Review ArticleFormation & dissociation of methane gas hydrates in sediments: A critical review
Graphical abstract
Introduction
Present-day civilization is excessively dependent on conventional energy sources (viz., coal, oil, natural gas), while one of the world's largest source of energy lies untapped in the form of Methane Gas Hydrates, MGHs (Makogon, 2010; Boswell and Collett, 2011; Collet et al., 2014; Selvakkumaran and Limmeechokchai, 2015; Van der Hoeven, 2015; Chen et al., 2016). Hydrates of natural gas, consisting methane (CH4), ethane (C2H6) and propane (C3H8), is present beneath the sea-bed along the continental margins and in permafrost regions are known to hold twice the amount of carbon present in all accessible conventional resources combined (Makogon, 1982; Sloan, 1991; Kvenvolden 1993, 1999; Keith and Rogers, 2005; Soga et al., 2006; Copard et al., 2007). This carbon is primarily found in the form of CH4 entrapped in molecular cavities of water as polyhedral lattice, giving it an ‘ice-like structure’, whenever high pressure (P) and low temperature (T) conditions are encountered (Schoell, 1988; Sloan and Koh, 2007; Rajunath et al., 2010). Though, exploration of the naturally occurring MGHs started in the late 1900s, identification of their potential reserves remains a challenging task owing to the (i) inaccessibility of the site, (ii) sensitivity of MGHs towards change in P-T conditions, (iii) risk of geohazards viz., seabed subsidence, uncontrolled release of CH4 gas into the environment, drilling complexities and well-bore instability (Koh and Sloan, 2007; Koh et al., 2012). Besides, the research pertaining to the determination of the quantity of MGHs, Sh, in HBS has revealed that the parameters like stiffness and electrical resistivity of HBS could be utilised for identification of the potential MGHs reserves (Stoll and Bryan, 1979; Judge, 1982; Pearson et al. 1983, 1986; Klein and Santamarina, 2003; Shankar and Riedel, 2014).
Furthermore, extraction of methane from HBS is yet to be fully commercialized, due to the aforementioned geohazards, expected to happen during CH4 gas production (Koh and Sloan, 2007; Pant, 2010; Tan et al., 2016; Motghare and Musale, 2017). Based on the studies conducted by Nixon and Grozic (2007), Long et al. (2009), Yuan et al. (2011), Ning et al. (2012), Goto et al. (2016), it has been reported that dissociation of MGHs, during extraction process, reduces the geomechanical (read shear) strength of the HBS, which might trigger subsidence of the seabed that in turn would damage the installed paraphernalia and the environment (Gabitto and Tsouris, 2010; Moridis et al., 2011; Collet et al., 2014; Dangayach et al., 2015). Studies carried out to address these issues suggest that the Sh (ratio of volume of hydrates to that of the voids) and type of MGHs habitat formed (viz., pore-filling, load-bearing and cementing) play a major role in governing the shear strength, permeability and compressibility of the HBS (Masui et al., 2005; Priest et al., 2009; Waite et al., 2009; Clayton et al., 2010; Yun and Santamarina, 2011; Uchida et al., 2012; Bu et al., 2017). In addition to geomechanical instability, dissociation characteristics viz., rate (Rd) and P-T conditions for dissociation are function of sediments and their matrix characteristics (Uchida et al. 1999, 2002; Smith et al., 2002; Babu et al., 2013; Mekala et al., 2014; Chong et al., 2015; Saw et al., 2015). Hence, laboratory synthesis of MGHs, for attaining a certain Sh along with the habitat of MGHs and to understand the dissociation characteristics becomes a much desirable exercise that would be a pre-cursor for initiating exploration activities for extraction of CH4 gas from the HBS.
In this context, studies were conducted by Hammerschmidt (1934), Englezos et al. (1987a), Joseph et al. (2017) who have synthesized MGHs in bulk-water, under controlled thermodynamic conditions. The basic intention of such an exercise was to (i) enhance storage of natural gas in tanks by converting it into hydrates (Nogami and Watanabe, 2008) and (ii) prevent hydrates formation in the gas pipelines, (Gudmundsson et al., 1996). However, these applications are limited to the synthesis of MGHs in bulk-water and it has been realized that there is a marked difference in synthesizing MGHs in the bulk-water and in the sediments matrix, primarily due to ‘inter-molecular interaction’ between the sediments and the pore-fluids (pore-water and hydrate forming gases), as reported by earlier researchers (Everett, 1961; Handa and Stupin, 1992; Adamson and Gast, 1997; Liu and Flemings, 2011).
Recent studies on MGHs have revealed that their formation (represented by Sh and rate of formation, Rf) in sediments depends on the dp, d, SSA, Sw, S, mineralogy, along with thermodynamic conditions (Blackwell, 1998; Uchida et al., 2002; Katsuki et al., 2006; Lu et al., 2011; Zang et al., 2013) and their dissociation (represented by Rd) depends on η, d, S (Kono et al., 2002; Babu et al., 2013; Chong et al. 2015, 2016b). However, these studies are site-specific and hence to comprehend the effect of the aforementioned governing parameters on formation and dissociation processes, parametric study becomes of utmost important, if not critical.
In order to understand the effect of parameters associated with sediment and their matrix characteristics on formation and dissociation (Sh, Rf and Rd), existing literature has been critically synthesized and an attempt has been made to demonstrate the mechanisms governing variation of Sh, Rf and Rd. The analysis, relationship and concept proposed in this manuscript prima facie will aid in estimating Sh and understanding the mechanisms, governing its dependency on sediments and their matrix characteristics.
Section snippets
Formation of MGHs
Formation, as well as dissociation of naturally occurring MGHs, is governed by heat & mass transfer through the sediments and the thermodynamic parameter, P-T conditions (Gabitto and Tsouris, 2010; Moridis et al., 2011; Gao et al., 2018). Heat and mass transfer in naturally occurring MGHs depends on the sediments and their matrix characteristics and thermodynamic conditions are a function of depth of the HBS and geothermal gradient (Misyura, 2016; Merey and Longinos, 2018). Furthermore,
Concluding remarks
Based on a critical synthesis of the existing literature, it has been realized that in order to simulate natural MGHs habitats in laboratory, aside pressure-temperature (thermodynamic) conditions, sediments and their matrix characteristics, and pore-solution viz., mineralogy, pore diameter, sediment diameter/mean diameter and specific surface area, density, initial water saturation, salinity, mineralogy and volume of sample are important. Furthermore, the influence of pore-diameter and salinity
Acknowledgements
The first and second authors would like to acknowledge grant of fellowship received from Gas Hydrate Research & Technology Centre (GHRTC), Oil and Natural Gas Corporation (ONGC) of India under PAN IIT-ONGC collaboration.
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