The inner sleeve that prevents the direct contact between the mineral oil used as confining fluid and the rock sample is perforated by 16 electrodes coupled to a data acquisition system. Axially, perspex buffer rods electrically isolate the sample from the cell. The inner temperature sensor was placed on the outer sidewall of the sleeve at the sample height, to provide accurate monitoring of the sample temperature.
We followed the method of Waite et al. Ellis, ; Priest et al. Our hydrate formation method and experimental setup represent gas hydrate systems where localized gas reaches the base of the GHSZ. The presence of air affects the saturation calculation, and some gases present in the air, such as CO 2 , can also form hydrate. Hence, the sample was only partially filled with brine, with the remaining pore space available for subsequent methane gas injection e.
The remaining pore space Methane gas was then injected to achieve a pore fluid pressure of Here the pressure drops due both to cooling and to hydrate formation.
The formation of hydrate also generates a slight increase in temperature caused by exothermic hydrate formation Hwang et al. This pressure increase is likely due to the interplay between the cooling of the system and the exothermic effect of hydrate formation. Cooling and the consumption of pore fluids due to hydrate formation both lead to a decrease in pressure. The net effect depends on the balance between the rate of cooling and the rate of hydrate formation.
Hydrate dissociation was initiated by increasing the temperature above the GHSC to room temperature. The initial pore fluid pressure for the third cycle was We tracked the evolution of the saturations of gas, brine, and hydrate from the changes in pore fluid pressure and temperature using the real gas equation the PT method , and independently from electrical resistivity measurements the electrical resistivity tomography [ERT] method.
We calculated continuously the saturations of the three phases gas, brine, and hydrate from the changes in pore fluid pressure and temperature using the real gas equation. This method does not assume that hydrate formation continues until the limiting phase is exhausted e. This allows us to deduce the physical processes that occur throughout the cycle of hydrate formation and dissociation. Our method assumes a closed system and conservation of the molar mass of methane and water in the sample pore space.
Methane can be present in hydrate, dissolved in brine, or as gas. Water can be present as liquid in the pore space brine or in hydrate pure water. The pore volume in the sample and inner volume of the input gas pipe were measured before starting the experiment and were assumed to remain constant throughout the experiment. A change in temperature can also change the sample's pore volume. The pores of the Berea sandstone can be occupied by gas, water, or hydrate.
Hydrate can only form in the pore space of the sample and no hydrate forms in the gas input pipe because it is outside hydrate stability conditions. The nonideal gas law is 1 where p is gas pressure, V is volume, n is the number of moles of methane gas, R is the universal gas constant, and T is temperature. Initially, there was no hydrate in the sample. The sample pore volume V ts was independently measured with a pycnometer, and a known volume of water V w 0 , measured using a syringe pump, was injected into the sample.
In our method, we accounted for the dependency of methane solubility in water, denoted by b , on temperature and salinity Tishchenko et al. The total number of moles of methane in the system n t was therefore 7. Once the temperature decreases below that for hydrate stability, hydrate starts to form from the methane and water in the sample's pore space. This process reduces the sample's gas pressure, generating an inflow of methane gas from the pipe to regain equilibrium of pore fluid pressure.
The net result is an overall decrease in the gas pressure. From this new gas pressure and the pipe and sample temperatures, T p and T s , we can calculate the number of methane moles in each phase. For a gas pressure p 8 9 10 11 12 13 where V h is the volume of hydrate, n h is the number of moles of hydrate, M h is the molecular mass of hydrate, D h is the density of hydrate, and c is the hydration number i.
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The total number of moles of methane in the system remains constant, so n h can be obtained from The hydrate saturation is given by We estimated the saturation of resistive material in the pore space from measured bulk resistivity of the sample. As both hydrate and gas are resistive compared to the conductive brine, it is not possible to obtain the individual saturations of gas and hydrate separately by this approach.
To determine this saturation, the first step is to calculate the saturation of brine. Several approaches can be used to estimate the saturation of brine from measured electrical resistivity e. This interpretation is complicated in the presence of clay minerals as they have charge deficiency. These counter ions can move along the grain water surface under the influence of an external electric field.
A wide variety of formulations have been developed to account for both surface and bulk conduction.
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The earlier models described the effect of surface conduction in terms of the volume of shale, while more recent models attempt to account for the physics of the diffuse ion double layer surrounding clay particles e. B represents the average mobility of the counter ions near the grain surfaces, CEC is the cation exchange capacity, and D o is mineral grain density. The empirical parameters m and n are the cementation coefficient and the saturation exponent, respectively.
Zhan et al.
Glover et al. The experiments of Glover et al. Since our sample has a low clay content of 2. As natural hydrate can be found also in clay rich sediments, appropriate clay conduction models should be used in such studies. For example, Clavier et al. Kan and Sen modeled clays as periodic arrays of charged insulating cylinders or spheres, immersed in symmetrical monovalent electrolyte.
Revil and Glover discussed the theoretical framework of surface conduction predictions. Revil et al. For a more detail review on conductivity models refer to, for example, Glover , de Lima and Sharma , and Mavko et al. The cation exchange capacity of quartz can be neglected due to its large size hence small amount of surface change per unit mass in comparison to clay minerals D.
This method of calculating CEC assumes a mixture of sand and clay and does not account for the various clay morphologies e. We used 0. The value of m was determined by fitting the initial resistivity for the known initial brine saturation, found to be 2. As hydrate forms, the value of m increases Chen et al. Spangenberg modeled the variation of n with water saturation, during hydrate formation.
In our experiment water saturation started from The results of the PT method provide evidence for the coexistence of methane gas, methane hydrate, and brine in our experiments. The maximum relative error in saturation calculated using the PT method is less than 0. At the maximum hydrate saturation, application of small perturbations in the confining pressure could have ruptured hydrate shells, trapping gas, and allowed further hydrate formation Fu et al.
Such perturbations were not applied, so our estimate of coexisting gas may represent an upper bound for our experimental setup. We used a hydration number of 6. In section 1 , we listed several mechanisms allowing three phase coexistence of methane gas with hydrate. Here we discuss some of the mechanisms that are relevant to our experimental study. Methane hydrate may form when methane gas and water are in contact and have conditions favorable for hydrate formation. Our experimental pressure and temperature conditions of 8.
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This calculation does not consider the effect of porous medium properties such as pore size, surface structure, and mineral composition that can all affect the GHSC e. Some experimental results suggest that surface structure and mineral composition may have little effect on GHSC Riestenberg et al. Therefore, we consider only capillary effects. Clennell et al. In small pores, the curvature is high and the excess free energy is also high. In big pores, the curvature is low, and so is the excess free energy. We rearranged equation 8 of Clennell et al.
For our experimental pressure of 8. The pore size varied from Note that only a small part 7.
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Sun and Duan developed a thermodynamic model for the effect of pore size and salinity on hydrate stability. We used their open online calculator models. We note also that it is very unlikely that hydrate formation can reduce the effective pore size to values close to the minimum pore radius because that could only happen when the pore is almost completely occupied by hydrate. When hydrate forms, it can create a physical barrier between methane gas and water that prevents further hydrate formation. Isolated pockets of gas or brine could exist in some pores.
Near seafloor sediments on the southern summit of Hydrate Ridge offshore Oregon, USA contain porous hydrates that likely formed when methane gas bubbles became coated with a hydrate film as they moved upward within the sediments and coalesced together Suess et al.
Hence, gas remains trapped within the hydrate film, even though hydrate stability conditions prevail. We propose that this is the dominant mechanism for coexisting gas in our experimental setup. This mechanism does not involve three phase thermodynamic equilibrium, as the trapped methane gas inside the hydrate shell is not in physical contact with water outside the shell. During drilling in a pockmark offshore Nigeria, Sultan et al.
These authors attributed this flow to rapid influx of gas along fractures in the GHSZ, which leads to rapid hydrate formation, primarily along the inner surface of fractures, leading to isolation of free gas from the surrounding pore fluid. This mechanism is similar to the one that we propose, but at much larger scale. Gas trapped within hydrate films would diffuse out over geological time scales, so our experimental results might not reproduce well hydrate formation in nature. However, in a dynamic natural system with ongoing gas flow into the GHSZ, methane bubbles with a hydrate film enveloping them can also be present.
Further evidence for such gas trapping comes from a laboratory study of methane production by hydrate dissociation by heating that showed an abrupt peak in methane production rate, while the rate of water production remained almost constant Tang et al. A similar abrupt peak in methane production has been observed also using depressurization Xiong et al.
The PT method can differentiate between methane gas and hydrate because gas and hydrate have different volumetric densities section 3.