Page 128 - Introduction to Petroleum Engineering

114 PETROLEUM GEOLOGY
6.5.1 Volumetric Oil in Place

The volume of original hydrocarbon in place (OHIP) in an oil reservoir is original oil
in place (OOIP). It is calculated using the expression
7758φ Ah S
N = ooi (6.1)
B oi
where 7758 is a unit conversion factor, N is OOIP (STB), ϕ is reservoir porosity
(fraction), A is reservoir area (acres), h is net thickness of oil zone (feet), S is initial
o
oi
reservoir oil saturation (fraction), and B is initial oil formation volume factor (RB/
oi
STB). Initial oil formation volume factor is the volume of oil at reservoir conditions
divided by the volume of oil at stock tank conditions. Associated gas, or gas in solu-
tion, is the product of solution gas–oil ratio R and N.
so

Example 6.5 Oil in Place
An oil reservoir has average porosity = 0.15 in an area of 6400 acres with a net
thickness of 100 ft, initial oil saturation of 75%, and initial oil formation
volume factor of 1.3 RB/STB. Use the volumetric OIP equation to estimate
OOIP.
Answer
OOIP is

N = OOIP = 7758φ Ah S
ooi
B
oi
7758 ×(015. )×(6400 acres )×(100 ft )×075.
= ≈ 430millionSTB
1 13RB/STB
.

6.5.2 Volumetric Gas in Place
OHIP for a gas reservoir is original free gas in place:
7758φ Ah S
G = ggi (6.2)
B gi
where 7758 is a unit conversion factor, G is original gas in place (SCF), ϕ is reservoir
porosity (fraction), A is reservoir area (acres), h is net thickness of gas zone (ft), S
g
gi
is initial reservoir gas saturation (fraction), and B is initial gas formation volume
gi
factor (RB/SCF). Initial gas formation volume factor is the volume of gas at reservoir
conditions divided by the volume of gas at standard conditions. Equation 6.2 is often
expressed in terms of initial water saturation S by writing S =− S . Initial water
1
wi
wi
gi
saturation can be determined from well log or core analysis.