Natural gas hydrates are increasingly encountered in offshore projects as they move into ever deeper water. Occurring from the seabed until several hundred meters in depth, hydrates are metastable, and dissociate back to gas and water if their high pressure- and low temperature stability conditions are affected by any kind of intervention. Hydrate dissociation weakens the seabed severely, and therefore, it is important to know accurately at which local pressure and temperature condition it is taking place. Experimental data on the phase equilibrium of seabed hydrates in the presence of porous media are at present limited to 11 MPa. This study presents a novel method of determining the hydrate phase boundary in porous media by controlled dissociation. Experimental phase boundary data of freshwater and 3.03 wt % NaCl hydrates up to 17 MPa pressure have been determined. The freshwater and seawater hydrate phase boundaries in porous media fit a set of simple empirical equations.

The combination of energy source and geohazardous potential of natural gas hydrate has raised the need to understand the processes related to hydrate dissociation within the sediment. In this article, several existing methane hydrate dissociation apparatuses are listed and their sample size capabilities given. A new design for line dissociation tests by combined electrical heating and pressure reduction from a miniature wellbore is presented. The 180-mm-diameter × 225-mm-length hydrate-bearing soil samples can be tested over a wide range of near in situ conditions, with pore pressures of up to 15 MPa, temperatures as low as −5°C and a maximum effective stress of 5.5 MPa. During testing, the pore pressure, local temperature changes, vertical strain, the extracted gas and water volumes, and radial density changes are measured. The devices used are shown and described in detail. The sample formation steps by the water excess method are described, and the dissociation behaviour of a typical 40 % hydrate-saturated sand sample during local heating and pressure reduction are illustrated.

In a previous study using a single wellbore production system, it was demonstrated that a combination of depressurization and wellbore heating is more efficient than depressurization alone, where the endothermic dissociation process rapidly consumes the specific heat of the formation, leading to a sharp decrease in the dissociation rate. This study extends the work on gas production and explores the feasibility of a novel dual wellbore production scheme, where heating and depressurization are conducted on separate wellbores. The drawback with combining heating and depressurization on a single wellbore is that the produced fluids are flowing in an opposite direction to the heat from the wellbore, and this forced convection may slow the dissociation process. Gas production tests are carried out using the dual wellbore system with different combinations of pressure and temperature at the depressurization and heating wellbores, respectively. The ensuing experimental results showed that both increased depressurization and heating can lead to optimized gas production. A production scheme with a higher depressurization compared to a lower depressurization at the same wellbore heating is generally more energy-efficient, while a higher wellbore temperature at the same depressurization resulted in more gas produced but no improvement in efficiency. Although a dual wellbore scheme has been an established practice in the petroleum industry, this is likely to be the first employed in the hydrate gas production tests.

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