Code implementation and testing

The actual program has been coded for a two-dimensional system, but can easily be extended to three dimensions, as the equations have shown. All equations are solved implicitly for the pore pressure P, the gas saturation S_g and the hydrate saturation S_h at each grid point with the Newton-Raphson method using a Cartesian grid system. A flow chart of the code can be seen in Figure 4.

The time is denoted by n, the number of Newton iterations by it, and the time steps by $\Delta t$. Each new $\Delta t$ is calculated automatically based on the following relationship Aziz (1995):

$$\Delta t^{n+1} = \Delta t^n \left( {{(1+ \omega) \eta_{ijk}}... ... {\delta_{ijk} + \omega \eta_{ijk}}}\right)_{min_{over_{ijk}}}$$ (30)

The desired change of either pore pressure P or saturations S_g and S_h over $\Delta t^n$ is given by $\eta_{ijk}$, the actual change by $\delta_{ijk}$ and $\omega$ is a tuning factor with values between zero and unity. Usually, $\eta_{ijk}$ has the same value for all grid blocks, but different values for the different variables (in this case, pressure P, gas and hydrate saturation S_g and S_h). So a different $\Delta t^{n+1}$ is calculated for each variable and the minimum is used for the subsequent time step.

 

flow
Figure 4 Flow chart of the simulation code.

Initially, the system is characterized by a given pore pressure, temperature, gas and hydrate saturation. Perturbation of the pressure at time t=0 or an initial system, which is not in equilibrium, will initialize fluid-flow through the system. The boundary conditions depend on the initial system, and assume either constant pressure boundary at z=0 and no-flow boundaries everywhere else, or no-flow boundaries everywhere.

The code is still in a test phase. As of now, it is working for only two-phase flow and such small hydrate saturation that neither hydrate growth nor dissociation can occur. It is still unstable and non-converging for hydrate saturations bigger than 0.01. Additional work is required to correct this problem.

I conducted a simple test on the functionality of the code for at least two-phase fluid flow with the third, solid hydrate phase being approximately zero. The initial system consisted of a no-flow boundary setup of water over gas with a negligible small hydrate saturation everywhere. The initial pore pressure was comparable to hydrostatic pressure, and the temperature was chosen so that neither hydrate growth nor decomposition could be expected for the pore pressure range. The density difference of gas and water, the pressure gradient and the effect of gravity result in a fluid flow stimulation in which the gas migrates upward and the water travels down.

The grid system is a 10x10 system, each grid block having dimensions of 10 m x 10 m. The top of the grid system is at about 1000 m depth. The grid blocks within the first 40 m are filled with 100% water, the last 60 m are filled with 100% gas. To avoid any instabilities resulting from a vanishing third flow equation (hydrate equation), a constant hydrate saturation of 0.001 was assumed throughout the system. The temperature was chosen to be 10 degrees, and the initial pore pressure followed the relation P_ij= 1000 kg/cm * 9.81 m/s² * z_ij. The viscosities were assumed to be constant with $\mu_g = 0.4$ cp and $\mu_w = 1.0$ cp. The relative permeabilities were simply expressed by $\kappa_{rg} = s_g^2$and $\kappa_{rw} = (1-S_g-S_h)^2$. The densities were assumed to be slightly compressible with $\rho_g = 0.006 + p/10^5$ and $\rho_w = 1.0 + p/10^4$. The absolute permeability was a random permeability field with values between 10 mD and 10000 mD for each grid block.

Figure 5 shows the gas saturation results of the fluid-flow initialized by the instable gas-water system. 100 % gas saturation is displayed as white, 0 % as black. At time 0, the water is above the gas and fluid flow is stimulated. After 1 day, gas and water have started to migrate depending on low and high permeabilities in the different grid blocks. Following the permeability pattern, the two fluids begin to mix. After 155 days, the two phases have nearly equilibrized and finally they reach their equilibrium state with gas above water. As soon as this equilibrium state is reached, there is no more flow through the system.

 

gaswater-ann
Figure 5 Fluid flow stimulation by an instable system of water over gas. Results are shown for the gas saturation. 100 % gas saturation is displayed as white, 0 % as black.

The test shows that the code appears to work correctly for at least a two-phase fluid flow. More work is still required to improve upon the incorporation of a third-phase with more than 0.001 saturation everywhere.