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Natural gas hydrate deposits in subsea sediments and permafrost have garnered significant interest as an unconventional energy resource and could potentially help in a sustainable energy transition. Among various methane production strategies, gas exchange by CO2or a N2/CO2mixture stands out as a promising method, as it provides hydrate structural stability due to CH4replacement with CO2, leading to simultaneous methane production and carbon dioxide sequestration. Insights into the kinetics of methane replacement using a mixture of N2and CO2(simulated flue gas) are not studied enough in the open literature. Direct flue gas injection (DFI) in hydrate reservoirs can circumvent the costly carbon capture step, thereby reducing overall operational expenses. In this direction, we have investigated the effect of the injected flue gas composition on methane production and CO2sequestration from lab-simulated hydrate-bearing sediments. Initially, methane hydrate formation experiments were carried out at ∼275.15 K in porous media having 35% porosity and 75% water saturation. After the initial methane hydrate saturation phase, the gas exchange reaction (methane production) was carried out by injecting a gas mixture containing N2and CO2with three different N2/CO2molar ratios (3:1, 1:1, and 1:3) and pure CO2by maintaining ∼4 MPa driving force based on the corresponding hydrate equilibrium pressure. It has been observed that N2assisted in CH4replacement from hydrate cages, improving CH4recovery to a certain extent depending on the N2/CO2molar ratio. The maximum CH4recovery is observed for the injected gas mixture with a N2/CO2molar ratio of 1:3. Although pure CO2in the fluid state led to some improvement in CH4recovery (comparable to N2/CO2= 1:1), it is not advisable owing to cost and operability concerns. We anticipate that DFI can provide energy security, enhanced geothermal stability, and CO2sequestration potential in marine basins.

Detailed studies on asphaltenes from Indian sources are scarce. In this study, nC7asphaltenes were extracted from an atmospheric tower bottom and a low waxy crude oil using IP143-based method and characterised using elemental analysis, nuclear magnetic resonance, Fourier-transform infrared spectroscopy, matrix-assisted laser desorption-time of flight mass spectroscopy, X-ray diffraction, and scanning electron microscopy. Results show that both asphaltenes differ widely in structures and molecular weights. It was deduced that both asphaltenes might have formed from kerogen degradation process resulting in higher oxygen content. Lower molecular weight and structural parameters (NMR) of atmospheric tower bottom asphaltene could be due to degradation of aliphatic part during preheating before atmospheric distillation process. Although dispersed, the XRD derived aromaticity of petroleum asphaltenes seems to vary inversely with stacking height and average diameter of the aromatic sheets, possibly due to compact structure of the cluster resulting from decreased stack height. [Received: September 29, 2019; Accepted: November 9, 2019]

Information on the high-pressure rheological properties of methane hydrate sediment slurry is an important precursor to understanding their impact on the mechanical behavior of host sediments during natural gas production from hydrate reservoirs. However, rheological analysis of gas hydrate slurries in the presence of natural sediments containing complex minerals and salts is not yet available in the open literature. The multiphase nature of hydrate system is another challenge when it comes to the high-pressure rheological study. Because the conventional Couette geometry does not ensure proper mixing of solid, liquid, and gas phases during hydrate formation and rheological measurements in a multiphase system, in this work, it has been modified to enhance the mixing capabilities. In this work, the rheological experiments on methane hydrate formation in sediment sample collected from the Krishna-Godavari Basin of offshore India have been performed. Rheological investigations were carried out using three different sediment concentrations of 10, 35, and 50 wt %, each at varying hydrate formation temperatures and pressures of 273.15, 275.15, 283.15, and 285.15 K and 10 MPa, respectively. Viscosity profiles of the methane hydrate sediment slurry containing three sediment concentrations at each temperature were analyzed for varying shear rates during hydrate formation. Also, rheological studies have been performed on methane hydrate sediment slurry during hydrate dissociation using thermal stimulation. Experimental case studies were conducted to understand the effect of varying hydrate, sediment, and water volume content in the slurry on the viscosity profile. The work is an endeavor to explain the underlying causes of the rheological behavior of hydrate slurries in marine sediments.

Supercritical fluids find use in various areas in oil and gas production operations due to their unique properties. Supercritical CO2(sc-CO2) with its reduced viscosity, increased diffusivity, and liquid like density, is attractive for enhanced hydrocarbon recovery, shale gas fracturing and drilling (especially in underbalanced condition). Apart from enhanced hydrocarbon recovery CO2injection is also desirable due to associated CO2storage through various trapping mechanisms. CO2-EOR/EGR and storage processes are affected by coupled geochemical, petrophysical and geomechanical alterations due interaction of sc-CO2with reservoir rock/fluids Along with diffusion/dispersion of sc-CO2in the fluid filled pore spaces. CO2-monitoring combined with lifecycle analysis can help to ensure safety and optimize the CO2-EOR/storage project, considering the environment. However, many technical, financial and policy challenges need to be overcome to realize the success of the CO2-EOR/storage project and achieve carbon neutrality. This review paper discusses such challenges, the current mitigation practice, gaps and future direction of research. This holistic review would help gain insights into the sc-CO2-EOR/storage mechanisms, selection criteria, various limitations, mitigation and optimizations while applying the method for unconventional reservoirs with heterogeneity and varying fluid properties.