PHYSICAL GEOCHEMICAL ASPECTS OF NATURAL GAS HYDRATE AND IMPLICATIONS FOR DISSOCIATION AND EXPLOITATION

Michael D. Max, Sarah A. Holman, John P. Osegovic, Audra L. Ames, and Shelli R. Tatro
Marine Desalination Systems, LLC, 1601 3rd St. South, St. Petersburg, FL, 33701, USA

In addition to the basic components of conventional natural gas prospects, which include reservoir lithologies with appropriate porosity and permeability, appropriate sealed traps, access to drilling and extraction, and the presence of a large enough volume of the resource in a concentrated enough form to allow for commercial extraction, gas hydrate deposits have some unique features. Principal among these is that hydrate deposits comprise both trap and reservoir, which may be associated with additional deposits of trapped free gas. Because marine hydrate generally occurs in unlithified, geologically weak marine sediments between about 150 meters and 750 meters below seafloor, rather than in geologically strong traps already containing conventional gas and petroleum deposits at elevated pressures and temperatures, special consideration must be given to the changing nature and strength of the trap and reservoir relationships as recovery of gas proceeds and parts of the trap itself are produced. In deposits where hydrate forms a diagenetic cement that may strengthen the sediment matrix, containment of the free gas produced through dissociation will lead to weakening of the trap and sediment matrix. In addition, production of gas from hydrate results in a decrease in overall matrix volume because water constitutes only 80% of the volume of hydrate, which may lead to sediment collapse, while overly high gas overpressures may lead to blowout. Therefore, a balance between maintenance of gas pressures and back flooding may be necessary to prevent breaching of the dissociation areas. Understanding growth and dissociation mechanisms of solid hydrate masses, which occur in ‘rich’ deposits having high pore fills of hydrate is key to commercial extraction of natural gas from hydrate in a safe manner.

Growth of solid hydrate requires a surface of crystalline hydrate to be in contact with water. The hydrate must grow outward, into the water space, drawing on dissolved hydrate-forming gas (HFG) reactants. The difference in chemical potential between the aqueous solution and the hydrate crystal, where the presence of solid hydrate influences the apparent solubility of the HFG in the surrounding water, is the fundamental driving mechanism for hydrate growth under these conditions. The interfacial water thus has a high potential to provide the HFG to accelerate growth of additional hydrate. The process of accelerated hydrate growth from gas-saturated water will continue so long as there is an abundant supply of gas saturated water, although the interface water gradient zone may broaden so that diffusion of HFG across the zone increases as a function of time. The concentration gradient that forms in the presence of hydrate is a powerful driving force that is not present at a water-HFG interface.

Hydrate growth experiments demonstrate that where free gas and seawater are brought together, hydrate forms only at their immedate boundary which creates only thin hydrate shelled bubbles. With further hydrate growth, which may cause aggregation, depending almost entirely on slow solid diffusion mechanisms. Gas moving through the gas hydrate stability zone (HSZ) in a hydrate-armored channel will not allow time for further hydrate growth, and much gas may reach the sea floor and escape into the sea. Formation of large amounts of solid hydrate appears to be best accomplished, not where free gas and water are brought together, but where dissolved HFG is brought into the vicinity of hydrate, which can then grow into the water space. The presence of free gas, therefore, may be less important than the nature of the local groundwater system that can transport dissolved HFG. Where groundwater can percolate through sediments, they provide an excellent means for distributing dissolved HFG reactants. Experiments have shown that the growth of solid hydrate, and consequently the development of high hydrate pore fills, takes place only where the HFG is dissolved in water at levels near saturation. Thus, a hydraulic system composed of pore water exposed to gas under conditions where the gas can be dissolved in water to near saturation-level conditions without forming hydrate and connected through pore space with sediment regions in which hydrate is stable, provides the best means for hydrate growth. The rate of hydrate growth, or removal of dissolved reactant from the pore water, is dependent on the surface area of hydrate exposed to the pore water containing the dissolved HFG reactant. High concentrations of hydrate in the McKenzie Delta area of Canada and offshore SE Japan appear to be characteristic of hydrate deposition in well bedded-differentiated sandy or coarse sediments having good porosity and high original permeability.

At present, it appears that the largest concentrations of hydrate in muddy, passive margin sediments are associated with major deposits of free gas trapped beneath the hydrate stability zone (HSZ) in seafloor bathymetric culmination. Abundance of overpressured gas immediately below the HSZ is at least superficially related to thick sections in which hydrate occurs. Although large amounts of free gas may be trapped below the HSZ, gas pathways through the HSZ allow gas to reach the surface without forming hydrate, as is evidenced by recognition of widespread gas seeps in the seafloor. Models for the recycling of gas produced at the base of the HSZ, owing to marine sedimentation that causes the HSZ to physically move upward as the seafloor sediments thicken or to climate factors that cause lowering of sea level or warming of ocean water, suggest that a significant amount of it enters the HSZ and reforms into hydrate. This cycling would provide for a long-term system for the conservation of natural gas in hydrate. The general manner of formation of high concentrations of hydrate, however, suggest that free gas will not naturally form large amounts of solid hydrate when it locally develops the potential to migrate into a water-rich area within the field of hydrate stability.

If there is a good supply of HFG reactants at a water interface of polycrystalline hydrate growing throughout a sediment host, hydrate can grow rapidly into the water space. Potentially, the entire water space may become filled so long as HFG reactants can be supplied to the hydrate. The best way of maintaining a good supply of HFG is by a process where water circulation can be maintained. Circulating water that is enriched in HFG will directly transport reactants for hydrate growth to the maximum extent possible. As hydrate grows in pore water space, which has the effect of reducing porosity and possibly permeability, maintaining good supply of HFG in circulating pore water becomes increasingly difficult. Where water circulation is substantially reduced but strong, local gas fluxes are present, diffusion of HFG reactants through the pore water can also occur, but much slower hydrate growth can be anticipated than where there is circulating transport of HFG.

In contrast to this best mode for hydrate growth, where hydrate may form and grow primarily as a film on sediment grains, or where in other situations pore water is not a good media for hydrate transport, substantially reduced permeability will provoke an early crisis for the supply of HFG after only minimal hydrate growth, even in conditions where porosity may be high. Hydrate growing primarily along grain boundaries would seal narrow grain-to-grain constrictions, which could result in the sediment-hydrate ‘rock’ being essentially impermeable, while considerable isolated pore water remains trapped in larger diameter pores. Vein works of hydrate in secondary porosity may also act to block the movement of groundwater. In this instance, further growth of hydrate within the trapped pore water will depend on solid diffusion of HFG through existing hydrate and into the trapped and isolated pore water. This is likely to result in very slow hydrate growth and relatively low pore fill.

The manner of hydrate dissociation is key to the recovery of its constituent natural gas because of resulting porosity and permeability relationships. Solid hydrate appears to dissociate only at its surface where it is in contact with gas or fluid outside of the field of hydrate stability. For instance, cores that have been recovered during deep ocean drilling or have been dredged or floated to the sea surface from the seafloor, persist for considerable periods of time and appear to be mechanically stable at atmospheric pressure after being brought to the surface from pressure depths as high as 30 MPa. Hydrate does not appear to fracture and rapidly decompress in the presence of very reduced pressures even through it may be well outside of its stability field. Just as hydrate appears to grow best at the margin of any solid hydrate exposed to pore water that transmits the temperatures or pressures that are conducive to hydrate stability and drive growth, dissociation is a surface phenomenon. Thus, in a hydrate deposit that is ‘rich’, that is where porosity and bulk rock percentages of hydrate are high, the surface area of hydrate will be naturally limited. Some method of increasing the surface area to hydrate mass ratio will almost certainly be required to insure a good rate of gas production from the hydrate. Where a sediment has very high pore fill concentrations of hydrate and the hydrate-sediment reservoir as a whole has little or no permeability, it will be necessary to provide for the development of widespread circulation in the reservoir to allow sufficient gas derived from the hydrate to concentrate, overpressurize, and flow in order to be recovered.

Thus, in order to insure that sufficient surface area is developed in deposits having high hydrate pore fill, it may be necessary to employ some type of hydrate-specific fraccing mechanism. Fraccing in a conventional deposit stimulates the rate of recovery. In a hydrate deposit that is rendered tight by the growth of hydrate in pore space, fraccing may be required to provide additional surface area of hydrate to stimulate dissociation. This porosity enhancement could occur at the interface between sediment grains by electromagnetic stimulation or by employing chemical or explosive fraccing techniques that are presently used by the hydrocarbon extraction industry. The sediment-hydrate reservoir could be treated as a mechanical whole or the different physical properties of the hydrate and sediment constituents could be manipulated to provide optimum gas recovery.

Another factor of gas hydrate reservoirs that may not be reflected in conventional hydrate reservoirs is the issue of containment. In a conventional gas reservoir, the hydrocarbon is contained by its geological trap. In a hydrate reservoir, the gas is trapped in the hydrate, which takes the form of diagenetic cement. Ideally, hydrate is subject to dissociation, the evolved gas ponds and can be drawn off by suitable collectors, probably of a multibranching horizontally drilled type. Where conditions of dissociation are imposed in a hydrate deposit in which permeability of the sediments would be naturally good, gas that is evolved through the dissociation may not be naturally contained if migration pathways occur. The model of producing gas at the base of a HSZ, which can be trapped by the overlying sediments in which hydrate has blocked porosity, which was developed primarily in connection with the large trapped gas deposits beneath hydrate off the U.S. SE coast, the model may not apply where there is little evidence of trapped gas. Lack of evidence of trapped gas may indicate not the lack of significant gas but the lack of a trap. Gas evolved from the hydrate in this circumstance may not remain associated with the hydrate and may flow away from collectors. Consideration should be given to the artificial trapping of evolved gas that might otherwise flow away from collectors by careful analysis of the geology and the placement of stimulation and collection apparatus. Production of temporary porosity ‘dams’ may also be necessary.

Acknowledgements: Part funded by DARPA Contract No. NBCHCO10003. Marine Desalination Systems, LLC, allowed release of this laboratory results.