Combining Pressurized Nitorgoen Gas w/ Hot Oil

[jgarcia78]

I have a case where we could be potentially mixing pressurized nitrogen gas with hot oil. If we were simulating a field condition between a casing slip hanger and spool, and the void test on the outside of the hanger between the two spools was 15ksi nitrogen and was to seep into the casing and into the hot oil in the casing, how would you go about calculating the effects? The hot oil is a fixed volume with no pressure on it.

 

[BigInch]

I envision it like this.

The oil would compress as the expanding N2 hit it according to its bulk modulus. Final pressure of the oil and nitrogen mixture would be determined by solving for the volume at a resulting pressure of the two fluid components equalling the volume of the casing as determined by its expanded ring diameter, due to the increase of internal pressure and the possible reduction of external pressure. You may want to solve this in a transient pressure mode, if the "seep" is "fast". Fast is relative and depends on the introduced flowrate of N2 verses both the rate of displacement and rate of compression of the oil inside. Temperature of the N2 outside would tend to drop as it expanded and entered the tubing thereby possibly tending to cool the oil it contacted. If the oil can flow and displace to other parts of the tubing to allow the N2 to enter, the pressure rise will be slower. If the oil is relatively restrained from flowing down the tubing (and out to surrounding strata ??), its rise in pressure in the restrained volumes would experience a "faster" increase in pressure due to the N2 entering faster than an equal amount of oil could be displaced. The tubing would experience a corresponding stress change and (possibly) rapid hoop stress/strain reversal as internal pressure increased and (possibly) external pressure reduced which would additionally correspond to how fast the internal pressure is building. That would tend to travel the tubing as pressure builds in a wave inside the tubing, moving away from the seep. As pressures increased, density and bulk modulus of the oil ahead and pressurized oil behind would dictate that the wave speed would also tend to increase in as it runs down the tubing. Frictional pressure drop across the length of any flowing oil column along the tubing would equal the N2 seep pressure behind the wave minus the pressure of the static oil ahead of the wave. How the N2 would flow with the oil, would depend on the rate of admittance and the inclination of the oil. It may tend to bubble up or foam, if slow, or actually maintain a 2 phase flow interface of some kind if admittance is faster.

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