Stephen Asks PERM:
- When conducting MICP analysis, I had always assumed that mercury was the non-wetting phase and air was the wetting phase. However, it the sample is evacuated of air prior to mercury injection, what is the wetting phase?
- If the relative permeability to mercury (or HC) is zero at 100% saturation of the wetting phase, how does the non-wetting phase get into the pores to begin saturating the sample?
Dr. Jonathan Bryan from PERM Answers:
- When we evacuate the air, the wetting phase is actually Hg vapour – we are measuring mercury and its own vapour in equilibrium with each other. Interestingly, we still use the Hg-air value of capillary pressure. A google search of MICP shows that this is the value always used; it doesn’t seem like IFT between Hg and Hg vapour is easily available. Probably the values are close, so it is a good approximation.
- In order for the NWP to enter into the sample, the pressure of the NWP needs to be higher than the WP pressure. Specifically, it needs to be high enough to exceed the capillary pressure of the largest pores. Remember that wettability and capillary pressure refer to the wetting state of the rock, i.e. it wants to be filled with the WP in contrast to the NWP. So if you put a rock in contact with NWP, nothing will happen. The NWP pressure needs to increase and exceed the large pore capillary pressures, and at this point it now has enough pressure to push its way into these large pores.
Dr. Jonathan Bryan