Determination of methane concentrations in water in equilibrium with sI methane hydrate in the absence of a vapor phase by in situ Raman spectroscopy

Abstract

Most submarine gas hydrates are located within the two-phase equilibrium region of hydrate and interstitial water with pressures ( P ) ranging from 8 to 60 MPa and temperatures ( T ) from 275 to 293 K. However, current measurements of solubilities of methane in equilibrium with hydrate in the absence of a vapor phase are limited below 20 MPa and 283.15 K, and the differences among these data are up to 30%. When these data were extrapolated to other P–T conditions, it leads to large and poorly known uncertainties. In this study, in situ Raman spectroscopy was used to measure methane concentrations in pure water in equilibrium with sI (structure one) methane hydrate, in the absence of a vapor phase, at temperatures from 276.6 to 294.6 (±0.3) K and pressures at 10, 20, 30 and 40 (±0.4%) MPa. The relationship among concentration of methane in water in equilibrium with hydrate, in mole fraction [ X (CH 4 )], the temperature in K, and pressure in MPa was derived as: X (CH 4 ) = exp [11.0464 + 0.023267 P − (4886.0 + 8.0158 P )/ T ]. Both the standard enthalpy and entropy of hydrate dissolution at the studied T–P conditions increase slightly with increasing pressure, ranging from 41.29 to 43.29 kJ/mol and from 0.1272 to 0.1330 kJ/K · mol, respectively. When compared with traditional sampling and analytical methods, the advantages of our method include: (1) the use of in situ Raman signals for methane concentration measurements eliminates possible uncertainty caused by sampling and ex situ analysis, (2) it is simple and efficient, and (3) high-pressure data can be obtained safely. 1 Introduction Most submarine gas hydrates are crystalline compounds of water and methane coexisting with seawater at the seafloor or with formation water in the sediment column. Seafloor pressure ( P ) and temperature ( T ) conditions for the coexistence of hydrate and water are commonly at higher pressures and lower temperatures than the three-phase (Lw–H–V) univariant coexistance curve for pure methane hydrate (H), aqueous solution of seawater salinity (Lw), and pure methane vapor phase (V), as shown in Fig. 1 for the most of DSDP (Deep Sea Drilling Project) and ODP (Ocean Drilling Program) samples ( Booth et al., 1996 ). Two-phase divariant coexistence governs hydrate dissolution in methane under-saturated water, with the concentration of methane in water playing a key role in providing the necessary fugacity gradient along with variations in temperature and pressure to drive dissolution or precipitation of hydrate. Thus, the knowledge of gas solubility in the presence of hydrate is essential for: (1) studying the formation and accumulation of submarine hydrates ( Rempel and Buffett, 1997 ); (2) determining the dissolution rates of hydrates in under-saturated seawater ( Zhang and Xu, 2003; Rehder et al., 2004 ); and (3) predicting the occurrence, distribution and evolution of methane gas hydrate in porous marine sediments ( Xu and Ruppel, 1999 ). Previous measurements of gas solubility in the presence of hydrate (e.g., Yang et al., 2001; Servio and Englezos, 2002; Kim et al., 2003 ) are mostly by traditional sampling and analytical methods. Commonly, the amount of fluid extracted for analysis and the mole fraction of methane were determined on the basis of a P – V – T – x calculation, or analyzed at one atmosphere with gas chromatography. Such ex situ methods have some limitations: (1) sampling and ex situ analysis may disturb one or more of the P – V – T – x properties of original solution in the experimental system such that the acquired concentration of methane in the fluid may not accurately represent the original composition, (2) the experimental measurements are rather complex and relatively time consuming, and (3) the experimental working pressures are limited by safety concerns, such that at present, no data are available above 20 MPa conditions, equivalent to ocean water depths of less than 2 km ( Fig. 1 ). In an effort to extend our knowledge of methane–hydrate stability to conditions in the deep ocean and within the sediment column we measured the concentrations of methane in water in equilibrium with hydrate at pressures equivalent to water depths of about 4 km and temperatures up to 294.6 K. Methane hydrates were synthesized in a high-pressure capillary optical cell ( Chou et al., 2005 ), and Raman spectroscopy was used for in situ measurements of dissolved methane in the aqueous solution adjacent to hydrate crystals under conditions of two-phase equilibrium at temperatures from 276.6 to 294.6 K and pressures at 10, 20, 30 and 40 MPa. This in situ method verified that the concentration of methane in water in equilibrium with hydrate decreases with decreasing temperature and increasing pressure. The pressure effect appears to increase with increasing temperature at a rate higher than those predicted by current theoretical models. 2 Methods 2.1 Experimental apparatus and procedures A capillary high-pressure optical cell (HPOC; Chou et al., 2005 ), in combination with the USGS heating–cooling stage ( Werre et al., 1980 ), was used for Raman spectroscopic study of the methane–water system at various pressures and temperatures. Fig. 2 shows the schematic diagram of the experimental system. The HPOC was constructed from a square cross-section flexible fused silica capillary tube (300 μm × 300 μm with 50 μm × 50 μm cavity and about 25-cm long). One end of the tube was sealed by using a hydrogen flame and inserted into the sample chamber of the USGS heating–cooling stage. The enclosed end of the tube was located near the center of the window (∼2 cm OD) of the heating–cooling stage, where the temperature could be controlled and maintained by a stream of nitrogen gas, and read by a K-type thermocouple with an accuracy of ±0.1 K. The other end was epoxied...