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Reserve Estimation
Volumetric Reserve estimation involves integration of geological data with reservoir geometry .Recoverable reserve is the volume of hydrocarbon that can be profitably produced from the reservoir. Reserve estimation can be done by estimating OOIP (original oil in place) and OGIP (original gas in place) which is total volume of hydrocarbon before production.
OOIP is estimated using below equation:-
OOIP(STB)=7758*H*A*ø*So/Bo
where
Unit of OOIP is in stock tank barrel (STB)
7758 = conversion factor from acre-ft to bbl
A = area of reservoir in acres
h = height or thickness of pay zone in ft’s
ø = porosity in decimals
So= Oil Saturation in decimals (1-Sw)
Bo = formation volume factor for oil at initial conditions (reservoir bbl/STB)
from lab data; a quick estimate is Bo=1.05 +(X*0.05) , where X is the number of hundreds of ft3 of gas produced per bbl of oil [for example, in a well with a GOR of 1000, Boi = 1.05 + (10 × 0.05)]
Basic volumetric equation to estimate OGIP is:-
OGIP (SCF)=43560*H*A*ø*So/Bg
where
- Unit of OGIP is Standard Cubic feet (SCF)
- 43560 = conversion factor from acre-ft to ft3
- Bg= formation volume factor for gas at initial conditions (RES ft3/SCF)
Recoverable reserves depends on efficiency of reservoir drive mechanism (recovery factor) and are fraction of the OOIP or OGIP .
The basic equation used to calculate recoverable oil reserves and recoverable gas reservoir is:-
Recoverable oil reserve (STB) =OOIP *RF
Where RF= recovery factor
Recoverable gas reserve (SCF) =OGIP *RF
Recovery factor for gas is near unity for dry gas reservoir.
Formation Volume Factor:-
Gas FVF: Gas Formation volume factor relates gas volumes in the reservoir to the produced volume at standard conditions. This factor is primarily used for the conversion of surface measured volumes to reservoir conditions.
Oil FVF: Oil formation volume factor is oil and dissolved gas volume at reservoir conditions divided by oil volume at standard conditions.
As most of the oil and gas production measurements are made at the surface, formation volume factors are required for the conversion of measured surface volumes to reservoir conditions. Oil formation volume factors are usually greater than 1.0. The primary reason is that oil in the formation incorporates dissolved gas which comes out of solution in the wellbore with dropping pressure.