Separator Test and Flash/Differential PVT Conversion: 24-Pet-A2 Petroleum Reservoir Fluids, December 2019, Q7

Question 7: Separator Test and Flash/Differential PVT Conversion (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Separator test

Given. $V_{o,b}=201.156$ cc (saturated oil, res. T); $V_{sep,liq}=150.833$ cc (200 psig/75°F); $V_{stb}(75\,{}^{\circ}\text{F})=136.591$ cc; $V_{stb}(60\,{}^{\circ}\text{F})=135.641$ cc; $G_{sep}=0.51383$ scf; $G_{stb}=0.15186$ scf.

Separator test process train: saturated reservoir oil is flashed through a single separator stage, then the separator liquid shrinks further to stock-tank oil; gas is released at both stages.

Approach. Every ratio below is expressed per unit stock-tank oil volume (the STB reference), converting the lab's cc-scale sample volumes to a "per barrel" basis via $1$ bbl $=158{,}987.3$ cc.

(i) $B_{ofb}$, oil FVF at the separator/bubble-point condition. $$B_{ofb} = \frac{V_{o,b}}{V_{stb}(60\,{}^{\circ}\text{F})} = \frac{201.156}{135.641} = \boxed{1.4830\ \text{bbl/STB}}$$
(ii) Total GOR at separator pressure. $$R_{s,total} = (G_{sep}+G_{stb})\times\frac{158{,}987.3}{135.641} = 0.66569\times1172.16 = \boxed{780\ \text{scf/STB}}$$
(iii) Separator GOR. $$R_{sep} = G_{sep}\times\frac{158{,}987.3}{150.833} = \boxed{542\ \text{scf/SP bbl}}$$
(iv) Stock-tank GOR. $$R_{stb} = G_{stb}\times\frac{158{,}987.3}{135.641} = \boxed{178\ \text{scf/ST bbl}}$$

Quantity	Value
(i) $B_{ofb}$	1.4830 bbl/STB
(ii) Total GOR	780 scf/STB
(iii) Separator GOR	542 scf/SP bbl
(iv) Stock-tank GOR	178 scf/ST bbl

(b) Converting differential data to flash (field) units at 4500 and 1600 psig

Given. From the sample's CCE relative-volume table (bubble point 2620 psig): $V/V_{sat}(4500\text{ psig})=0.9703$. From the differential-liberation table at the bubble point: $B_{odb}=1.600$, $R_{sdb}=854$ scf/STB; at 1600 psig: $B_{od}=1.445$, $R_{sd}=544$ scf/STB. From part (a): $B_{ofb}=1.4830$ bbl/STB, $R_{sfb}=780$ scf/STB.

Approach. Above the bubble point, undersaturated liquid compression is identical whether or not gas has been removed, so $B_o$ scales directly with the CCE relative-volume ratio and $R_s$ stays fixed at $R_{sfb}$. Below the bubble point, the standard McCain/Craft&Hawkins conversion rescales the residual-oil-basis differential data onto the flash (STB) basis using the ratio $CF=B_{ofb}/B_{odb}$.

Conversion factor. $$CF = \frac{B_{ofb}}{B_{odb}} = \frac{1.4830}{1.600} = 0.9269$$
At 4500 psig (above $P_b=2620$ psig): undersaturated, use the CCE relative-volume ratio directly. $$B_o(4500) = B_{ofb}\times\frac{V}{V_{sat}}(4500) = 1.4830\times0.9703 = \boxed{1.439\ \text{bbl/STB}}$$ $$R_s(4500) = R_{sfb} = \boxed{780\ \text{scf/STB}}\quad(\text{no gas evolves above } P_b)$$
At 1600 psig (below $P_b$): differential-to-flash conversion. $$B_o(1600) = B_{od}(1600)\times CF = 1.445\times0.9269 = \boxed{1.339\ \text{bbl/STB}}$$ $$R_s(1600) = R_{sfb} - (R_{sdb}-R_{sd}(1600))\times CF = 780 - (854-544)(0.9269) = \boxed{493\ \text{scf/STB}}$$

Pressure	$B_o$, bbl/STB	$R_s$, scf/STB
4500 psig	1.439	780
1600 psig	1.339	493

Topic. Converting differential-liberation PVT data (residual-oil basis) to field (flash/separator) units using a single-stage separator test.

Key relations. Above $P_b$: $B_o(p)=B_{ofb}\times[V/V_{sat}](p)$, $R_s(p)=R_{sfb}$. Below $P_b$: $B_o(p)=B_{od}(p)\cdot CF$, $R_s(p)=R_{sfb}-(R_{sdb}-R_{sd}(p))\cdot CF$, where $CF=B_{ofb}/B_{odb}$.

Why this works. A differential liberation test removes gas at every step and reports volumes on a residual-oil basis, which does not match how gas is actually released in the reservoir/wellbore/separator system (closer to a single flash); the conversion factor $CF$ rescales the differential data's "1 unit of residual oil" onto the true field STB, calibrated at the one point (the bubble point) where both a flash (separator) and a differential measurement exist for the same fluid.

Common pitfall. Applying the $CF$ scaling above the bubble point (it isn't needed there – a single liquid phase compresses identically regardless of test type) or forgetting that $R_s$ is constant (equal to $R_{sfb}$) for all pressures above $P_b$.

Source. Craft & Hawkins, Applied Petroleum Reservoir Engineering, Ch.2 (differential-to-flash data conversion); McCain, Properties of Petroleum Fluids Ch.5.

Question 7 of 7: Separator Test and Flash/Differential PVT Conversion

Question 7: Separator Test and Flash/Differential PVT Conversion (20 marks)

(a) Separator test

(b) Converting differential data to flash (field) units at 4500 and 1600 psig