An oil reservoir exists at its bubble-point pressure of (3,000 psia) and temperature of [tex]$\left(160^{\circ} F \right)$[/tex]. The oil has an API gravity of [tex]$\left(42^{\circ}\right)$[/tex] and gas-oil ratio of ([tex]$600 scf / STB$[/tex]). Initial oil formation volume factor is 1.396 bbl/STB. The following additional data are also available:
Reservoir area = 640 acres. Average thickness = 10 ft.
Connate water saturation = 0.25
Effective porosity = 15%.
The OIIP in STB IS:
a. 4 MM STB
b. 4001.260 M STB
c. 400 M STB
d. 4 MMM STB
Answer (1)
Calculate the bulk volume: V b = 640 acres × 10 ft = 6400 acre-ft .
Convert bulk volume to cubic feet: V b = 6400 × 43560 = 278 , 784 , 000 ft 3 .
Calculate pore volume: V p = 278 , 784 , 000 × 0.15 = 41 , 817 , 600 ft 3 .
Calculate hydrocarbon pore volume: V H C = 41 , 817 , 600 × 0.75 = 31 , 363 , 200 ft 3 , and find the original oil in place: O II P = 1.396 31 , 363 , 200 = 22.466 MM STB .
Explanation
Understanding the Problem We are given the following data for an oil reservoir:
Reservoir pressure: 3 , 000 psia
Reservoir temperature: 16 0 ∘ F
Oil API gravity: 4 2 ∘
Gas-oil ratio: 600 scf/STB
Initial oil formation volume factor: 1.396 bbl/STB
Reservoir area: 640 acres
Average thickness: 10 ft
Connate water saturation: 0.25
Effective porosity: 15%
We need to calculate the original oil in place (OIIP) in STB.
Calculating Bulk Volume First, we calculate the bulk volume of the reservoir:
V b = A re a × T hi c kn ess = 640 acres × 10 ft = 6400 acre-ft
Converting Units Next, we convert the bulk volume from acre-ft to f t 3 :
V b ( f t 3 ) = V b ( a cre − f t ) × 43 , 560 ( f t 2 / a cre ) = 6400 × 43560 = 278 , 784 , 000 ft 3
Calculating Pore Volume Now, we calculate the pore volume:
V p = V b × P oros i t y = 278 , 784 , 000 ft 3 × 0.15 = 41 , 817 , 600 ft 3
Calculating Hydrocarbon Pore Volume Then, we calculate the hydrocarbon pore volume:
V H C = V p × ( 1 − Wa t er S a t u r a t i o n ) = 41 , 817 , 600 ft 3 × ( 1 − 0.25 ) = 41 , 817 , 600 × 0.75 = 31 , 363 , 200 ft 3
Calculating OIIP Finally, we calculate the original oil in place (OIIP) in STB:
O II P = O i l F or ma t i o n V o l u m e F a c t or V H C = 1.396 bbl/STB 31 , 363 , 200 ft 3 = 22 , 466 , 475.64 STB
Note that we are dividing by the oil formation volume factor to convert from reservoir barrels to stock tank barrels.
Expressing OIIP in MM STB To express the OIIP in MM STB (millions of stock tank barrels), we divide by 1 , 000 , 000 :
O II P = 1 , 000 , 000 22 , 466 , 475.64 = 22.466 MM STB
Final Answer The original oil in place (OIIP) is approximately 22.47 MM STB. Therefore, the closest answer from the options is:
None of the provided options are correct. The correct answer is approximately 22.47 MM STB.
Examples
Understanding the original oil in place (OIIP) is crucial for reservoir management and field development planning. For instance, if a reservoir engineer needs to estimate the economic viability of a new oil field, knowing the OIIP helps in forecasting production rates and ultimate recovery. This estimation guides decisions on infrastructure investment, such as drilling platforms and pipelines. Moreover, accurate OIIP calculations are essential for regulatory reporting and determining reserves, which impact a company's financial statements and investor confidence. By accurately assessing the initial resource, engineers can optimize extraction strategies and maximize the field's economic potential.
