Koval fractional flow

[frank1010111]

Is your feature request related to a problem? Please describe.
There's information in how the water/oil ratio is changing. We ought to capture that with a Koval fractional flow model. We can compare it to the Buckley-Leverett solutions.

Describe the solution you'd like
The equation

$$f_w=\begin{cases} 0, & t_D < 1/K_v \\ \frac{K_v - \sqrt{K_v/t_D}}{K_v - 1}, & 1/K_v \le t_D \le K_v \\ 1, & t_D > K_v \end{cases}$$

where $K_v = H_k E$, $H_k$ is the heterogeneity, $E = \left(0.78 + 0.22 \left(\mu_o/\mu_w\right)^{1/4}\right)^4$ is the effective viscosity ratio.

Describe alternatives you've considered
Buckley Leverett with straight-line relative permeabilities might be able to do this.

Further reading: https://doi.org/10.2118/217973-PA