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.
Keywords: Methane gas hydrate, Gas injection, CH4–CO2 exchange, Quantitative analysis, Numerical simulation
Natural gas approaches recovery zones via pressure gradients in conventional gas reservoirs. In such reservoirs, the rate of gas production depends on permeability of the formation and also the pressure gradients between the reservoir and production well(s).1-3 Exploration and production of gas from hydrate-bearing reservoirs require an additional energy to dissociate the crystalline water lattice that makes up the structure of the hydrate. Various methods have been suggested for producing methane gas from hydrate deposits, thermal stimulation, depressurization, chemical injection of inhibitors and CO2 or mixed CO2 and N2 exchange.4-6
Visual experiments of the dissociation process in glass micro-models illustrated that the hydrate becomes colloidal and migrates with the injected brine during dissociation process.7 Tang and Kotov8 made a conclusion that lower injection rates and temperatures result in higher recovery energy ratios, so does higher initial hydrate saturations. Komai.9 observed that more than 90% of CH4 in hydrate phase can be exchanged by CO2 within 12 hours in their experiment conducted. Panter proved that raising the amount of N2 in gas phase of N2 + CH4 hydrate system, shifts the equilibrium phase boundary to lower temperature and higher-pressure conditions.10
Ohgaki. observed that fraction of mole of CO2 in hydrate phase was higher than those in gas phase during the process of exchange.11 Seo quantified this observation by revealing that gas phase mole fractions of the hydrate formers (i.e., CH4 and CO2) above 40% CO2 gave hydrate phase fractions of mole of CO2 in hydrate phase higher than 90%.12 Minagawa proposed the idea of electrical heating assisted depressurization technology.13 Gupta of India Dhanbad Production University proposed the CO2 swapping assisted depressurization technology in an attempt to ensure an environmentally friendly production of CH4.14 Yuan conducted an experiment on hydrate-bearing sediment samples to look into conditions that favors the production of methane from gas hydrate reservoirs with gaseous CO2.15 In experiments performed by Ota. the system temperature and pressure are set to 275K and 3.30MPa respectively.16 These are similar to the figures in the experiments of Yuan.15
In this paper, simulation studies which model unconventional methane gas hydrate recovery methods such as CO2 injection, N2 + CO2 gas injection methods and CH4-CO2 replacement technology is conducted. It is also to compare the injection of appropriate gaseous phase mixtures (of CO2 and N2) as opposed to pure CO2 injection and deduce its effect on the behavior of the reservoir rock for successful CO2-CH2 swapping.
Computer Modeling Group’s CMG STARS reservoir simulation software was selected to conduct the simulation study in this thesis. Establishment of the model was done by referring to the reservoir and operation parameters of Ignik Sikumi Field production trial. The injected gas used is a mixture of CO2 and N2. The method adopted was the depressurization injection from a single well to demonstrate the CO2-CH4 exchange concept. The simulation involved the injection of on CO2 gas and then injection of a mixture of N2 and CO2 gas into the reservoir for CH4 production.
Assumptions
In carrying out the simulation, several assumptions were made which included
- 1. The reservoir is uniform, homogenous and can be represented by a series of cells
- 2. Hydrates that exist in the reservoir are pure CH4 hydrates
- 3. Hydrate exists in equilibrium with excess water
- 4. The system is adiabatic and transfer of heat to and from confining strata is not necessary
- 5. The influence of gravity can be ignored
- 6. Movement of mass is limited to only liquid and gas. Solids cannot flow
Energy conservation model
The conservation equations of each component and the energy are shown in equations (1) and (2). For the flowing component (i.e., CH4, CO2, N2 or H2O), the conservation equations are;
Where Vf is the volume of the mobile phases; Vb is the apparent cell volume; µA and µG is the densities of the aqueous and gas phases, respectively; wi and yi are the percentage mass of components in the aqueous and gas phases, respectively; is the number faces of neighboring cells; (A/l)c is the ratio of effective area and distance between the interfaces; kc is the effective permeability at the interface; pAand pG is the the pressures at the aqueous and gas phases, respectively; nt is the number of chemical reactions;snii and sni is the stoichiometric coefficients of the product and reactant of component, respectively; rn is the rate of volumetric reaction; qi is the mass source from the well.
For the equations of energy conservation:
Where Vr is the volume of rock (solid inert matrix, rock grains); cs is the concentration of total solid; Ur is the energy per volume of rock; UA, UG and Us are the energies of the aqueous, gas and solid phases, respectively; HA, HG and Hrn are the enthalpies of the aqueous phase, gas phases and reaction respectively; kc is the effective thermal conductivity at the interface; T is the temperature; qc is the heat source from the injection/production wells.
Permeability model
The reservoir absolute permeability is modeled with respect to the hydrate saturation. In addition, relative permeability of the mobile phase changes with the effective phase saturation. The fluid phase permeability, effective phase saturation and actual phase saturation are defined in equations (3), (4) and (5) respectively:
Where ka0 is the reservoir absolute permeability without the presence of the gas hydrate; φ and φf is the reservoir porosity and fluid porosity respectively; m is the model parameter which is set to 4.3413 by changing Civan’s permeability-porosity relationship.17
The flow of the mobile phases follows Darcy’s law, and relative permeability models are revealed in equation (7):
Capillary pressure model
The capillary pressure model of the gas phase and the aqueous phase is shown in equation (8)
Where pc is the capillary pressure; pc0 - model parameter, which is set to 104 Pa and k - model parameter which is set to 0.77437.19
Kim based on experimental results suggested the generally used CH4 hydrate dissociation kinetic model.20 In their model, rate of dissociation corresponded to particle surface area of hydrate and methane fugacity difference at equilibrium and dissociation pressures. When setting the fugacity coefficient, the fugacity can be approximated with an equivalent pressure equal to 1. An assumption was made that formation and dissociation of CH4 hydrate and CO2 hydrate follow the Kim-Bishnoi model. The CH4/CO2 hydrate dissociation rate is expressed as follows:
Where cHyd is the quantity of mole of the CH4/CO2 hydrate per unit volume; k0d is the intrinsic constant rate of dissociation of CH4/CO2 hydrate; AHS is the specific area of reaction, which is 750,000m2/m3 i.e., assuming hydrate particles are regular spheres having diameter of 8lm;21 ρw is the density of aqueous phase, 1000kg/m3; h is the density of CH4 hydrate or CO2 hydrate, 919.7kg/m3 or 1100kg/m3. 22 ΔEd is the activation energy of dissociation reaction, which is 81kJ/mol and 102.88kJ/mol for CH4 hydrate and CO2 hydrate, respectively; R is the gas universal constant; c is the equilibrium pressure; y is the mole fraction of CH4/CO2 in gas phase; K is the equilibrium ratio, as follows,23,24
Where Pg is the gas phase pressure; a1, a2 and a3 are the model parameters and are calculated based on the experimental results of Adisasmito.25
Results and Discussion
Reservoir parameters
The formation of the reservoir consists of unconsolidated sand and was modelled based on the reservoir parameters of hydrate-bearing units according to Ignik Sikumi trial data. The formation consists of the impermeable overlying layer, the Hydrate-Bearing Layer and the impermeable bottom layer having a thicknesses of 32ft. The model size is 500ft 500ft 100ft with a grid division of 51×51×10, as shown in Figure 1. The hydrate reservoir was produced by gas injection which firstly composed of pure CO2 gas and then a mixture of 22 mol% CO2 and 78 mol% N2. At the initial CH4 hydrate saturation of 72% and water saturation of 28%. The remaining parameters are shown in the Table 1.
Simulation results for pure CO2 injection
Effect of temperature on CH4 production (CO2 injection)
At the beginning of the simulation, the temperature around the injection well is higher because of the exothermic nature of mixed hydrate (CH4-CO2-hydrate) formation reaction, as shown in Figure 1a. The reason for the temperature rise is due to two thermodynamic processes which are the specific enthalpy of injected CO2 and the exothermic nature of CO2 dissolution in water. As the simulation advances, the rise in the temperature of the reservoir gradually spreads to areas further away from the wellbore corresponding to the advancement of CH4-CO2 reaction, as shown in Figure 1b. At the end of the simulation, because of the heat exchange with the surrounding strata, it is observed that the temperature around the wellbore area declines becoming lower than the temperature of the entire reservoir, as shown in Figure 1c. This proves that the supply of heat is relevant for continuous dissociation of hydrate. Greater methane production at high temperatures when gas is injected proves that high temperatures favor both the thermodynamics and the kinetics methane recovery. The increase in the amount of methane production with temperature is accredited to hydrate equilibrium pressure. At increased temperatures hydrate equilibrium pressure is higher suggesting a greater density of gas phase in equilibrium with the hydrate phase. In order to obtain a given vapor phase concentration of methane, there needs to be the release of more methane from hydrate at higher temperatures rather than at lower temperatures.26
Due to the endothermic nature of the process of methane hydrate dissociation, there is heat exchange with the surrounding strata. This leads to the cooling of the reservoir especially around the wellbore area. As a result of this, the rate of hydrate dissociation and subsequent production reduces as production advances. This effect is as shown in Figure 2, the daily rate of methane hydrate production drops from 160m3 at the initial stages of the production period when the reservoir temperature was a little under 5oC to less than 60m3 at the end of the production period at a temperature of less than 1oC
Effect of hydrate saturation on CH4 production (CO2 injection)
The decrease in hydrate saturation around the wellbore area implies the dissociation of CH4 hydrates at the end of the simulation. The reduction in the saturation of CH4 hydrates around the wellbore area up till 500 confirms the swapping of CH4 with CO2 of the initially CH4 hydrate lattices. However, compared to the CH4 saturation at the end of the simulation for N2+CO2 injection, more hydrate is dissociated when N2+CO2 is injected indicating that the addition of N2 gas enhances the dissociation and subsequent production of CH4 gas.
As shown in Figure 3, it is clearly seen that as more CO2 is injected into the reservoir with time, the CH4 hydrate saturation reduced. This is because a higher injection of the amount of CO2 usually means that more CO2 will be available to react with and dissociate more hydrate to produce methane gas. At the end of production period, more than half of the initial CH4 hydrate in the reservoir has been dissociated because as the amount of injected CO2 increases, there is the inducement of the dissociation of more hydrate.
Effect of porosity on CH4production (CO2 injection)
The result from Figure 4 shows that a porosity of 0.4 gave the highest volume of CH4 production. However an interesting observation was made. Porosity value of 0.3 yielded a higher production volume of CH4 than porosity value of 0.5. This clearly shows that there is variation of result depending on the porosity value chosen which indicates a copious unpredictability connected to porosity changes.
Simulation Results for N2+CO2 injection
Temperature influence on CH4 production (N2+ CO2 injection)
The temperature of reservoir reduces with time of production because the endothermic process of hydrate dissociation. At higher temperatures, the hydrate easily becomes dissociated because it is shifted from its equilibrium and hence higher gas rates is associated with warmer reservoirs. In the simulation the reservoir temperature is 2.5°C. For sensitivity analysis study this temperature is changed to 1°C and 4°C.
As expected, higher gas production is associated with higher reservoir temperature and this can be ascribed to more heat present in the system. As shown in Figure 5, it is observed that a reservoir temperature of 1oC yielded a total CH4 production volume of 10,000m3 over a period of eighteen months as compared to 83,000m3 over the same period when the temperature was set to 4oC. The increase in amount of CH4 produced as temperature increases proves that the supply of heat is important for continuous hydrate dissociation. Apart from the latent heat present in the Hydrate-Bearing Layer, the latent heat present in the overlying and bottom layers influence the dissociation of CH4 hydrate.
Effect of reservoir permeability on CH4 production (N2+CO2 injection)
Figure 6 displays the effect of permeability on the production of CH4. Higher rates of gas production are observed with a rise in permeability. In the simulation three cases were examined in which absolute permeability was changed from 1000mD to 750mD and 500mD. A reservoir permeability of 1000mD produced a total volume of 85,000m3 of CH4 over a period of 19 months whiles permeability of 500mD produced a total CH4 volume of 61,000m3 over the same period.
This phenomenon occurred because; as the duration of production increases, the permeability and the fluid porosity of reservoir also significantly increase because the dissociation of CH4 hydrate which enhances seepage condition increases thus favors gas flow in the hydrate reservoir during injection or production.
Effect of reservoir porosity on CH4 production (N2+CO2 injection)
For this simulation, porosity values of 0.3, 0.4 and 0.5 were used, as shown in Figure 7. No peculiar trend was seen in the reservoir characteristics with respect to changes in porosity. Effect of porosity on the rate of CH4 production relies on the values chosen. If porosity values of 0.2, 0.3 and 0.4 are chosen, different result will be observed. Conventional thought would propose that higher porosity yields to higher CH4 production rates due to greater pore volume in the reservoir. Depending on the selected porosity value for the simulation, there is a variation in the result from the reservoir.
Effect of hydrate saturation on CH4 production (N2+CO2 injection)
Higher initial hydrate saturation means that there is more methane in the reservoir. In hydrate reservoir simulation, higher hydrate saturation resulted in lower production rates. Hydrate dissociation is endothermic and results in the cooling of the reservoir. This therefore means that higher hydrate saturation quickly cools the reservoir stopping further dissociation of hydrate which yields to reduced gas production rates.
As shown in Figure 8, initial saturation of hydrate of 0.7 gave higher CH4 production than initial saturation of hydrate of 0.8 signifying that a decrease in initial hydrate saturation results in a rise in the rate of production of CH4.
Comparison between pure CO2 injection and N2+CO2 injection
Tornado plot is used for sensitivity analysis to investigate the main (linear) effects, interaction effects, and quadratic (nonlinear) effects of each reservoir parameter on the volume of CH4 produced. The Y axis in the tornado plot is parameter effect (linear, interaction and quadratic effects) and the X axis denotes response change in volume of CH4 produced. The tornado plot shows the actual predicted response change in volume of CH4 produced as the parameter travels from a smallest sample value to the largest sample value. Permeability in the horizontal direction (PermH) has the highest positive effect, followed by porosity and permeability in the vertical direction. Bottom-hole pressure has a negative effect.
The maximum and minimum values of volume of CH4 produced obtained from the range of factors considered can also been seen in Figures 9 and 10. It is observed that the most influencing factors are the main (linear) effects of permeability in the horizontal direction (PermH), porosity, injection pressure and permeability in the vertical direction (PermV). There are interaction effects between porosity and permeability in the horizontal direction, bottom-hole pressure and permeability in the horizontal direction, injection pressure and permeability in the horizontal direction. It can be observed from the comparison of the tornado plot between N2+CO2 injection and CO2 injection that all the parameters have stronger effect on N2+CO2 injection than CO2 injection. This occurs because there is a greater production of CH4 in N2+CO2injection.
In this paper, simulation and modeling are carried out to understand the process of exchange of CH4 hydrate to CO2 hydrate in the use of CH4-CO2 exchange methods for the production of CH4. The effect of conditions such as reservoir temperature, temperature of injected gas, reservoir pressure including pressure of injected fluid, reservoir hydrate saturation, mole of injected fluid, on CH4 production was studied. This helped understand the various mechanisms involved in the production of gas from hydrates. From this study, the following conclusions were made.
- 1. The direct use of N2+CO2 gas mix instead of pure CO2 shifts the equilibrium phase boundary to lower temperature and higher-pressure conditions and therefore facilitates methane hydrate dissociation. Nitrogen also speeds up the process of depressurization and enhances CO2 exchange. A higher initial mole of gas injected into the system will cause an increase in the driving force of CH4 hydrate dissociation which will yield a higher rate of formation of CO2-CH4 hydrate and subsequently yield a greater CH4 production.
- 2. High temperatures enhance both kinetics and thermodynamics of methane production hence an increase in CH4 production with increase in both the reservoir temperature and the injected gas temperature.
- 3. Permeability controls the flow of both gas and water by influencing pressure propagation in the reservoir therefore higher rates of gas production are associated with a rise in permeability of reservoir.
- 4. During the process of exchange of CH4-CO2 hydrate when low dosage methanol is present, CH4 recovery is enhanced. When methanol is present, formation of hydrate film is delayed at gas–liquid interface enabling additional molecules of CO2 gas to reach the hydrate surface. Higher concentrations CO2 at the surface enables greater diffusion into methane hydrate. Because of the enhancement of the thermodynamic force, more molecules of CO2 are able to replace CH4.
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The authors confirms that this article content has no conflict of interest
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