Amir Asks PERM:
If you flood two identical cores (let’s say with poro=25%, perm=200 md) one with water and another one with immiscible gas (like associated gas), for which case do you expect a higher residual oil saturation?
Dr. Jonathan Bryan from PERM Answers:
Let’s start by assuming that we have a water wet reservoir. In this case, flooding with water is an imbibition process. Water will come through the smallest pores first and displace mobile oil in the larger pores out of the core. Eventually oil is trapped by being surrounded with water, so depending on the nature of the porous medium (i.e. connectivity of pores and PB/PT aspect ratio) trapping of oil may be large or small. If the reservoir is oil wet, we generally expect residual oil saturation to be lower since oil trapping will occur in the smaller pores, and oil will remain continuous (mobile) for longer.
In the case of gas displacing oil and water, gas is always the non-wetting phase. Gas will therefore come through the center of the pores, and oil/water will drain around the edges of the gas. So in this case, we always expect a low residual oil saturation. But please note: this assumes that invasion of gas is gravity stable (slow). If gas injection is too fast, gas will finger through the oil and large parts of the system will be bypassed. So in this condition, residual oil saturation will be much higher, but this is due to viscous bypassing of oil and not capillary trapping.
Dr. Jonathan Bryan