Production of Sedimentary Methane Hydrates by Depressurization

Abstract

The voluminous amounts of hydrates found in nature are distributed across all continents and pose a huge possibility for future energy harvest. In fact, the total amount of energy stored in hydrates is predicted to be in the same range as conventional fossil fuels combined. Successful exploitation of this energy supply will serve as a vast and relatively clean source of fossil fuel for the remaining of the fossil era. Natural gas hydrates consist of guest molecules encapsulated inside cavities formed by hydrogen-bonded water molecules. The origin of the encaged gas may be biogenic or thermogenic with methane being the most common hydrate former in the earth. Methane hydrates are stabilized at elevated pressures and low temperatures and are mainly located in onshore permafrost regions and in marine sediments offshore.This paper review 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 costeffective 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 function of pressure-decrements below dissociation pressure. A 2D porous silicon wafer micromodel was used to gain insight on the mechanisms of hydrate dissociation. Direct visualization of hydrate melting induced by both depressurization and heating is reported from pores replicating authentic sandstone pores.

Cyclic Depressurization-Driven Enhanced CH4 Recovery after CH4–CO2 Hydrate Swapping

“…3−5 Among the various proposed methods to produce natural gas from hydrate reservoirs, depressurization is suggested as the most economical and feasible approach that can be used on a commercial scale. 6 There are many variations proposed within the depressurization technique to optimize gas production, including constant-rate depressurization, 7 cyclic depressurization (CD), 8,9 slow multistage depressurization, 10−14 and depressurization combined with gas injection. 15,16 To address concerns about geo-mechanical instability due to hydrate dissociation during depressurization, a novel technique has been proposed in which CO 2 or CO 2 -rich gas is injected into a gas hydrate reservoir to produce CH 4 and store CO 2 in a geological formation.…”

Pore-Scale Visualization of CH4 Gas Hydrate Dissociation under Permafrost Conditions

“…37 The thickness of the hydrate film between the gas−liquid interface has previously been estimated to be 10− 20 μm. 29,38,39 The gas bubbles were usually fully consumed in areas with low gas saturation, whereas gas remained coated by hydrate films in areas with high gas saturation and limited availability of water. 29 Hydrate formation in the water phase was also observed when liquid water was saturated with dissolved CH 4 .…”

“…29,38,39 The gas bubbles were usually fully consumed in areas with low gas saturation, whereas gas remained coated by hydrate films in areas with high gas saturation and limited availability of water. 29 Hydrate formation in the water phase was also observed when liquid water was saturated with dissolved CH 4 . 31,40 Crystallization of hydrates in the water phase led to transparent hydrate crystals (HC) that were easily distinguishable from the dark-gray-colored hydrate films because of the difference in the refractive indexes between gas, water, and hydrates (gas hydrate = 1.35, water = 1.33, gas = 1).…”

“…Hydrate dissociation is also accelerated in the presence of mobile free gas due to increased mass transfer and added convective heat flow to the overall heat transfer. 29 The endothermic nature of the dissociation makes the mobility of the gas phase in the pore space an essential factor as immobile free gas acts as a heat insulator compared to water and restricts the heat transfer to the gas hydrate dissociation front, thus leading to the low dissociation rate. Heterogeneous hydrate saturation influences the gas production rate such that gas recovery is slower for high hydrate saturation and total gas produced is dependent on the overall saturation.…”

“…26 Studies have also shown the presence of a metastable hydrate in equilibrium with unfrozen water below 273 K and formation of supercooled water during hydrate dissociation below 273 K. 27,28 Two-dimensional micromodels offer a unique opportunity to perform pore-scale visualization during formation and dissociation of hydrates at high resolution using artificial sedimentary rock. 29 The current micromodels are fabricated to work in a high-pressure environment equipped with actual geological and topographical properties and pore-scale geometry of real rocks. Micromodels provide nondestructive, quick, and low-cost experiments compared to other pore-scale techniques, such as X-ray computer tomography (CT) scanning and magnetic resonance imaging (MRI).…”