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, CH₄ 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 CH₄ 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.

Recent advances on shale oil and gas exploration and development technologies

In this paper, we used a method of combining reservoir stimulation technique (RST) with depressurization to investigate the conversion efficiency of marine natural gas hydrate (NGH) reservoirs in the Shenhu area, on the northern slope of the South China Sea, which differ in intrinsic permeability and initial NGH saturation conditions. We also analyzed the influence of the variable-stimulation effect on marine NGH conversion efficiency in different accumulation conditions, providing a reference scheme for improving the NGH conversion efficiency in the Shenhu area. In this work, we performed calculations for the variations in CH₄ production rate and cumulative volume of CH₄ in different initial NGH saturation, intrinsic permeability, and stimulation effect conditions. Variance analysis and range analysis methods were used to analyze the significance of these key factors and their interaction. Furthermore, we investigated the sensitivity of stimulation effect on NGH conversion efficiency. The simulation results showed that the stimulation effect has a significant influence on NGH conversion efficiency, and the influence of interaction between these three factors was not obvious. Possibly most importantly, we clarified that the sparsely fractured networks (N = 3) had a better effect for higher NGH conversion efficiency under higher saturation conditions. For lower permeability cases, the influence between sparsely (N = 3) and densely (N = 5) fractured networks were similar.

Low-temperature co-current oxidizing pyrolysis, which can achieve high recovery of hydrocarbons without significant oil loss, has great potential to reduce the huge external energy required for oil shale conversion. However, this promising method is far from being fully understood, especially the unknown competing mechanism of different types of inorganic minerals in promoting or inhibiting hydrocarbon generation. In this study, the raw Huadian oil shale (HD-R), its carbonate-free (HD-C-F), and carbonate-silicate-free (HD-CS-F) samples obtained through acid treatment are used to investigate the effects of associated minerals on the oil shale co-current oxidizing pyrolysis. The results of shale oil yields of HD-R, HD-C-F, and HD-CS-F were 41.53, 22.38, and 33.97%, respectively, indicating that silicates inhibited, while carbonates catalyzed the formation of shale oil during the co-current oxidizing pyrolysis. Meanwhile, silicates increase the alkane content and decrease the alkene content in shale oil via promoting the combination of hydrogen radicals and alkyl radicals. On the contrary, alkali metals and alkaline earth metals in carbonates inhibit the binding activity of hydrogen radicals and alkyl radicals, concurrently enhancing the release of hydrogen-free radicals of alkyl radicals to form more alkenes. The removal of carbonates could enhance the conversion of organic carbon into hydrocarbons, and the silicates will strengthen the conversion process. It is hoped that this experiment can further enrich and perfect the basic theory of oil shale pyrolysis and provide a reliable reference for the pretreatment of oil shale conversion.

Pyrolysis behavior and pyrolysate characteristics of Huadian oil shale kerogen catalyzed by nickel-modified montmorillonite