Sanja Asks PERM:
I would like to have better understanding on contact angle and wetting properties for both shale and carbonate reservoirs.
I am a little unfamiliar with the testing. Can you please advise?
Dr. Jonathan Bryan from PERM Answers:
To get wettability data we can run several different tests. The contact angle test is nice because it gives a visual indication of wettability, but the problem with this method is that it requires the solid surface to be perfectly smooth. If there is surface roughness then the contact angle you measure may be wrong, so often people will go to model systems (e.g. marble to represent your rock) but now you only study wettability from fluids and not from the effect of the solid itself.
In order to measure wettability the better test is the Amott-USBM method. In this method we start with a core that is saturated with oil and irreducible water. We first put the core in contact with oil and measure how much oil spontaneously invades (if any). Then we force oil through the core and measure the saturation change from forced oil invasion. Then we do the same with water: measure the invasion of water first through spontaneous soaking and then through forced displacement. The wettability of the sample is determined on the basis of the ratio of how much of one fluid spontaneously can invade the core vs. the total invasion (spontaneous + forced).
This should work well with carbonates or anything with “conventional” permeability. The difficulty we face in shales is that the pore spaces are so small that it is hard to establish all of the various end point saturation states. For example, how would we establish the initial state (oil + irreducible water) in a very tight sample, and what pressure drop is required to run forced invasion of the different fluids. Wettability in shales is still an area that we are investigating currently; we don’t think that there are good methods currently existing. The other thing to keep in mind is that these reservoirs are generally very high pressure systems. So even when you run a frac program and now the rock is in contact with water (frac fluid), the oil pressure may still be high enough to overcome capillary pressure and keep water from spontaneously invading the system, so there may or may not be spontaneous water invasion even in a water wet system. This is something we are still investigating as well.
Dr. Jonathan Bryan