Experimental Investigation of Hydrate Formation and Dissociation in Consolidated Porous Media

Abstract

Summary This paper describes the first uniform formation of natural gas hydrates in Berea sandstone cores by use of a flow system. The hydrate formation was monitored by pressure drop and changes in electric resistance along three cores of three different permeabilities. Formation procedures, such as annealing to ensure uniformity, and the effects of surfaces on equilibria are discussed. Hydrates were dissociated through depressurization, as is done for gas recovery from the Messoyakha hydrate reservoir in the USSR. During the dissociation, two types of rate behavior were observed as functions of the displacement of the dissociation pressure from the equilibrium value. Conclusions and implications for the production of natural gases from hydrate deposits are discussed. Introduction Natural gas hydrates are inclusion compounds in which certain-molecular-weight gases stabilize the cages formed by hydrogen-bonded water molecules under favorable conditions of pressure and temperature. X-ray diffraction studies1 have shown that small molecules of nonpolar gases can stabilize either of two specific structures called Structures I and II. Fig. 1 shows a unit cell of each structure. A unit cell of Structure I contains eight cavities (two small and six large) and is formed by 46 hydrogen-bonded water molecules, while a unit cell of Structure II contains 136 water molecules and encloses 24 cavities (16 small and 8 large). At maximum occupancy (each cavity is filled with one gas guest molecule), 1 mol of hydrate of either structure yields about 0.15 mol gas and 0.85 mol water. The molecules of such gases as methane, ethane, propane, isobutene, CO2, H2S, and N2 are known to stabilize the microcavities formed by either of the two hydrate structures. The formation of either Structure I Structure II is related to the ratio of the guest molecule size to the cavity size and to the thermodynamic conditions of temperature, pressure, and gas composition. A more comprehensive exposition of hydrates and their structures may be found elsewhere.2 Gas hydrates were considered nuisances for the gas industry since Hammerschmidt's3 discovery that hydrate formation causes plugging of gas pipelines. Following discoveries of substantial gas hydrate deposits in the USSR4 and in the Canadian permafrost,5 however, gas hydrates began to receive attention as a potential energy resource. Bearing in mind that 1 m3 of PV filled with hydrate can yield a maximum of 184 std m3 of gas, substantial amounts of gas could be produced from these hydrate deposits. Makogon6 and Kvenvolden7 independently estimated the amount of hydrated gas to be two orders of magnitude greater than the current proved natural gas reserves in the world. The only reported production of a gas hydrate field is that from the Messoyakha field6 in the USSR. Verma et al.8 found that gas hydrates can also form from water/liquid-hydrocarbon mixtures. It is believed9 that hydrate formation has caused some North Slope oil deposits to increase in gravity and viscosity by stripping the light components (CH4 to C4H10) of these oils in a process called denuding. Several studies10–14 have been made to simulate natural gas hydrate formation in nature and to investigate possible schemes to produce these deposits. Previous laboratory attempts,15–17 however, have shown little success at quantifying uniform hydrate formation and dissociation in consolidated porous media. The objectives of this work were to form methane gas hydrates in Berea sandstone core samples and to measure the dissociation rate of such hydrates at constant pressure and temperature in an attempt to simulate gas hydrate production by a slow depressurization process. Experimental Apparatus and Procedure Fig. 2 is a diagram of the apparatus used for hydrate formation and dissociation in Berea sandstone core samples. The core was enclosed with heat-shrink plastic tubing and contained within a stainless-steel pressure sample bomb. An external pressure, which was a minimum of 1 MPa greater than that within the core, was maintained on the outside of the heat-shrink tubing with a manual hydraulic pump. The electric resistance was measured with four pairs of electrodes implanted at equal distances along the core length under the heat-shrink tubing, as shown in Fig. 3. In addition to pressure drop, measurement of the electric resistances provided another method to check for both the amount and uniformity of hydrate formation. Data acquisition was accomplished with a Kiethley-500™ series system connected to an IBM-XT™ personal computer. Ref. 18 gives a more detailed description of the apparatus. Experimental Procedure. The core sample was initially evacuated and saturated with 1.5 wt% NaCl solution. Then, several PV's of brine solution were circulated under high pressure through the core to ensure full saturation and stability. Later, gas injection began at the experimental pressure (7 to 8 MPa) and temperature (273.7 K). Gas injection continued until the desired water and gas saturations were established in the core. The produced water and gas volumes were closely monitored during this step. The outlet valve was then closed, and gas injection was maintained during hydrate formation; when no more gas uptake was possible, the inlet valve was closed. At this stage, the bath temperature was maintained at 273.7 K to allow hydrate formation to continue at a pressure always in excess of the equilibrium value for a period of 4 to 34 hours. When no change in the pressure and electric resistance with time was observed, the hydrate formation was assumed to have ceased. In many instances, especially with the lower-permeability cores, a pressure drop of up to 2.8 MPa across the core was detected after the hydrate was formed. Then, one or more cycles of an annealing process were performed to eliminate this pressure drop and to ensure more uniform hydrate distribution along the core. In the annealing process, hydrate was dissociated and reformed by cycles of heating (to 279.8 K) and cooling (to 273.7 K) the core. After hydrate formation, dissociation began at constant outlet pressure and bath temperature. The rate of gas produced during this step was measured by water displacement into a graduated cylinder over 1-minute intervals. At the end of the dissociation process, the amount of water produced during this step was determined. Experimental Procedure. The core sample was initially evacuated and saturated with 1.5 wt% NaCl solution. Then, several PV's of brine solution were circulated under high pressure through the core to ensure full saturation and stability. Later, gas injection began at the experimental pressure (7 to 8 MPa) and temperature (273.7 K). Gas injection continued until the desired water and gas saturations were established in the core. The produced water and gas volumes were closely monitored during this step. The outlet valve was then closed, and gas injection was maintained during hydrate formation; when no more gas uptake was possible, the inlet valve was closed. At this stage, the bath temperature was maintained at 273.7 K to allow hydrate formation to continue at a pressure always in excess of the equilibrium value for a period of 4 to 34 hours. When no change in the pressure and electric resistance with time was observed, the hydrate formation was assumed to have ceased. In many instances, especially with the lower-permeability cores, a pressure drop of up to 2.8 MPa across the core was detected after the hydrate was formed. Then, one or more cycles of an annealing process were performed to eliminate this pressure drop and to ensure more uniform hydrate distribution along the core. In the annealing process, hydrate was dissociated and reformed by cycles of heating (to 279.8 K) and cooling (to 273.7 K) the core. After hydrate formation, dissociation began at constant outlet pressure and bath temperature. The rate of gas produced during this step was measured by water displacement into a graduated cylinder over 1-minute intervals. At the end of the dissociation process, the amount of water produced during this step was determined.