Horizontal Flowline — Pressure Traverse to the Facility: 24-Pet-B2 Oil and Gas Evaluation and Economics, December 2015, Q4

Question 4: Horizontal Flowline — Pressure Traverse to the Facility (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.

Given. $D=2.5$ in, $L=10{,}000$ ft, horizontal; $q_o=800$ STBD; GOR $=4000$ SCF/STB; $p_{wh}=1000$ psia; horizontal flowing-pressure-gradient traverse curve for 800 STBD/2.5 in provided.

Find. The pressure at the production facility (downstream end of the flowline).

Approach. This is a graphical multiphase-flow-correlation read, not a closed-form calculation — enter the given horizontal traverse curve (Gilbert/Beggs-Brill-family gradient curves, specific to this rate, GOR and pipe size) at the upstream pressure and length, and read off the corresponding downstream pressure directly.

Fig. 1 — Horizontal flowline schematic. The exam source itself annotates this chart with the worked reading (“the pressure traverse gives surface facility pressure to be 650 psia”), so the facility pressure is taken directly from that reading rather than re-derived from a single-phase formula, which would not capture the two-phase (oil + 4000 SCF/STB solution/free gas) flow behaviour in the line.

Read the traverse curve. Entering the given 800 STBD, 2.5 in horizontal gradient curve at $p_{wh}=1000$ psia and following it out to $L=10{,}000$ ft: $\boxed{p_{facility}=650\ \text{psia}}$.

Quantity	Value
Pressure at the production facility	650 psia

Check: the facility pressure is a direct graphical read off the supplied horizontal flowing-pressure-gradient curve (as the source itself confirms), not an independently computed value — multiphase pressure-traverse correlations (Beggs-Brill, Hagedorn-Brown, etc.) require iterative, chart-embedded friction/holdup correlations that are exactly what the provided curve already encodes for this specific rate, GOR, and pipe size.

Topic: Multiphase pressure-traverse (gradient-curve) analysis for surface flowlines.

Key relations: Two-phase pressure drop $=\Delta p_{elevation}+\Delta p_{friction}+\Delta p_{acceleration}$, with two-phase friction/holdup obtained from empirical correlations (Beggs-Brill, Gilbert) that are pre-packaged into rate/GOR/diameter-specific gradient curves.

Why this works: Multiphase flow behaviour (liquid holdup, flow regime, friction factor) is far too complex for a simple single-phase Darcy-Weisbach calculation; published gradient curves pre-solve the governing correlations for a fixed rate, GOR and pipe size so the engineer only needs to look up the answer.

Common pitfall: Applying a single-phase gas or liquid pressure-drop formula to a line carrying both oil and a large associated-gas rate (4000 SCF/STB here is well above solution GOR at these pressures, so free gas is present) — the flow regime and pressure gradient are then qualitatively different from either single-phase case.

Source: Beggs, Production Optimization Using Nodal Analysis; Brown, The Technology of Artificial Lift Methods, Vol. 1 (gradient curves).

Question 4 of 7: Horizontal Flowline — Pressure Traverse to the Facility

Question 4: Horizontal Flowline — Pressure Traverse to the Facility (20 marks)