80000 STB of oil and 60×106 SCF of gas have been produced from a reservoir disco

Question

80000 STB of oil and 60×106 SCF of gas have been produced from a reservoir discovered at 4200 psia, and the reservoir pressure is currently at 3400 psia. Two geological descriptions have been proposed for this reservoir and it is necessary to ascertain which is correct. The rst description indicates 245000 STB of oil in place with a gas cap containing 1000×106 SCF, while the second description species 675000 STB of oil and 1500×106 SCF of gas in the cap. Which description is correct? PVT data:

P (psia) Bo (rbbl/STB) Bg (rbbl/1000 SCF) Rs (SCF/STB)

4200 1.34 0.86 1250

3400 1.28 0.89 900

Explanation / Answer

The value of m(ratio of gas cap gas volume to oil volume bbl/bbl) was determined for the two geological descriptions.

The value of m for the 1st case (m1)= 0.2776

The value of m for 2nd case(m2)= 0.1511

The material balance equation for oil reservoirs was applied for the values of m ,and the value of initial oil in place was found to be closer to 245000 STB of oil.

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