Yahya from Southern California Asks PERM:
I’m currently working on forced imbibition during hydraulic fracture process in tight gas formations.
To investigate this phenomena, the reliable relative permeability for water/gas and capillarity pressure data play an essential role. I was interested in the method which you applied for calculating those data for unconventional reservoirs.
I really appreciate in advance and thank you very much for your time.
Dr. Jonathan Bryan from PERM Answers:
Thanks for your question. This is a very interesting one.
As I am sure you are aware, these tests are not easy to do. In order to get to secondary imbibition, we need to be able to start with a dry core (non-trivial) and then vacuum saturate with water, which is also very difficult in a low permeability rock. Often times vacuum saturation does not lead to proper filling of pore spaces, so we are already starting from the wrong point. But even at that time, to drive the system back to Swirr is very difficult. In conventional core we would run centrifuge technology to spin water out of cores, but for tight media we are not able to achieve and overcome the high capillary pressures to get sufficient water out of the rock. This leads in turn to very high Swirr estimates, and again you would be starting your secondary imbibition from the wrong point.
So experimentation is very difficult in these system, but it is not impossible. There are work-around techniques such as high pressure gas or water injection, which we could employ instead of centrifuging. Data like this could be generated if you are interested.
But on a bigger scale, a better question would be whether or not we think that water imbibition from frac jobs would actually happen. Let’s run a thought experiment together:
- Most of these systems are very high pressure, on the order of 500 – 1000 psig pore pressures. If the reservoir pressure is too low for these low porosity systems, then there wouldn’t be sufficient gas so usually the target is very high pressure reservoirs.
- Let’s assume IFT is on the order of 72 mN/m (air-brine) and that the system is strongly water wet (contact angle = 0). With these assumptions, in order for water to get past this high pore pressure, the pore size would have to be on the order of 1 nm. If pores are larger than this, the reservoir pressure would drive water back out again, effectively cleaning out the damage from the fracs.
So we think that an important question you would need to answer is whether your pores are in fact this small. If so, then yes there is potential for water blockage from frac treatments. But if pores are larger, then this effect will probably not be too significant.
As always, we welcome your thoughts and feedback.
Dr. Jonathan Bryan