Formation and Dissociation Kinetics of Methane Hydrates in Seawater and Silica Sand

Abstract

Understanding the kinetics of gas hydrate formation and dissociation in porous media has become important since their discovery in permafrost locations and marine sediments. Natural gas hydrates are now recognized as a huge potential source of methane gas. The present work is focused on understanding the kinetics of methane hydrate formation and dissociation in pure water and seawater. Methane gas hydrate formation and dissociation kinetics were studied in Toyoura sand (100–500 μm) with pure water at 8 MPa (driving force of 4.2 MPa) and seawater at 8 and 10 MPa (driving force of 6.2 MPa) and a temperature of 277.15 K. For the present work, 3.03 wt % saline seawater obtained from Pulau Tekong (Singapore) is used. The methane hydrate formation kinetics in Toyoura sand and 100% pure water saturation at 277.2 K and 8.0 MPa was found to agree well with the literature works. For experiments conducted at 277.2 K and a driving force of 4.2 MPa, water conversion to hydrate for the experiments conducted with pure water was 72%, whereas for the experiments conducted with seawater, it was only 11.6%. While the role of salts as thermodynamic inhibitors is well-known, our study implies that, in the presence of porous media, the presence of salts significantly affects the kinetics of hydrate formation, resulting in a 6 time reduction in the conversion and also a significant reduction in the rate of hydrate formation. Subsequently, the hydrate samples were dissociated by employing thermal stimulation at a constant pressure of 4.8 MPa. Hydrates were thermally stimulated by two different driving forces (ΔT = 20 and 10) and the dissociation characteristics, and production rates were observed and determined. On the basis of the recovery curves obtained from all of the experiments conducted for water as well as seawater, we observed distinctive dissociation behaviors for the hydrates in seawater and hydrates in pure water.

Review of natural gas hydrates as an energy resource: Prospects and challenges

“…The dissociation kinetics of methane hydrate in seawater/ pure water and porous media have also been studied [104]. The methane hydrate samples were thermally stimulated by changing the water bath temperature to 10°C and 20°C above equilibrium temperature under a constant pressure of 4.8 MPa.…”

“…As gas are released from thermal stimulation which in turn increases the system pressure, the system equilibrium will shift towards a higher temperature. Works performing dissociation at constant pressure have been able to quantify this driving force by considering the temperature difference between the reactor environment and the hydrate equilibrium point [103,104]; alternatively, using heat flux to quantify thermal driving force may be appropriate, as it can shed the light on the exact amount of energy provided via thermal stimulation to the hydrate sample as well as the effectiveness of thermal stimulation as a dissociation method for methane hydrate. 2.…”

“…The techniques of heat delivery include the changing of environment temperature (water/air bath heating) [102][103][104][105], injection of hot fluid into the hydrate sample [101], supplying heat via point sources [107] and combustion of in situ methane [106,108]. While water/air bath heating is a technique that work in the laboratory setting, it will not be feasible for use in the field.…”

“…While it has been well established that NaCl is a thermodynamic inhibitor, recent research found that the salt also inhibits hydrate formation and dissociation kinetically [104]. It is worth investigating in-depth how NaCl at various concentrations affect not only the equilibrium but also the formation and dissociation behaviour of CH 4 hydrate to evaluate its usefulness as an inhibitor or agent to promote dissociation during gas production, since hot brine injection has also been proposed as a means of thermal stimulation.…”

Investigation into optimization condition of thermal stimulation for hydrate dissociation in the sandy reservoir

“…To make the utilization of this resource economically viable, it is imperative to exploit the gas from the hydrate reservoir effectively and safely. The major potential methods for producing gas from hydrates are depressurization [11][12][13], thermal stimulation [14][15][16][17][18], inhibitor stimulation [19][20][21][22], and carbon dioxide replacement [23][24][25]. The depressurization is considered as the most economically feasible and the least energy intensive method.…”