PETROLEUM RESERVIOR ENGINEERING 3. (30 points) The gas cap of the Nameless Field
ID: 114724 • Letter: P
Question
PETROLEUM RESERVIOR ENGINEERING
3. (30 points) The gas cap of the Nameless Field had a bulk volume of 14000 acre-feet when the reservoir pressure had declined to 700 psig. Reservoir temperature is 150 °F. Core analysis shows an average porosity of 20 percent, and an average interstitial water of 25 percent. It is desired to increase the recovery of oil from the field by repressuring the gas cap to 1400 psig. Assuming that no additional gas dissolves in the oil during repressuring and the compressibilities of water and formation are negligible, calculate the required gas volume in SCF (at conditions of 14.7 psia and 60 °F) to be injected in repressuring. The gas z-factors for both the eservoir gas and the injected gas are 0.9 at 700 psig and 0.8 at 1400 psig, both at 150 F.Explanation / Answer
1 acre-feet= 1233.48 cubic meter
1 psig= 0.069 bar
First condition,
Pressure, P1= 700 Psig
Temperature, T1=150 F
Volume, V1= 14000 acre-feet
Porosity due to gas=20 %
Interstitial water= 25 %
Second Condition,
Pressure, P2= 1400 Psig
Temperature, T2=60 F
z1 factors for reservoir gas at pressure 700 psig, 150F =0.9
z2 factors for injected gas at pressure 1400 psig, 150F=0.8
We have been asked to calculate gas needed to injected in repressing SCF( 14.7 psia, 60F).
First we need to calculate the amount of gas volume present because that is only component compressible, rest component i.e. water and petroleum are non compressible, so there volume will not change.
Volume of gas in field at condition 1, i.e T1= 150 F, P1= 700 psig = Vgas= porosity * Actual field volume
= 0.2*14000
=2800 acre-feet
Now applying, real gas equation for gas present in the field,
(Z1*P1*V1/T1)=(z2*P2*V2/T2)
Now temperature in field is being kept constant.
So, Equation becomes,
(Z1*P1*V1)=(z2*P2*V2)
0.9*700*2800=0.8*1400*V2
V2=1575 acre-feet
this is the amount of real gas at P2=1400 psig, T2=150F.
In the question it is asked to calculate the volume of gas at SCF i.e Pressure, P3= 14.7 psia, and temperature=T3=60F)
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