Jitendra Sangwai

Energy is an essential commodity for the survival and socioeconomic development of the human race. The energy supply sector primarily comprises of industrial, commercial, and domestic applications. The foremost challenges faced by the energy supply sector are growing consumption levels, limited accessibility, environmental concerns, viz-a-viz, climate change, and pollution of water and air resources. As conventional resources of energy have started to decline and are expected to get exhausted by 2040, the main focus has been shifted to unconventional sources [1]. In this category, natural gas resources such as gas hydrate, shale gas, coal bed methane will provide tremendous potential for meeting the demand. Gas hydrates are ice-like crystalline substance formed by a framework of water and natural gas molecules. Recent exploration programs by various agencies such as United States Geological Survey (USGS), National Gas Hydrate Program (India), Japanese Methane Gas Hydrate R&D have proved that massive amount of gas hydrate deposits lying across marine settings and permafrost environments. Hydrate deposits are currently estimated to be 5 × 1015 m3 of methane gas [2]. If this untapped resource of energy becomes feasible for the economic production, it could increase natural gas reserves to multifold. Moreover, this would be considerably greater than the total amount of all fossil fuels together. As reported by USGS, gas hydrates hold more than 50% of the entire world’s carbon. It has been estimated that commercial production of methane from 15% of natural gas hydrate can fulfill the energy requirement of the entire world for next 200 years [3]. Hence, natural gas hydrates are considered to be the vital sustainable energy resource. Many pilot production tests have been completed and are underway to recover methane from gas hydrate deposit across the world [4]. Preliminary studies and pilot tests have shown promising results in terms of methane recovery from natural gas hydrates by employing methods such as thermal stimulation, depressurization, inhibitor injection. Ongoing gas hydrate research programs throughout the world and advances in technology will certainly help to cater any technical challenges in order to potentially harness the huge amount of energy stored in the form of natural gas hydrates.

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.

Microorganisms play an important role in the formation of methane hydrate in subsea environment. Studies involving the effect of biosurfactants produced by microorganisms on methane hydrate formation kinetics are not well understood. The present work investigates the influence of cell free solution containing biosurfactant obtained during the cultivation of microorganisms on the formation kinetics of methane gas hydrate. Two microorganisms, viz., Pseudomonas aeruginosa CPCL and Bacillus subtilis YB7 have been used to produce biosurfactants namely, rhamnolipid and surfactin, respectively. The performance of the cell free solution containing various concentrations (200, 400, 600, 800 and 1000 ppm) of biosurfactant to form the methane gas hydrate was analyzed by adding it into the pure water system and compared with synthetic surfactant, sodium dodecyl sulfate (SDS). It has been observed that the introduction of biosurfactant into pure water system improves the formation kinetics of methane hydrate and reduced the induction time. Addition of 200 ppm of rhamnolipid solution in pure water system has resulted in 47.3% of methane gas to hydrate conversion with an induction time of about 0.23 h, whereas pure water showed 45.1% conversion with an induction time of about 5.77 h. The same concentration of surfactin and SDS have resulted in 42.7 and 33.3% of methane gas to hydrate conversion, respectively. Biosurfactants studied here shows efficient and better performance than their chemical counterpart, namely SDS. This study also provides information on the optimum biosurfactant concentration for the improved formation kinetics of methane hydrate. The results suggest that the utilization of environment friendly biosurfactant can be used as an effective kinetic promoter for the methane hydrate formation suitable for optimum storage and transportation of natural gases.

In this work, the effects of Al2O3, SiO2, Ag and Cu nanoparticles on the kinetics of CO2CH4 hydrate formation process were experimentally studied by measuring the amount of gas consumed and the rate of gas consumption. A suspension of 0.1 wt%, 0.2 wt % and 0.3 wt% of each nanoparticle was injected into the hydrate formation reactor, while pressure and temperature were maintained at 4.0 MPa and 274.15 K, and the magnetic stirrer speed was set at 350 rpm. The CO2CH4 hydrate formation process was studied in both pure water and water containing a 0.1 wt%, 0.2 wt% and 0.3 wt% of each nanoparticle suspension. The results showed that these nanoparticles had a positive effect on hydrate formation. These effects varied from one nanoparticle to another. It was observed that nanoparticles of SiO2 had the most positive effect on CO2 gas consumption, particularly at a concentration of 0.3 wt%. At this concentration the average amount of gas consumed was about 45% higher than that in pure water during the dissolution and 77% during crystallization. Cu and Al2O3 nanoparticles had an intermediate effect with improvement in gas consumption by 1%-15% during dissolution; while it had an important impact on gas consumption during hydrate crystallization with an improvement of 30%-65%. Ag nanoparticles had no significant effect during these two phases.

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.