An Experimental Study of Methane Hydrate Growth and Dissociation in Porous Media

Abstract

Natural gas hydrate (NGH) is a solid phase consisting of water and natural gas. NGH represents a justified vast global energy resource, and is widespread in on-shore artic and sub-seafloor environments. Due to the worlds increasing demand for energy, and pressing environmental issues, methane (CH4) production from NGH represent a favorable option towards more energy security and sustainability. This thesis presents a series of experiments conducted on sandstone core samples. The main focus of the experiments was to gather data from hydrate formation and different gas productions schemes in porous media. The results from the experiment contribute to an in-house database which is used to develop numerical models for hydrate systems. Bentheimer sandstone was used in all the core scale experiments. All the cores were saturated with 3.5wt% NaCl prior to the experiments and had fairly high initial water saturation in the range of 0.57-070. CH4 hydrate formation was conducted in 15 different Bentheimer sandstone cores. The core temperature during formation was in the range of 0-4°C, depending on the experiment. Final hydrate saturations ranged from 0.40-0.64 which is consistent with previous experiments. The impact on growth patterns due to salinity variations was inspected by comparison with previous experiments with 0.1wt% NaCl. Salt lowers the water activity and shifts the hydrate stability line towards lower temperatures. A clear trend showed higher initial growth rate in the low salinity hydrate formations. The salinity of the formation water showed no clear correlation with the final hydrate saturation in the concentration range investigated. Pressure induced dissociation was conducted by 0.7bar pressure steps on 10 Bentheimer cores. There was an expected correlation between core temperature and dissociation pressure. The amount of gas released (2-12ml) on each pressure step was lower than expected and 8-12 pressure steps were needed to produce all the methane. Similar experiments previously conducted reached a full recovery after three pressure steps (0.7 bar each), but these had a lower salt concentration in the formation water (0.1 wt%). The large amount of pressure steps needed for full recovery is believed to be caused by a decrease in salinity as water molecules are released during dissociation, thus shifting the hydrate phase boundary to lower pressures. Even more pressure steps were needed to fully dissociate the sandstone core containing a mixed hydrate of CO2 and CH4. 14 small (0.7bar) and 1 large (11.8bar) pressure steps was executed to fully dissociate the hydrate. This was due to the variation in melting pressure depending on the molar fractions of the two components inhabiting the hydrate. Pure CO2 hydrate and pure CH4 hydrate has an estimated melting pressure of 24.35 and 45.03 bars in bulk at 4°C and 3.5wt% NaCl, respectively. Dissociation was first observed at 42.9 bars in the experiment. Methane production through CO2/CH4 resulted in an estimated CH4 recovery of 0.25 from hydrate. Due to high excess water saturation (0.31) after hydrate formation the N2 was co- injected with CO2 with the ratio 60(N2)/40(CO2) to prevent CO2 from initiating hydrate formation with the excess water. The applied production scheme was a flush-sequence where the fluid mix was injected in several intervals ranging from 2-8.8 hours. A Gas Chromatograph was used together with a mass flow meter to determine to composition and amounts of the produced gas.