Jitendra Sangwai

Subsea sequestration retains a huge potential in terms of the long-term viability of stable CO2 storage and, therefore, can contribute to global carbon neutrality by addressing global warming challenges. However, macroscopic parameters such as salinity, porosity, sedimentary types, and additives play a vital role in tapping the fullest potential of subsea CO2 sequestration. This aspect offers a wide range of opportunities for discussion and will open new avenues for future development. Therefore, there is a wide scope for discussions in this area, which will lead to new technological innovations in the future. CO2 sequestration in subsea sediments in solid hydrate form is discussed in terms of interaction chemistry and macroscopic environmental effects on pore-scale hydrate formation and growth. This Perspective presents insights related to CO2 hydrate formation and its long-term stability with relevance to porous media, CO2-sedimentary interactions, the effect of additives, and possible cost estimates for large-scale CO2 storage in oceans. Insights into hydrate formation behavior and the effect of physicochemical parameters (interfacial tension, water saturation, organic matter, salinity, and the chemical nature of the sediments) have been additionally outlined. Light is shed on the economics of transportation and injection using cost estimates from the literature along with the challenges and outlook associated with the current technologies. The chemical interactions between CO2 and hydrate-bearing sediments, additives, and marine environments would aid in understanding hydrate formation in subsea sediments.

The release of coal mine methane (CMM) or coal bed methane (CBM) to the atmosphere leads to the wastage of a valuable energy resource and contributes to the greenhouse effect. In the present work, CMM has been assumed as a mixture of methane and nitrogen (CH4-N2, 30:70 mol%), and this gas mixture has been separated by the hydrate-based gas separation (HBGS) process. Formation of sI hydrate with 70% N2 in the mixture requires significantly higher pressure and thus not suitable for scale-up. Thus, in this work with a suitable thermodynamic promoter, sII and sH hydrates were formed to study the methane separation from CMM gas mixture at moderate temperature and pressure conditions. sII hydrate formation was carried out using polar THF (Tetrahydrofuran) and non-polar cyclopentane (CP) at different molar concentrations and at 274.2 K temperature. sH hydrate formation was facilitated using polar tert-butyl-methyl-ether (TBME) and non-polar neo-hexane (NH) at different molar concentrations and at 274.2 K temperature. Further, water-soluble hydrate promoter sodium dodecyl sulfate (SDS) was used at 500 ppm to enhance the rate of hydrate formation and thus to achieve better separation efficiency in a given time. For the CMM gas mixture, sII hydrate formation showed better methane recovery compared to sH hydrate formation, whereas sH hydrate formation showed a better separation factor compared to sII hydrate formation. Hydrate dissociation was also carried out to recover the hydrated gas via depressurization and thermal stimulation to compare the effect of polar and non-polar hydrate formers.

The measurement of the rheological properties of gas hydrate slurries necessitates the high pressure rheometer that can provide a proper mixing inside the pressure cell during hydrate formation from two multiphase fluids, water and gas. However, the hydrate formation is highly challenging in conventional cup and bob geometry due to its plane surface. To overcome this, the present work focuses on the study of high pressure rheology for hydrate slurries formed from water-heptane (C7H16) system using a high pressure cell in Anton-Paar® (MCR-52) rheometer and a modified Couette geometry which enables the measurement of rheological studies of multiphase hydrate system. It was observed that the hydrate slurries exhibit shear thinning behavior. The present study provides an important information about the rheology of methane hydrate slurries formed from multiphase systems for flow assurance applications.

Thermodynamic hydrate inhibitors (THIs) are water-loving additives that in an aqueous phase preferentially interact with water and inhibit hydrate nucleation at given temperature and pressure conditions. Hydrate inhibition is one of the flow assurance challenges in oil and gas facilities, primarily for the production and transmission pipelines. In this era of a shift towards offshore production of oil and gas that is usually accompanied by large water cuts in production pipelines, the use of THIs requires a huge onboard inventory (of these additives); further, associated higher recovery costs and environmental issues has resulted in the identification and use of low dosage hydrate inhibitors (LDHIs). Thus, as the name suggests they are used in lower concentrations and do not present any space/storage problem on the offshore platforms. However, LDHI works differently, it primarily inhibits the growth and agglomeration of hydrate crystals thus ensuring flow assurance for a given period of time. Hence, an effective LDHI, may or may not delay hydrate nucleation, however, it should certainly reduce the kinetics of hydrate growth and should act as an anti-agglomerant. This work comparatively investigates the effect of three such additives on methane gas hydrate formation kinetics at very low concentrations. Sodium Dodecyl Sulphonate (SDS) is a well-studied molecule for gas hydrate formation kinetics, while SDS is a good anti-agglomerant (and has been used as anti-agglomerant in multiple gas hydrate studies), it is also a well-known hydrate promoter. In this work, additives having some similar molecular fragments to that of SDS were tested for better gas hydrate inhibition insights at molecular level. Sodium Benzene Sulphonate (SBS), Sodium Dodecyl Benzene Sulphonate (SDBS) and SDS was screened for methane gas hydrate formation experiments. A comparative kinetic study with these additives at two different concentrations (0.05 M and 0.1 M) has been presented for methane gas hydrate formation at 5 MPa (at the beginning of the experiments) and 274.15 K in n-heptane with 50% water cut, in a stirred tank isothermal reactor. The presence of a benzene ring in the molecular structure of the additive along with varying lipophilic tail and hydrophilic head on the methane gas hydrate formation kinetics are thoroughly investigated.