What You Will Do in This PBL
This PBL consolidates all four topics of Module 01 — PVT Behaviour & Phase Envelopes, Formation Volume Factor, Permeability & Relative Permeability, and Drainage Geometry, by applying them to a single authentic gas-condensate appraisal scenario. The well is GK-22 in the Gashaka field, a rich gas-condensate discovery offshore West Africa. You will work through five discrete, sequenced sub-problems, each mapped to a Module 01 topic, then integrating your results into a field recommendation.
Learning Objectives
By the end of this PBL you will be able to:
- Classify the GK-22 reservoir fluid from PVT data, locate the initial reservoir state on a p-T phase envelope, and confirm whether single-phase gas deliverability or a two-phase model applies at initial conditions.
- Derive Bg from the real gas law using Z-factor and pseudo-critical properties; convert surface gas rate to reservoir voidage in res bbl/d.
- Determine effective permeability keff = k·krg(Swi) for the gas phase and defend why the absolute permeability k must not be used in the flow equation.
- Compute the full geometric factor G = ln(re/rw) − ¾ + S for a circular drainage area and apply the non-Darcy (turbulence) skin correction D·q for a high-rate gas well.
- Assemble the pseudo-steady-state gas deliverability equation, calculate AOF at pwf,min, and derive condensate rate and voidage at that AOF.
- Construct a p/Z depletion plot from the provided data, estimate IGIP, and comment on drive mechanism.
- Run a six-variable sensitivity study on k, h, Z, re, rw, and S; identify which parameters dominate the AOF uncertainty.
- Present a written FEED-gate engineering recommendation that distinguishes theoretical AOF from a defensible design rate, quantifies uncertainty, and flags the condensate banking risk.
Why GK-22 Is More Complex Than a Simple Oil PI Problem
A classic oil PI scenario is well-structured: identify fluid state, pick Bo, read k from core, calculate ln(re/rw), assemble. GK-22 introduces genuine engineering complexity at every step:
- Phase classification matters: the fluid sits just 1,150 psia above its dew-point. The wrong flow model (Vogel instead of single-phase gas deliverability) produces an incorrect AOF. SP-1 makes this explicit.
- Z > 1 at high pressure: GK-22 operates at 7,350 psia where the gas compressibility factor is 1.40, greater than ideal. Engineers who assume Z ≈ 1 underestimate Bg by 40%. SP-2 quantifies this.
- Condensate banking is a long-term risk: once p̅ falls below the dew-point (6,200 psia), retrograde condensate accumulates near the wellbore and krg collapses from 0.94 toward 0.30–0.45. SP-3 analyses the implication.
- Non-Darcy skin is significant at high gas rates: at 12.4 MMscfd the turbulence skin D·q = 4.34, larger than the mechanical skin S = 2.5. Ignoring it overstates AOF by ~30%. SP-4 develops the correction.
- Integration requires iterative thinking: the AOF feeds back into D·q, making the deliverability equation mildly nonlinear. SP-5 guides you through this and produces the final recommendation.
Gashaka Field Appraisal Memo: GK-22
Well GK-22 has successfully completed a short-duration DST on the Lower Gashaka Sand (LGS). The Gashaka FEED gate closes in 48 hours and the FPSO facilities team requires engineering input before the separator train specification is locked. Please provide the following from the attached PVT report and core analysis:
- Confirmation of fluid type, is the LGS fluid a dry gas, wet gas, rich gas-condensate, or volatile oil? Justify from the phase envelope data.
- Absolute Open Flow (AOF) in MMscfd using the pseudo-steady-state Darcy deliverability equation with non-Darcy correction, evaluated at pwf,min = 3,500 psia (subsea tree rating limit).
- Associated condensate production rate in STB/d at the AOF gas rate.
- Reservoir voidage rate in res bbl/d required for the aquifer assessment and material balance.
- A six-parameter sensitivity table (±30% on k, h, re; ±1 bar on Z; ±2 units on S) so the facilities team understand the P10/P50/P90 range on separator sizing.
- A p/Z depletion projection using the initial data point and two further pressure nodes with IGIP estimate and commentary on whether early pressure support is warranted.
Note: The DST was shut in before pseudo-steady state was established. Use core-derived k and log-derived h as primary inputs. DST-derived skin should be treated as provisional pending full PTA include in sensitivity.
Field & Well Context
The Gashaka field is a Cretaceous sandstone gas-condensate accumulation in ~1,200 m water depth on the West African margin. The Lower Gashaka Sand (LGS) is the primary reservoir: a clean, high-net-to-gross aeolian sandstone with excellent reservoir quality. GK-22 is the second appraisal well following the GK-18 discovery. The field will be developed via a subsea production system tied back to the Gashaka FPSO, with gas export via 24" pipeline and condensate stabilised and loaded by tanker.
- Reservoir TVD: 4,820 m (15,810 ft) · Cretaceous aeolian sandstone · N:G ≈ 0.88
- Drive mechanism: suspected volumetric gas expansion, no OWC in either appraisal well
- FPSO export: Gas pipeline arrival pressure 1,200 psia · Condensate stabilisation to RVP < 12 psi
- Hydrate risk: seabed temperature ~5°C (41°F), MEG injection required; pwf,min set partly by hydrate avoidance margin
Engineering Workflow: Five Steps to the FEED Gate
- SP-1 · PVTClassify fluid & verify phase state at p̅
- SP-2 · FVFCalculate Bg, voidage & p/Z
- SP-3 · PERMDetermine keff & banking risk
- SP-4 · GEOMCompute G with D·q skin
- SP-5 · INTEGAssemble AOF, sensitivity & recommendation
Well GK-22: Complete Data Pack
All data below are consolidated from the GK-22 drilling report, wireline log interpretation, core analysis (routine + SCAL), and DST fluid sample PVT report. This is the single source of truth for all five sub-problems. Each SP highlights the specific rows relevant to its step, do not use data from a later SP ahead of time.
Rock & Reservoir Parameters (Core + Logs + DST Horner)
| Parameter | Symbol | Value | Units | Source & Confidence |
|---|---|---|---|---|
| Initial average reservoir pressure | p̅ | 7,350 | psia | DST Horner plot: high confidence |
| Reservoir temperature | Tres | 285 | °F (745 °R) | MDT gauge, ±2°F |
| Net pay thickness (LGS) | h | 94 | ft | Neutron-density log, Vsh < 0.15 cut-off |
| Absolute permeability | k | 38 | mD | Routine core: arithmetic mean, 12 plugs (±40%) |
| Porosity (log-derived) | φ | 0.21 | fraction | Density log, ρma = 2.65 g/cc |
| Irreducible water saturation | Swi | 0.18 | fraction | SCAL centrifuge capillary pressure |
| krg at Swi (end-point) | krg(Swi) | 0.94 | — | Steady-state gas/brine core flood (SCAL) |
| Drainage radius (appraisal spacing) | re | 2,980 | ft | 640-acre provisional well spacing |
| Wellbore radius | rw | 0.354 | ft | 9⅝" production casing ID |
| Mechanical skin (DST provisional) | S | +2.5 | — | DST radial flow analysis: provisional; PTA pending |
| Non-Darcy coefficient | D | 0.00035 | d/Mscf | Jones correlation (estimated: not measured) |
| Min. flowing BHP (subsea tree) | pwf,min | 3,500 | psia | Tree rating + hydrate avoidance margin |
PVT Laboratory Data (DST Recombined Sample, 285 °F)
| Parameter | Symbol | Value | Units | Note |
|---|---|---|---|---|
| Fluid classification | — | Rich gas-condensate | — | To be confirmed in SP-1 |
| Initial dew-point pressure | pd | 6,200 | psia | CCE experiment: p-V break at 285°F |
| Cricondentherm | Tcric | 320 | °F | Phase envelope maximum temperature vertex |
| Gas specific gravity | γg | 0.65 | air = 1 | Recombined wellstream at separator conditions |
| Z-factor at p̅ = 7,350 psia | Z | 1.40 | — | Standing-Katz chart: Tpr = 2.006, ppr = 11.01 |
| Gas viscosity at p̅ | μg | 0.0312 | cp | Lee–Kesler correlation at 7,350 psia, 285°F |
| Initial condensate yield (CVD) | Rv | 112 | STB/MMscf | Separator test: 52°API condensate |
| Condensate API gravity | °APIc | 52 | °API | Light white condensate: S.G. = 0.771 |
| Margin above dew-point at p̅ | Δpd | +1,150 | psia | p̅ − pd = 7,350 − 6,200 · single-phase gas initially |
Derived Parameters (Calculated: Verify in SPs)
| Parameter | Symbol | Value | Units | How Derived |
|---|---|---|---|---|
| Gas pseudo-critical temperature | Tpc | 371.3 | °R | Standing: 187 + 330(0.65) − 71.5(0.65)² — derive fully in SP-2 |
| Gas pseudo-critical pressure | ppc | 667.7 | psia | Standing: 706 − 51.7(0.65) − 11.1(0.65)² — derive in SP-2 |
| Pseudo-reduced temperature | Tpr | 2.006 | — | 745 / 371.3: derive in SP-2 |
| Pseudo-reduced pressure | ppr | 11.01 | — | 7,350 / 667.7: derive in SP-2 |
| Gas FVF at p̅ | Bg | 0.000714 | res bbl/scf | 0.005035 × Z × T / p: derive fully in SP-2 |
| Effective permeability (gas) | keff | 35.7 | mD | k × krg(Swi) = 38 × 0.94: derive in SP-3 |
| Basic geometric factor | Gbasic | 10.788 | — | ln(2,980/0.354) − 0.75 + 2.5: derive in SP-4 |
| Non-Darcy skin at DST rate | D·qDST | 4.34 | — | 0.00035 × 12,400 Mscfd: derive in SP-4 |
| Total geometric factor at DST | Gtotal | 15.13 | — | Gbasic + D·q = 10.788 + 4.34: derive in SP-4 |
| Initial p/Z | p̅/Z | 5,250 | psia | 7,350 / 1.40, anchor point for p/Z plot in SP-5 |
KWL Planner: Activate Prior Learning
Spend 10 minutes completing the KWL table below before opening any sub-problem. The K column surfaces hidden assumptions, teams frequently discover they have not thought carefully about what "single-phase gas" really means for a flow equation. The W column is where the real learning opportunity lives. Leave the L column completely blank at the start. It is filled in after each SP as you complete it.
From Topics 1.1–1.4 and your engineering experience:
- Gas-condensates have dew-point pressures, not bubble points
- Bg comes from the real gas law: Bg = 0.005035 ZT/p
- keff = k × krg(Swi) for gas at initial water saturation
- PSS geometric factor: ln(re/rw) − ¾ + S
- Gas wells experience non-Darcy turbulence at high rates
- p/Z vs. Gp is a straight line for a volumetric gas reservoir
What is still uncertain before you begin?
- Is p̅ = 7,350 psia above or below the dew-point, and what flow model does that mandate?
- Why is Z = 1.40 (greater than 1) and what does it mean for the Bg calculation?
- Should we use krg = 0.94 or 1.0? Does 6% matter in a field decision?
- How large is the D·q skin compared to the mechanical skin of +2.5?
- What happens to krg once p̅ drops below the dew-point of 6,200 psia?
- How do we estimate IGIP from a single p/Z data point?
Complete after each SP: leave blank now
- After SP-1: _______________
- After SP-2: _______________
- After SP-3: _______________
- After SP-4: _______________
- After SP-5: _______________
Think-Pair-Share: Hub Opening Questions
Before launching SP-1, discuss these two questions (3 min each). Do not look up answers yet. Articulate your current thinking:
GK-22 has p̅ = 7,350 psia and dew-point pd = 6,200 psia. The cricondentherm is 320°F and Tres = 285°F. A junior engineer says: "I'll use the Vogel equation, it's the standard for gas-condensate wells." Is this correct? What does single-phase gas above the dew-point mean for the inflow equation? What changes when p̅ eventually crosses 6,200 psia?
Core analysis gives k = 38 mD and krg(Swi) = 0.94 → keff = 35.7 mD. Another engineer says: "The difference is only 6%, within measurement uncertainty. Just use 38 mD and skip the step." Is this argument ever valid? What principle is violated if you use absolute permeability in a system where connate water is present?
Sub-Problems: Launch Sequence
Each sub-problem is a self-contained learning unit with its own problem narrative, highlighted data slice, step-by-step task list, guided calculations, knowledge check (MCQ), and a carry-forward box. Complete them in order. SP-5 requires your carry-forward values from SP-1 through SP-4.
Use the p-T phase envelope, dew-point pressure pd = 6,200 psia, and condensate yield Rv = 112 STB/MMscf to classify the GK-22 fluid. Confirm whether the reservoir is in single-phase gas or two-phase gas+condensate at p̅ = 7,350 psia and justify the correct inflow model. Identify the dew-point threshold that will trigger retrograde condensate banking during field life.
Derive pseudo-critical properties from γg = 0.65 using Standing correlations. Calculate Tpr and ppr, verify Z = 1.40 from the Standing-Katz chart, then compute Bg in res bbl/scf and res bbl/Mscf. Convert the DST rate of 12.4 MMscfd to reservoir voidage in res bbl/d and condensate rate in STB/d. Build a 3-row p/Z table at 7,350, 5,000, and 3,500 psia.
Calculate effective permeability keff = k × krg(Swi) = 38 × 0.94 = 35.7 mD and defend why absolute k must not be used. Critically evaluate the "6% doesn't matter" argument. Then analyse the condensate banking scenario: estimate how krg deteriorates as p̅ falls below the dew-point to 5,500 psia and assess the deliverability impairment factor at late field life.
Calculate Gbasic = ln(re/rw) − ¾ + S = 10.788 for the 640-acre drainage area. Apply the non-Darcy skin correction S′ = D·q at the DST rate (12.4 MMscfd) to obtain Gtotal = 15.13. Quantify how much turbulence costs the AOF relative to mechanical skin alone. Evaluate the PI benefit of stimulation (S → 0) versus rate restriction.
Assemble all SP-1 through SP-4 carry-forward values into the full gas deliverability equation. Solve for AOF at pwf,min = 3,500 psia. Calculate condensate rate and voidage at AOF. Complete the p/Z plot and estimate IGIP. Run the six-variable sensitivity study and write the FEED-gate engineering recommendation memo, including the condensate banking risk caveat.
Delivery Map & Assessment
This PBL is designed for a single half-day facilitated workshop or two self-paced online sessions with a synchronous debrief. Total learner effort is approximately 4 hours excluding the facilitated debrief.
| Stage | Mode | Duration | Assessment |
|---|---|---|---|
| Hub: Field brief + data pack | Individual read | 15 min | — |
| Hub: KWL + Think-Pair-Share | Team (2–3 learners) | 10 min | KWL sheet submitted |
| SP-1: Fluid classification & phase state | Individual → team check | 30 min | 5 MCQ auto-marked + carry-forward confirmed |
| SP-2: Bg, voidage, p/Z table | Individual → team check | 25 min | 5 MCQ auto-marked + carry-forward confirmed |
| SP-3: keff & banking analysis | Individual → team check | 25 min | 5 MCQ auto-marked + carry-forward confirmed |
| SP-4: Geometry + non-Darcy | Individual → team check | 25 min | 5 MCQ auto-marked + carry-forward confirmed |
| SP-5: AOF, sensitivity, recommendation | Team then individual write-up | 60 min | Recommendation memo + sensitivity table |
| Facilitated debrief | Tutor-led plenary | 30 min | Participation + L-column completion |
Final Deliverable: FEED-Gate Engineering Note
At the end of SP-5, each engineer submits a one-page FEED-gate note (max 500 words + one results table) covering: (1) confirmed fluid classification; (2) AOF in MMscfd at pwf,min = 3,500 psia; (3) condensate rate at AOF in STB/d; (4) reservoir voidage at AOF in res bbl/d; (5) IGIP estimate from p/Z; (6) six-variable sensitivity table (P10/P50/P90 AOF); (7) recommended design rate with justification; (8) condensate banking risk flag and recommended mitigation. This is assessed against the SP-5 marking rubric by the facilitator.