Module 01 is the entry point for the WELLPROD™ Programme. Its four topics are not independent subjects, they are four sequential inputs to a single engineering calculation: the gas deliverability equation. Each topic establishes one term of the numerator or denominator, and the GK-22 PBL problem set is the exercise in which those four terms are assembled for the first time.
Why GK-22 is used throughout
A single consistent scenario is used for all examples, worked solutions, and the PBL problem set for two reasons. First, it builds cumulative familiarity, by Topic 1.4 you know the well intimately and can focus on the new concept rather than decoding new numbers. Second, it demonstrates integration: the numbers from Topic 1.1 feed Topic 1.2 which feeds Topic 1.3 which feeds Topic 1.4. Engineering is not a collection of independent calculations, it is a chain, and GK-22 makes that chain visible.
GK-22 is a rich gas-condensate discovery in the Gashaka field, offshore West Africa. Its complexity, HPHT conditions, Z-factor above 1.0, near-dew-point reservoir pressure, non-Darcy turbulence, and a provisional skin that masks significant uncertainty, ensures that every shortcut in the methodology is exposed and every calculation decision is consequential.
Read this first: GK-22 has two producing intervalsThe same well, two distinct datasets
GK-22 penetrates two stacked reservoirs, and this module deliberately uses both. Keep them separate whenever you carry a number forward:
GK-22 gas-condensate interval (Topics 1.1–1.2): k = 38 mD · h = 94 ft · φ = 0.21 · Swi = 0.18 · p̅ = 7,350 psia (initial) · pd = 6,200 psia · rw = 0.354 ft · 640-acre (re = 2,980 ft)
GK-22 oil leg (Topics 1.3–1.4): k = 8.2 mD · h = 53 ft · φ = 0.19 · Swc = 0.22 · p̅ = 5,651 psia · pb = 3,250 psia · rw = 0.328 ft · 580-acre (re = 2,835 ft)
The upper gas-condensate interval anchors the PVT and FVF work (single-phase gas above its dew point); the deeper oil leg anchors the permeability, relative-permeability and drainage-geometry work (an undersaturated oil above its bubble point). They are the same well but not the same rock or fluid, using the gas-condensate k or rw in an oil-leg calculation (or vice-versa) is a common and consequential error.
Learning integrationHow the four topics build to the PBL
Each topic maps 1:1 to one sub-problem in the GK-22 problem set. Complete the topic before attempting its corresponding sub-problem. The sub-problems are sequenced: SP-5 (integration) requires the outputs of SP-1 through SP-4.
1.1 PVT → SP-1: Phase Classification
1.2 FVF → SP-2: Bg & Voidage
1.3 Permeability → SP-3: keff & Banking
1.4 Geometry → SP-4: Gtotal & D·q
All topics → SP-5: AOF & Recommendation
Your learning path
1
Topic 1.1 · ~60 min · → SP-1
PVT Behaviour & Phase Envelopes
What kind of fluid is in the reservoir, and does it change phase during production?
The first decision in any well productivity calculation is choosing the correct flow model, and that decision is determined entirely by the fluid’s phase behaviour at reservoir conditions. Topic 1.1 establishes the five-fluid-type classification scheme, the p-T phase envelope, bubble-point and dew-point behaviour, and the retrograde condensate phenomenon that makes gas-condensate wells uniquely challenging over their producing life.
GK-22 context: condensate yield Rv = 112 STB/MMscf and a measured dew-point of 6,200 psia at 285°F. Initial pressure is 7,350 psia, 1,150 psia above the dew-point. You learn to recognise this as a rich gas-condensate above its dew-point at initial conditions, confirm single-phase gas flow prevails now, and predict when retrograde condensate will begin to impair krg.
Phase state: p̅ > pd → single-phase gas → Darcy gas deliverability applies
Phase state: p̅ < pd → two-phase (gas + condensate) → modified model required
GK-22 at initial: p̅ = 7,350 > pd = 6,200 psia → SINGLE-PHASE GAS ✓
p-T phase envelopeFive-fluid classificationBubble vs dew-pointRetrograde condensateCricondenthermInflow model selection
2
Topic 1.2 · ~75 min · → SP-2
Formation Volume Factor (B)
How much reservoir volume does each surface barrel actually represent?
Every production calculation involves two measurement worlds: the reservoir (high pressure and temperature, compressed fluids) and the surface (standard conditions). The Formation Volume Factor is the conversion between them. Topic 1.2 derives Bg from the real gas law, explains why the gas deviation factor Z cannot be assumed equal to 1.0 at GK-22 conditions, and introduces the p/Z material balance.
GK-22 context: at 7,350 psia and 285°F, Tpr = 2.006 and ppr = 11.01. Standing-Katz returns Z = 1.40, greater than 1.0 because molecular repulsion dominates at extreme pressure. Assuming Z = 1 gives Bg = 0.510 instead of 0.714 res bbl/Mscf, a 40% error that propagates straight into AOF.
Bg = 0.005035 × Z × T / p [res bbl/scf] (T in °R, p in psia)
GK-22: Bg = 0.005035 × 1.40 × 745 / 7,350 = 0.000714 res bbl/scf
Voidage at DST: qvoid = 12,400 × 0.714 = 8,854 res bbl/d
Real gas lawZ-factor & Standing-KatzPseudo-critical propertiesBg calculationOil FVF (Bo)p/Z material balance
3
Topic 1.3 · ~60 min · → SP-3
Permeability & Relative Permeability
Which permeability value enters the flow equation, and what happens to it over field life?
Permeability is never used raw in a multiphase-saturated reservoir. Topic 1.3 establishes the absolute / relative / effective hierarchy and explains why keff = k × krg(Swi) is always the correct value for the flow equation, then addresses the critical long-term risk: how retrograde condensate banking below the dew-point progressively destroys krg.
GK-22 context: core gives k = 38 mD and SCAL gives krg(Swi) = 0.94, so initial keff = 35.7 mD. The real risk is not the initial 6% reduction but the 60–70% krg impairment that develops as condensate builds when pressure crosses 6,200 psia.
keff = k × krg(Swi) = 38 × 0.94 = 35.7 mD [use keff, never k]
Banking: krg 0.94 → 0.35 below dew-point → AOF falls to 37% of initial
Absolute vs effective vs relativeEnd-point krg(Swi)Core analysis & SCALRel-perm curvesCondensate banking
4
Topic 1.4 · ~75 min · → SP-4
Drainage Geometry
How does the drainage geometry resist flow, and what role does turbulence play at high gas rates?
Darcy’s radial flow equation integrates to a geometric denominator G = ln(re/rw) − ¾ + S. Topic 1.4 derives this factor, explains the pseudo-steady-state correction, and introduces the non-Darcy (Forchheimer) turbulence term D·q that adds a rate-dependent apparent skin, making the deliverability equation implicit in rate and requiring iteration.
GK-22 context: with re = 2,980 ft and rw = 0.354 ft, ln(8,418) = 9.038. At the DST rate of 12.4 MMscfd the non-Darcy skin D·q = 4.34, 74% larger than the mechanical skin of +2.5. A 30% uncertainty in re changes G by only 3%, showing why skin control matters far more than well spacing for PI.
Gbasic = ln(re/rw) − ¾ + S = 9.038 − 0.750 + 2.500 = 10.788
Gtotal = Gbasic + D·q = 10.788 + 4.340 = 15.128 [at DST rate]
Ignoring D·q overestimates AOF by 15.13/10.79 − 1 = 40%
Radial flow derivationPSS vs steady-stateShape factorsSkin & geometric factorNon-Darcy flowre sensitivity
★
Capstone · ~4 hrs · → SP-5
Integration & the GK-22 deliverability recommendation
Once all four topics are complete, launch the GK-22 PBL hub. Five sequenced sub-problems take you from fluid classification through to a full FEED-gate engineering recommendation, assembling every parameter from Topics 1.1–1.4 into the complete gas deliverability equation. SP-5 produces the Absolute Open Flow, condensate rate, reservoir voidage, IGIP estimate, and sensitivity study that form the core of the deliverable.
Module learning outcomes
By the end of Module 01 you will be able to…
- Classify any reservoir fluid from PVT data, locate its initial state on a p-T phase envelope, and select the correct inflow model with a written justification.
- Derive Bg from the real gas law using Z from pseudo-critical properties, and apply it to convert surface rates to reservoir voidage and anchor a p/Z material balance.
- Calculate effective permeability keff = k × krg(Swi) and defend why absolute permeability must never enter a flow equation for a multiphase-saturated reservoir; estimate banking impairment below dew-point.
- Compute the geometric denominator Gtotal for a circular drainage area, applying the PSS correction and non-Darcy turbulence skin correctly, and assess AOF sensitivity to each component.
- Assemble the full deliverability equation from SP-1–SP-4 carry-forward values, solve iteratively for AOF at a specified minimum flowing BHP, and derive condensate rate, voidage and IGIP from p/Z analysis.
- Run a six-variable sensitivity study, identify the parameters that dominate the P10/P50/P90 range, and write a concise FEED-gate recommendation distinguishing theoretical AOF from a defensible design rate.
Integration reference, how each topic feeds the deliverability equation
The gas deliverability equation is the single destination toward which all four topics converge. The table maps each topic to its contribution, the GK-22 value it establishes, and the sub-problem that uses it.
qg [Mscfd] = keff · h · (p̅² − pwf²) / [1,424 · μg · Z · T · Gtotal]
| Topic | Equation term | What it provides | GK-22 value | Used in |
|---|---|---|---|---|
| 1.1 PVT Behaviour | Equation form | Confirms single-phase gas deliverability applies (p̅ > pd); rules out Vogel IPR and two-phase models | p̅ = 7,350 > pd = 6,200 psia | SP-1 → SP-5 |
| 1.2 Formation Volume Factor | Z, μg, Bg | Z validates the denominator; Bg converts surface rate to voidage; p/Z anchors material balance | Z = 1.40; Bg = 0.714; p̅/Z = 5,250 psia | SP-2 → SP-5 |
| 1.3 Permeability | keff · h | Effective permeability to gas at connate water; banking impairment forecast for field life | keff = 35.7 mD; krg → 0.35 below pd | SP-3 → SP-5 |
| 1.4 Drainage Geometry | Gtotal | Complete geometric denominator including mechanical skin and non-Darcy turbulence | Gbasic = 10.788; D·q = 4.34; Gtotal = 15.13 | SP-4 → SP-5 |
| SP-5 Integration | qg,AOF | Iterative AOF at pwf,min = 3,500 psia; condensate rate; voidage; IGIP; sensitivity | AOF ≈ 17.7 MMscfd; condensate ≈ 1,980 STB/d | FEED gate deliverable |
Suggested study schedule
Session A: Fluid properties (~3 hrs)
Topic 1.1 then Topic 1.2, then attempt SP-1 and SP-2. These two sub-problems are self-contained. Compare your carry-forward values against the guided answers before proceeding.
Session B: Rock & geometry (~3 hrs)
Topic 1.3 then Topic 1.4, then SP-3 and SP-4. Pay attention to the condensate banking analysis in SP-3 and the non-Darcy skin comparison in SP-4, the two most commonly misunderstood concepts.
Session C: Integration & PBL (~4 hrs)
With all four carry-forward values confirmed, complete SP-5: ~60 min calculation, 30 min memo, 30 min debrief. The facilitator guide lists the three most common cohort errors.
Before you start Module 02
Confirm from memory: the GK-22 AOF (MMscfd), the dew-point threshold (psia), the dominant sensitivity variable (k), and why Vogel is inapplicable. These recur throughout Course 01.
Reference
GK-22 gas-condensate interval (Topics 1.1–1.2). The oil leg used in Topics 1.3–1.4 has its own parameter set (k = 8.2 mD, h = 53 ft, φ = 0.19, p̅ = 5,651 psia).
Reservoir & rock: k = 38 mD (absolute) · krg(Swi) = 0.94 · keff = 35.7 mD · h = 94 ft · φ = 0.21 · Swi = 0.18 · re = 2,980 ft (640-acre) · rw = 0.354 ft
Conditions: p̅ = 7,350 psia (initial) · T = 285°F (745°R) · pd = 6,200 psia · Tcric = 320°F · S = +2.5 (provisional)
PVT: γg = 0.65 · Z = 1.40 at p̅ · μg = 0.0312 cp · Bg = 0.000714 res bbl/scf · Rv = 112 STB/MMscf (52°API condensate)
DST summary: qg = 12.4 MMscfd · pwf,test = 6,850 psia · qc = 1,390 STB/d · voidage = 8,860 res bbl/d · D = 0.00035 d/Mscf · pwf,min = 3,500 psia
Reservoir & rock: k = 38 mD (absolute) · krg(Swi) = 0.94 · keff = 35.7 mD · h = 94 ft · φ = 0.21 · Swi = 0.18 · re = 2,980 ft (640-acre) · rw = 0.354 ft
Conditions: p̅ = 7,350 psia (initial) · T = 285°F (745°R) · pd = 6,200 psia · Tcric = 320°F · S = +2.5 (provisional)
PVT: γg = 0.65 · Z = 1.40 at p̅ · μg = 0.0312 cp · Bg = 0.000714 res bbl/scf · Rv = 112 STB/MMscf (52°API condensate)
DST summary: qg = 12.4 MMscfd · pwf,test = 6,850 psia · qc = 1,390 STB/d · voidage = 8,860 res bbl/d · D = 0.00035 d/Mscf · pwf,min = 3,500 psia
Misconception 1“Single-phase gas flow means krg = 1.0”
Single-phase gas (p̅ > pd) means no free condensate is present, not that the rock is 100% gas-saturated. Connate water at Swi = 0.18 always reduces krg below 1.0. The correct permeability is always k × krg(Swi); using k instead overestimates AOF by 6.4% at initial conditions and establishes a wrong methodology that cannot accommodate later banking analysis.
Misconception 2“Z = 1 is a safe approximation for high-pressure gas”
At ppr < 4, Z is near 1.0. At GK-22 conditions (ppr = 11.01, Tpr = 2.006), Z = 1.40, 40% above ideal. Assuming Z = 1 underestimates Bg and voidage by 40% and overestimates AOF by ≈40%. At very high reduced pressures, repulsive forces dominate, so the gas is less compressible than ideal and Z must exceed 1.0.
Misconception 3“The Vogel equation is standard for gas-condensate wells”
Vogel’s (1968) equation was derived for solution-gas-drive oil reservoirs below bubble point. A single-phase gas reservoir above its dew-point has a linear relationship between q and (p̅² − pwf²): there is no Vogel curvature, and applying it misrepresents the inflow.
Misconception 4“±30% drainage-radius uncertainty makes the AOF unreliable”
Because re sits inside a logarithm, its uncertainty is heavily compressed: a 30% reduction changes Gbasic by ~3.2% and AOF by ≈3%. The dominant uncertainties are permeability, net pay, skin and Z-factor: drainage radius is the least important despite being perceived as large.
Required
Mathematics: natural logarithm (ln), basic algebra, unit conversion (psia, °R, mD, res bbl). No calculus, only the integrated form of Darcy’s law is used.
Required
Course 01 introduction: review the Programme Overview and the Well Productivity Framework before beginning Module 01.
Helpful
Chemistry / physics background: familiarity with the ideal gas law (pV = nRT) helps with Topic 1.2; basic fluid mechanics supports Topic 1.4. Neither is strictly required, both are developed from first principles.