(25a) Natural Gas Production From Gas Hydrates Coupled with Carbon Sequestration

Natural Gas production via Gas Hydrates:  A Parametric Study of Energy Efficiencies and Carbon Sequestration Potential.

Garrett Fitzgerald, Marco J. Castaldi

Abstract

While extensive fundamental research has been done, there has been little research focused on fuel production from hydrates coupled with CO₂ sequestration. A recently developed in-situ process for gas production from hydrates and COMSOL simulations show that it is possible to have an ideal energy efficiency of nearly 90% for land based reservoirs for hydrate loadings of 20%. Results from a 72 liter reactor vessel show production rates and efficiencies for hydrate loadings between 5 and 15 % pore volume conducted at both high [100 w] and low [20 w] relative heating rates.   During the heating tests various trends were observed relating hydrate loading and heating rate to gas production rate and overall efficiencies.  Results of multiple heating rates for in-situ dissociation tests also agree with published model simulations and indicate a low heat flux may preferred to yield the highest efficiency, however hydrate loading affects the optimal heating rate, this interaction is explored. We have now begun to quantify the energy associated with CO₂ hydrate formation and its ability to displace methane and remain sequestered. Recent findings of using CO₂ to displace methane in a hydrate reservoir during production of methane gas to be used as a fuel will be presented.  Multiple CO₂ injection rates are investigated. 

Introduction  

            Hydrates are non-stoichometric inclusion compounds which form ordered cages around light gas molecules that become entrapped in the lattice.  These compounds are formed in the presence of water and guest molecules at sufficiently low temperature and high pressure.  Natural gas hydrates are found in the form of methane hydrates around the globe in permafrost regions and in sediments below the deep-sea.  The worldwide estimates of methane hydrates are massive and wildly uncertain.  The USGS has estimated the in-place hydrates to be on the order of   400 million trillion cubic feet [TCF] , a staggering number compared to proven worldwide gas reserves of 5500 TCF.[ Demirbas et al. ]  These large numbers are however only estimates of in-place hydrates and it is widely accepted that the recovery of CH₄  from hydrate formations is a technically challenging task and has yet to be proven economically feasible.  Energy demand is expected to increase by 49% from 2007 to 2035 [IEO, 2010] and with this increase in consumption is an expected increase in CO₂ emission from 29 billion tons in 2007 to 44 billion tons in 2035.  Methane hydrates present a unique opportunity to address both the energy consumption and CO₂ production dilemma that is inevitably in our near future. 

            CO₂ can be stored as hydrate during the simultaneous conversion of methane hydrates into methane gas for production. Due to the favorable thermodynamics of CO₂ hydrates this methods offers several benefits including the potential for a carbon neutral energy source as well as the potential for increased economic feasibility associated with a CO₂ assisted methane production.  CO₂ injection also is advantageous to the continued geo-mechanical stability of the hydrate formation after CH₄ has extraction.  Exploiting the fact that CO­₂  is more thermodynamically stable than CH₄ at the temperature and pressures present in natural hydrate deposits, not only will the CH₄  selectively dissociate, the CO₂ will replace the previous CH₄ hydrate re-stabilizing the sediment.   The rates and extent to which this exchange process can occur is still unknown and is under current investigation and will be reported when data become available. 

Experimental

            There are currently three accepted methods for the recovery of CH₄ from hydrate deposits; thermal stimulation, formation depressurization and inhibitor injection.  In this paper we investigate the overall energy efficiencies of several different thermal stimulation methods.  Methane hydrate deposits vary widely in their defining characteristics such as sediment permeability, porosity, pore volume hydrate saturation, water saturation, and formation temperature and pressure.  Hydrate saturation plays a key role in the rate of heat and mass transfer that can be achieved during the dissociation process.  We have developed a large scale apparatus that is capable of dissociating hydrate via thermal stimulation, pressure reduction or a combination of the two at various hydrate saturations and under different heating conditions.   The apparatus used in these experiments is depicted in figure1; this 70 liter internal volume setup is rated to 2000 [14 mpa] psi and is equipped with a centrally located resistive heater and CO₂ injection port.  There are 12 k type thermocouples arranged to capture heat flow patterns in the radial and azimuthal directions. The sediment used in these experiments is 500 micron quartz sand that is probed with two gas sampling ports for gas composition analysis. 

Hydrate formation occurs in a stochastic and heterogeneous manner and thus the precise location of hydrate in the sediment is somewhat unknown. This uncertainty requires that great care be taken when forming the hydrate as well as during data analysis to insure no faulty conclusions are made.  When growing the hydrates in the lab we chose to use a gas injection method.  The sediment is partially saturated with water and brought to the desired hydrate formation temperature prior to injection.  When the entire system has reached thermal equilibrium CH₄ is injected from the bottom of the vessel and allowed to percolate up through the wet sediment and eventually fill the 2 liter headspace above the sediment.  When the desired overpressure above the hydrate stability zone (HSZ) is reached, gas flow is terminated and the hydrate begins to form.  The process is repeated when the pressure stabilizes indicating hydrate-gas-water equilibrium; the number of cycles is increased to achieve higher hydrate saturations. 

When the hydrate is formed with known hydrate, water, and gas saturations the heating test are initiated.  The heat flux in is set constant at either 20 [low] watts or 100[high] watts using a custom omega resistive heating element.  The thermal stimulation causes the hydrate to dissociate into the gas phase resulting in a pressure rise and gas accumulation in the head space.  The pressure is allowed to rise ~30 psi [200 kpa] at which point a back pressure regulator releases the gas until the pre-heating pressure is realized.  Mass flow meters and the Peng-Robinson   EOS are used to complete a mass balance and determine the amount of hydrate that dissociated from the thermal stimulation.  

Results

            The preliminary results from the heating test show that at higher hydrate saturations higher heat flux is more efficient; as well it was observed that the efficiency for the lower heat flux is greater at for the lower hydrate saturation.   These numbers are preliminary and will be verified in a second round of tests to assure accuracy.  Due to the low thermal conductivity of pure hydrate the overall conductivity of the formation decreases for increased hydrate loadings, this leads to a buildup of heat close to the heater which raises the local formation temperature significantly above the dissociation temperature.  This excessive heating of the non gas producing sediment is one source of the energy losses and decreased efficiencies associated with the increased hydrate saturation; it is essentially a competition between the dissociation of the hydrate and the thermal diffusivity of the sediment. 

Using an high heat flux can also suffer from the same issue of overheating the non-productive matrix, however at the high heating rate this was not observed for the higher hydrate loading because the increased flux was spent in dissociating the greater volume of hydrate present.  Test 4 was compromised and must be repeated.  The full set of data will be presented and discussed in detail.  

Test 1     Q=100 15.6 % Hydrate  eta = 69%	Test 2    Q=100 10.2% Hydrate eta=?
Test 3     Q=20 15.6 % Hydrate Eta=49%	Test 4    Q=20 10% Hydrate Eta = 71%

Results from in-situ heating and CO₂ sequestration will also be presented; similar tests to those described above will be reported for various heating rates and various CO₂ injection rates. 

Conclusion

CH₄ production via the thermal stimulation method was conducted using a resistive heating element in hydrate bearing sediment of different saturations.   Hydrate saturation and heating rate have important impacts on the efficiency of production.  Increased heating with higher hydrate loading was shown to be more efficient that lower heating at the same saturation. However the is competition between hydrate loading and heating rate and the heating rate must be designed to match the thermal diffusion rate in the hydrate bearing sediment. 

References:

Demirbas, Ayhan,   ‘Methane hydrates as potential energy resource: Part 1 – Importance, resource and recovery’ Energy Conversion and Management, Vol 51, 7 2010

IEO, 2010 International Energy Outlook 2010.< www.eia.gov/oiaf/ieo/index.html.> Energy Information Administration  2010. 

Boswell, Ray. ‘Resource potential of methane hydrate coming into focus’  Original Research Article
Journal of Petroleum Science and Engineering, Volume 56, Issues 1-3, March 2007, Pages 9-13