Experimental study on the formation and dissociation conditions of methane hydrates in porous media

Abstract

Gas hydrates are crystalline compounds formed when gas and water molecules are combined under low temperature and high pressure conditions. This study experimentally investigates the conditions leading to the formation and dissociation of gas hydrates formed by methane gas and pure water in porous media. Methane gas hydrates were formed in a cell packed with 0.177-mm (0.007 in) diameter single sand (U.S. Sieve Series Designation Mesh No. 80) and 0.420-mm (0.017 in) diameter single sand (U.S. Sieve Series Designation Mesh No. 40), which were saturated with pure methane gas and distilled deionized water. From the plots of pressure and temperature curves for the formation and dissociation of methane hydrates in porous media, the beginning and ending points of hydrate formation as the cell was cooled are investigated. The ending point of hydrate dissociation occurs as the cell is heated, so that the cell pressure increases at the conditions of hydrate dissociation. The initial conditions in this experiment were in the range of 82.4 bars (1,200 psi) to 102.7 bars (1,497 psi) of pressure and in the range of 24.3⁰C (75.7⁰F) to 27.3⁰C (81.1⁰F) of temperature. At the end of hydrate dissociation, the conditions of equilibrium phase was found approximately at a pressure of 88.8 bars (1,294 psi) and temperature of 14.5oC (58.1⁰F) in Runs 1 to 10, at a pressure of 91.8 bars (1,337 psi) and temperature of 17.4⁰C (63.3⁰F) in Runs 11 and 12, at a pressure of 86.5 bars (1,260 psi) and temperature of 17.31oC (63.2⁰F) in Run 13, and at a pressure of 93.2 bars (1,359 psi) and temperature of 15.9⁰C (60.6⁰F) in Runs 14 to 16. Temperature jumping data at the beginning point of hydrate formation and the variation with time of pressure and temperature during hydrate formation and dissociation were recorded. These experimental data may be used to improve predictive thermodynamic models of methane hydrates in porous media. The accurate prediction of methane hydrates in porous media may remove a hazard to drilling from seafloor hydrate slides resulting from the dissociation of gas hydrates. A predictive thermodynamic model would also allow the prediction of the onset of hydrate formation conditions in porous media, and the evaluation of methods to recover methane gas from gas hydrate reservoirs.