we carried out numerical simulations to investigate the effects on depressurization efficiency of a variably-fractured NGH reservoir, to make a first order assessment of fracking efficiency. We performed calculations for the variations in original NGH saturation, pressure distribution, CH 4 gas production rate, and cumulative production under different fracturing conditions. Our results show that the rate of the pressure drop within the NGH-saturated host strata increases with increased fracturing. The CH 4 gas production rate and cumulative production are greatly improved with fracturing. Crack quantity and spacing per volume have a significant effect on the improvement of NGH conversion efficiencies. Possibly most important, we identified an optimum fracking value beyond which further fracking is not required.

Influence of Reservoir Stimulation on Marine Gas Hydrate Conversion Efficiency in Different Accumulation Conditions

“…The problem of how to efficiently and economically exploit natural gas hydrates in low-permeability marine sediment reservoirs is an important issue. Being able to exploit NGHs from marine sediments, especially in low-permeability silt and clay sediments with a less producible capacity, will greatly expand the potential use of NGH as a gas resource [23].…”

“…In an ideal case, a hydrate deposit probably had a sufficiently brittle response to fracturing; our model would provide a base case with which actual testing can be compared in order to assess the likelihood of artificial fracturing when inducing additional permeability in semi-consolidated marine sediments, which, without NGH, would be expected to respond in a more mechanically-ductile manner [23].…”

“…RST has expanded to include low-permeability oil and gas shale, as well as tight sand reservoirs [28]. The depressurization method and RST combine to exploit NGHs, which will increase the transmission rate of depressurization within NGH deposits and promote NGH decomposition in fractured zones, in addition to being conducive to the discharge of methane [23,[29][30][31].…”

“…Su et al [22] used depressurization and thermal stimulation method to analyze NGH conversion efficiency from the SH2 site. Chen et al [23] used this model to investigate the effect of fracturing technology on the production efficiency of NGHs by depressurization from the SH7 site.…”

“…Clay and clastic limestone and secondary permeability related to structure also host NGH in some areas of the South China Sea [18]. The NGH-enriched reservoirs in the Shenhu area have poor permeability, which hinders the transfer of heat and pressure between the production wells and the reservoir, and reduces the conversion efficiency of the continuous dissociation of NGH [14,22,23]. The problem of how to efficiently and economically exploit natural gas hydrates in low-permeability marine sediment reservoirs is an important issue.…”