Multiscale Laboratory Verification of Depressurization for Production of Sedimentary Methane Hydrates

Abstract

Summary This study reviews how production of methane from hydrates can be triggered by dissociation of the hydrate structure. Techniques leading to dissociation of hydrates are summarized by pressure depletion, thermal stimulation, and injection of inhibitors. Depressurization is considered to be the most-cost-effective method and is easily implemented in gas reservoirs with overlying hydrate layers. Examples and status of pressure-depletion tests on field scale will be reviewed. In hydrate reservoirs not adjacent to gas zones, the success of pressure depletion is dependent on sufficient permeability to allow for pressure perturbations to reach within the hydrate reservoir and to allow for flow of dissociated gas. This effect has been investigated in this paper by performing controlled pressure depletions on hydrate-filled sandstone cores. Dissociation pressures at given temperatures have been quantified as well as recovery of methane as a function of pressure decrements lower than dissociation pressure. Hydrate dissociation was found to take place over a range of pressure values because of salinity changes in the water phase. A 2D porous silicon-wafer micromodel has been used to gain insight into the mechanisms of hydrate dissociation. Direct visualization of hydrate melting induced by both depressurization and heating is reported from pores replicating authentic sandstone pores. Thermal stimulation led to a more-uniform hydrate melting compared with pressure depletion, and depressurization was most effective when the hydrate was in direct contact with gas bubbles.

Salinity Effects on Pore‐Scale Methane Gas Hydrate Dissociation

“…This suggests that the multiple dissociation pressures needed to completely dissociate the methane gas hydrate in Figure during depressurization, where methane hydrate surrounded by water dissociated at a higher pressure compared with methane hydrate surrounded by methane gas, are due to pore water freshening and not local endothermic effects. In fact, previous methane gas hydrate depressurization experiments with distilled water show that the rate of dissociation is higher when hydrate is surrounded by gas because of increased mobility of the liberated gas (Almenningen et al, ). In addition, the observed dissociation pattern is influenced by surface roughness on the pore floor, where discrete brine droplets resided below the methane gas phase after drainage (Figure ).…”

“…Two methane gas hydrate growth patterns were observed: shell‐like growth along the gas‐water interface resulting in a porous hydrate with encapsulated methane gas in a shell of methane gas hydrate (column a, Figure ) or crystalline growth where all the free methane gas was consumed and the pore was filled with solid nonporous methane gas hydrate (column b, Figure ). The resulting hydrate configuration (porous or nonporous) was inferred from color analysis of the hydrate phase (Almenningen et al, ). The former shell‐type growth was most frequently observed, with methane gas inside the methane gas hydrate shell, presumably because of lack of water transport across the hydrate shell and/or insufficient pressure in the gas phase to maintain further growth (Almenningen et al, ; Peng et al, ).…”

“…The water‐wet nature of grains induced a curved interface between water and gas (Figure ) and made it possible to differentiate between fluid phases. Segmented images were used to calculate two‐dimensional fluid saturations, but pixel counting does not necessarily reflect the true volumetric saturation in the pore space because multiple phases can coexist in the same location in the vertical depth of 25 μm (Almenningen et al, ). Pixel counting assumes that each pixel is saturated with only one fluid phase and is an approximation to the volumetric saturation.…”

Magnetic Resonance Imaging of Methane Hydrate Formation and Dissociation in Sandstone with Dual Water Saturation

“…The relationship between signal intensity and water saturation during hydrate growth was thus investigated, and is shown for both experiments in Figure 3. The average water saturation during hydrate growth was calculated based on the amount of consumed methane gas at a constant pressure [17]. A hydration number of 5.99 was used [18].…”

Enhanced CH4-CO2 Hydrate Swapping in the Presence of Low Dosage Methanol

“…These deposits remain untouched and hold a reserve estimated to be twice the amount of known fossil fuels available [3][4][5]. To produce methane from a gas hydrate reservoir, several techniques have been suggested, including depressurization [6], thermal stimulation [7] and chemical inhibitor injection [8]. In comparison, depressurization is considered as the most cost-effective method to be commercially applied.…”