Hydrate CH4 Research Articles

Numerical Simulation Study of Methane Gas Hydrates Production by Using Gas Injection

Methane gas hydrate is a solid inclusion compound composed of gas and water, which is stable under high pressure and low temperature. However, the required natural gas exploration method is different from the usual traditional gas reservoir development. The exchange of methane with carbon dioxide is a leading unconventional technology for the production of natural gas hydrate. The production process of methane gas hydrate is simulated by using CMG STAR reservoir simulation software. The ability of CH4 in hydrate exchanged by injected CO2 gas and CO2-N2 mixed gas is compared, and the exchange situation of CO2-CH4 is evaluated. The results show that carbon dioxide injected into the process of methane hydrate formation has the ability to seal carbon dioxide in the form of carbon dioxide hydrate while recovering methane. A mixture of 78% nitrogen and 22% carbon dioxide produces 76.7% methane, while methane using only carbon dioxide produces 61%. Methane recovery increases with the increase of reservoir temperature and permeability. CH4-CO2 exchange technology through gas injection into hydrate reservoirs is feasible in practical oilfields, and natural gas production can be increased by injecting a mixture of nitrogen and carbon dioxide

Comparison between the replacements of CH4 in natural gas hydrate with CO2 and NH3

Natural gas hydrate (NGH), as a clean energy with great development potential, has been limited by the lack of efficient and safe exploitation methods for a long time. The CO2 displacement method proposed in recent years has not made substantial progress because of its poor permeability and low efficiency. The NH3 replacement method is proposed in this paper because of its strong permeability. We performed orthogonal molecular dynamics simulation of the replacement of CH4 in hydrate with NH3 and CO2 at different temperatures (245 K, 255 K, 265 K) and pressures (3 MPa, 5 MPa, 10 MPa). It is shown that:(1) Compared with CO2 molecules, NH3 and can penetrate into the hydrate layer effectively, and the resulting pore channels are conducive to the outward diffusion of CH4 molecules. (2) With the progress of the replacement process, the hydrate structure gradually decomposes but not completely, and the residual structure will inhibit the diffusion of CH4 molecules and lead to the agglomeration of CH4 molecules. (3) Within 1000 ps, the number of CH4 molecules replaced by NH3 is more than that of CO2 under the conditions of 245 K and 255 K, and less than that of CO2 under the conditions of 265 K. At the same temperature, the pressure does not affect the final comparison result.

Mesoscale Modeling of Exploiting Methane Hydrate by CO2 Replacement in Homogeneous Porous Media

In this study, the objective is to establish a general mesoscale model for replacement so as to precisely estimate the flux of methane hydrate dissociation and CO2 hydrate formation in the cage of hydrate. If homogeneous porous media is assumed, porosity (void fraction) of methane hydrate sediment can be obtained from silica packing in experiment. Based on considering the driving force of free energy in dissociation and formation processes, a new modeling of replacement process is established by considering the individual flux of hydrate surface at stable and unstable regions of CH4 hydrate. At stable CH4 and CO2 hydrates region, it provides a simple situation of discussing the recovery process which CO2 guest particle replaces CH4 in hydrate due to free energy difference. Here, the formation rate of CO2 hydrate is dominated by the dissociation rate of CH4. However, large amount of CH4 dissociated at the surface of hydrate at unstable region. The formation rate become dominated by the formation flux of CO2 hydrate itself. In addition, the high-pressure effect of accelerating the guest gas trapped in hydrate is also considered by applying kinetic theory of gases. By applying CFD method and unstructured grid, it is possible to consider momentum, concentration and thermal distributions in non-equilibrium state simultaneously. To compare with experimental results, flux of methane hydrate dissociation at high surface concentration in simulation are consistent with experiment.

Understanding effect of structure and stability on transformation of CH4 hydrate to CO2 hydrate

Understanding the transformation process of CH4 hydrate to CO2 hydrate is crucial to develop the CH4CO2 replacement technique for CH4 production and CO2 sequestration. Ab initio calculations show that the transformation will slightly distort the host lattice and decrease the binding strength of guest molecules, but it is a thermodynamically spontaneous process dominated by the entropic contribution. Moreover, ab initio molecular dynamics simulations suggest that the dynamics of the host lattice is independent on the guest molecules, while CO2 in hydrate exhibits slower translational and rotational motion than CH4 in hydrate.

Replacement of CH4 in Hydrate in Porous Sediments with Liquid CO2 Injection

AbstractThe dynamics of the replacement of CH4 in hydrate in porous sediments with liquid CO2 was investigated using a self‐developed experimental apparatus at different temperatures and initial pressures. The pressure increases steadily as the replacement reaction processes. The amount of the replaced CH4 is almost the same as that of the CO2 forming hydrate in the early stage and gradually becomes somewhat less in the later stage. The initial pressure has minor effects on the replacement rate, and temperature reduction causes a lower replacement rate. The experimental results suggest that the replacement rate is not related to the region of the temperature‐pressure conditions but is mainly affected by the fugacity differences of CH4 hydrate decomposition and CO2 hydrate formation.

Adsorption as a methane storage process in natural lake sediment

Methane stored in a natural lake sediment was investigated. Fresh sediment (8–25 cm in sediment depth) taken from Lake Biwa, Japan, contains 2.6 μmol/g d.s. (dry sediment) of CH4 on average. The sediment retained 0.31 μmol/g d.s. of CH4 even after air drying, which is 12% of the total CH4 in the fresh sediment. The higher the temperature for drying of the sediment, the smaller the amount of CH4 retained. The result of an adsorption experiment indicated that dried and completely degassed lake sediment adsorbs CH4. Clay particles in the sediment are possibly responsible for the adsorption of CH4. Methane observed in the fresh lake sediment was probably produced by bacteria in the past, then adsorbed on the sediment particles. Adsorption is another form of storage of CH4 in lake and ocean sediments, in addition to the well known forms of dissolved, bubble, and hydrate CH4.