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Gas cap saturated reservoir in MBAL (prediction)

Well Test Analysis, Pressure and Production Monitoring........ etc.

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Gas cap saturated reservoir in MBAL (prediction)

Postby gagvas » Fri Apr 11, 2014 6:54 pm

Hi all,

I'm trying to use MBAL to get some initial production estimation prior to real production. There is no production, hence no historic data, yet. The reservoir is a gas cap saturated oil one. Estimations for all necessary data to initiate prediction are available (permeabilities, gas cap-to-oil volume ratio, aquifer model, etc). Production is designed to start with perforating the oil zone and taking advantage of the existing gas cap to maintain pressure over time.

To proceed with a scenario like this, the producing well needs to be defined so that fluids are withdrawn from the oil zone solely. Therefore, gas saturation within the oil zone at the very first moment should be equal to zero as the oil is at its bubble point (assume gas cap is fully separated from the oil zone). Hence, no free gas flow should take place as the gas cap is supposed not to interact with the well. Equivalently, the produced GOR at the very first moment of the production is expected to be equal to the solution Rs of the oil zone.

As far as I am concerned, when in prediction mode, MBAL combines the material balance equation to the mobility ratio equations to provide estimations of produced fluid volumes over pressure (or time). However, after some experimentation, it was found that MBAL cannot distinguish between gas cap and gas that is liberated from the oil zone due to pressure reduction below the saturation one. Therefore, gas saturation is computed by taking into account ALL gas present in the reservoir (including the gas cap) and, as a result, it could be of the order of 50% instead of zero. That high gas saturation implies that gas permeability is computed as non-zero (from the user defined Corey curves), hence gas is assumed to flow within the well. Therefore, the produced GOR is huge even from the very first moment of the production.

Evidently, one could modify the gas relative permeability curves to ensure zero permeability values even at elevated gas saturations. This trick would keep gas production to a minimum but it would also "twist" the model completely. Is there any better way to simulate production by perforating the oil zone (solely) of a gas cap saturated reservoir?
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Re: Gas cap saturated reservoir in MBAL (prediction)

Postby FANARCO » Thu Jan 15, 2015 11:00 pm

Dear gagvas,

I know it is very late reply, but still can continue with your thread.

What happened in real production from this reservoir.
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