Formation & dissociation of methane gas hydrates in sediments: A critical review

Highlights

• Formation and dissociation of methane gas hydrates in sediments has been critically reviewed.

• Parameters associated with sediments and matrix characteristics have been identified and qualitatively analysed.

• Importance of driving force on formation of methane gas hydrates is highlighted

• Elaborate discussions on mechanisms governing methane hydrates formation in sediments is presented.

Abstract

Methane Gas Hydrates, MGHs, are perceived as a future source of energy for the sustenance of mankind owing to their abundance, usually found in continental margins and permafrost regions. However, identification of potential MGHs reserves, characterized by their high saturation in sediment pores, S_h, establishing their spatial distribution in hydrate bearing sediemnts, HBS, and their extraction, is extremely challenging. This calls for a proper understanding of the mechanisms and factors, associated with sediments and their matrix characteristics, viz., the pore-diameter, d_p, particle diameter, d, specific surface area, SSA, initial water saturation, S_w, salinity of the pore-water, S, mineralogy, porosity, η, and density, ρ and thermodynamic conditions, affecting the formation and dissociation of MGHs in sediments. In this context, several laboratory investigations have been conducted to mimic the natural process of MGHs formation, with an intention to gain an insight into (i) mechanisms involved in formations, (ii) response of HBS during dissociation, and (iii) CH₄ gas recovery rate under prevailing thermodynamic conditions. However, such studies have, usually, focused on the simulation of ‘specific reservoir conditions’ (viz., P-T, S, d) and hence acquiring a comprehensive picture about the (i) MGHs formation and dissociation process, and (ii) influence of above-mentioned parameters on these processes, becomes necessary. With this in view, a critical review of the literature pertaining to the synthesis of MGHs has been conducted and it has been demonstrated that in addition to thermodynamic conditions, d, d_p, S_w, S, SSA and mineralogy of the sediments have a significant influence on the formation as well as dissociation process. In this context, the discussion, analysis and relationship proposed in this manuscript prima facie will aid in estimating S_h and understanding the mechanisms governing its dependency on sediments and their matrix characteristics.

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, S_h, 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 S_h (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 (R_d) 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 S_h 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 S_h and rate of formation, R_f) in sediments depends on the d_p, d, SSA, S_w, 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 R_d) 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 (S_h, R_f and R_d), existing literature has been critically synthesized and an attempt has been made to demonstrate the mechanisms governing variation of S_h, R_f and R_d. The analysis, relationship and concept proposed in this manuscript prima facie will aid in estimating S_h and understanding the mechanisms, governing its dependency on sediments and their matrix characteristics.