Module 03 Introduction — Deconstructing Skin Factor: The GK-22 Well Performance Crisis

The GK-22 investigation

The Gashaka GK-22 well was drilled and completed 14 months ago in the Gashaka field, onshore Niger Delta. It was drilled with a water-based mud (WBM) system through the Agbada Formation reservoir sands and completed with an overbalanced cased-hole perforating programme (4 spf, 0° phasing). A drill-stem test after completion confirmed reservoir permeability of 85 md across 42 ft of net pay.

The well has been producing at 782 stb/d against a flowing pressure of 2,500 psi — significantly below what the reservoir quality would suggest. A pressure build-up test analysed by the Horner method returned a total skin of S′ = +14.

The asset team has requested a complete well-performance investigation: (1) diagnose what is causing the skin, (2) calculate every skin component to confirm the attribution, (3) design the optimal treatment programme, (4) quantify the production uplift and economic value, and (5) determine whether sand control is required and at what cost to productivity.

The investigation must culminate in a formal Well Performance Report with a recommendation to the Asset Leadership Team for a go/no-go decision on the intervention.

Learning integrationHow the six topics map to the six sub-problems

Each sub-problem is anchored to one Module 03 topic. Complete the topic before attempting its sub-problem. The sub-problems build sequentially — each answer feeds the next, and all six are required for the Well Performance Report.

3.1 Skin & PI → SP-1: Baseline 3.2 Total skin S′ → SP-2: Turbulence check 3.3 Formation damage → SP-3: Hawkins anatomy 3.4 Pseudo-skin → SP-4: Geometric check 3.5 Flow efficiency → SP-5: Forecast & economics 3.6 Sand control → SP-6: Sand decision & audit

Your learning path

Topic 3.1 · Skin Factor Concept · → SP-1

Establishing the baseline — the well’s true potential

What would GK-22 produce if there were no skin?

Before any treatment decision, establish the undamaged baseline. Calculate J_ideal, the actual PI, the production lost to skin, and the pressure wasted at the wellbore, then judge whether the current operating point is economic.

J_ideal = 0.00708·k·h / [μ·B·(ln(r_e/r_w) − 0.75)] = 1.380 stb/d/psi FE = 7/(7+S) = 7/(7+14) = 0.333 → J_actual = 0.460 stb/d/psi Δp_skin ≈ 1,038 psi (61% of the 1,700 psi drawdown wasted on skin)

Tasks: (a) J_ideal (S = 0) from Darcy radial flow; (b) J_actual from measured q and drawdown, verify J_ideal×FE with FE = 7/(7+14); (c) undamaged rate q_ideal at p_wf = 2,500 psi; (d) Δp_skin as a fraction of total drawdown; (e) the J ratio and what it means for the asset.

Darcy radial flowProductivity indexSkin–PI relationshipPressure-drop decomposition

Open Topic 3.1 →

Topic 3.2 · Total Skin S′ = S + Dq · → SP-2

Rate-dependent skin check — is any of the +14 turbulence?

Is the skin formation damage, or partly non-Darcy flow?

Before attributing the skin to damage, verify the non-Darcy contribution is negligible. Only single-rate data exists for GK-22, so use a theoretical D calculation; a supplementary gas-well dataset (FK-7) is provided to practise the two-rate separation method.

S′ = S + D·q GK-22: D·q ≈ +0.001 → S = S′ = +14 (all damage) FK-7 two-rate separation: D = 0.72, S = +5.44

Tasks: (a) non-Darcy coefficient D via the Katz β correlation; (b) Dq at q = 782 stb/d as a % of S′; (c) [FK-7] separate S and Dq from S′₁ = +11.2 @ 8 MMscfd and S′₂ = +18.4 @ 18 MMscfd; (d) predict Dq and S′ for FK-7 at 25 MMscfd — acid or reperforation? (e) confirm S = S′ = +14 for GK-22.

Forchheimer equationKatz β correlationTwo-rate skin separationGas-well IPR turbulence

Open Topic 3.2 →

Topic 3.3 · Formation Damage Skin · → SP-3

Formation damage audit — anatomy of the +14 skin

What permeability ratio and damage radius explain S_d = +14?

With the skin confirmed as formation damage, use Hawkins’ formula to construct the damage anatomy. Back-calculate r_s from the core-flood k_s/k, then build a two-zone composite model physically consistent with the drilling history and design the matrix-acid treatment.

Hawkins: S_d = (k/k_s − 1)·ln(r_s/r_w) k_s/k = 0.145, S_d = +14 → r_s = 3.77 ft Post-acid (76% k restoration): S_d → ~+1

Tasks: (a) back-calculate r_s from k_s/k = 0.145 and S_d = +14, verify forward; (b) two-zone model — Zone 1 (k_s1/k = 0.08, r_s1 = 0.60 ft) + Zone 2 (k_s2/k = 0.25), solve r_s2 so S_d1+S_d2 = +14; (c) acid chemistry (HCl preflush + HF mud acid) justified by mineralogy; (d) post-acid S_d from 76% restoration; (e) post-acid J and rate at p_wf = 2,500 psi.

Hawkins’ formulaTwo-zone composite modelDamage anatomyAcid treatment design

Open Topic 3.3 →

Topic 3.4 · Pseudo-Skin Components · → SP-4

Pseudo-skin verification — confirming zero geometric contribution

Could partial completion or deviation be hiding inside the +14?

Formally verify that no significant pseudo-skin exists. Calculate the partial-completion skin S_c and deviation skin S_c″ for GK-22, then work a contrasting 45°/50%-perforated hypothetical that reinforces why these checks matter.

S_c (b = h_p/h = 1.0) = 0 (Brons–Marting, by inspection) S_c″ (θ = 4°, k_v/k_h = 0.5) ≈ −0.005 (Cinco-Ley) Verdict: no geometric skin → S_d = S′ = +14

Tasks: (a) S_c for GK-22 (b = 1.0, h_D = 120) and confirm zero; (b) S_c″ via Cinco-Ley in full; (c) [hypothetical 45°, 21 ft perforated] S_c (Brons–Marting), S_c″ (Cinco-Ley), and Jones–Watts total with the h/h_p amplification; (d) state the pseudo-skin verdict and confirm S_d = S′ = +14.

Brons–Marting correlationCinco-Ley deviation skinJones–Watts multiplierPseudo- vs damage skin

Open Topic 3.4 →

Topic 3.5 · Flow Efficiency (FE) · → SP-5

Production forecast & economic justification

What is the treatment worth in barrels, dollars, and payback?

Translate the skin audit into a production forecast using Standing’s Flow Efficiency. Construct IPR curves for pre-treatment, post-acid, and post-acid + ICHGP conditions, then build the NPV justification — the core financial output of the report.

FE = 7/(7+S): 0.333 (S=+14) → 0.862 (S=+1.12) J: 0.460 → 1.190 stb/d/psi Δq = +1,240 stb/d (+159%) at p_wf=2,500 Payback < 5 days at $75/bbl ($1.05M treatment)

Tasks: (a) FE for current (S=+14), post-acid (S=+1), post-acid+ICHGP (S=+1.12); (b) J and an IPR table (p_wf 4,200→0 at 500-psi steps) for all three states; (c) plot the three IPRs and find each AOF; (d) uplift Δq at p_wf = 2,500 psi; (e) economics — daily revenue, payback, 24-month NPV ($75/bbl, $5/bbl opex, $1,050,000 cost); (f) break-even oil price and robustness.

Flow Efficiency (FE)IPR constructionAOF calculationNPV & paybackBreak-even analysis

Open Topic 3.5 →

Topic 3.6 · Sand Control Skin · → SP-6

Sand control decision & final skin audit

Is sand control required — and what does it cost in productivity?

Integrate sand-production risk with the gravel-pack skin penalty to complete the analysis. Make a defensible sand-control recommendation and assemble the final skin audit that anchors the report’s executive summary.

CDP = 62 psi → current drawdown ≈ 27× CDP: sand control mandatory Saucier (D_50,sand=215 µm): 12/20 mesh gravel S_g (ICHGP, 8 spf, clean) = +0.12 → S′_post = +1.12

Tasks: (a) CDP at S_w = 0.22 and 0.80, ratio of current drawdown to CDP, risk verdict; (b) gravel-pack design (Saucier mesh, 4→8 spf, screen type); (c) S_g for the ICHGP design (simplified and Darcy forms); (d) the complete final skin audit (S_d, S_g, S_c, S_c″, Dq) pre/post-treatment; (e) risk-adjusted NPV including sand-out avoidance (60% sand-out in 12 months without control, $3.5M workover, 90-day loss); (f) the one-paragraph executive recommendation.

CDP calculationSaucier gravel sizingGravel-pack skin S_gComplete skin auditRisk-adjusted NPV

Open Topic 3.6 →

★

PBL Hub · Integration deliverable

Launch the Module 03 PBL — GK-22 well performance crisis

Once all six topics are complete, open the full problem set: the locked GK-22 data pack, six sub-problem launch cards, the delivery map, and the assessment criteria. The package culminates in the Well Performance Report — a go/no-go recommendation to the Asset Leadership Team supported by quantified analysis from every sub-problem.

Launch GK-22 PBL →

Reference answer framework — GK-22 skin-audit case key numbers

The table gives the key numerical answers your sub-problem solutions should converge on. If a calculation differs materially, review the relevant Topic before finalising the report.

S′ = S_d + S_g + S_c + S_c″ + D·q and FE = 7 / (7 + S′)

Quantity	Symbol	Pre-treatment	Post-treatment (acid + ICHGP)	Ideal (S=0)	Source
Total skin	S′	+14.00	+1.12	0.00	DST / Topics 3.1–3.6
Formation damage skin	S_d	+14.00	+1.00	0.00	Hawkins / Topic 3.3
Gravel-pack skin	S_g	0.00	+0.12	0.00	Furui et al. / Topic 3.6
Partial-completion skin	S_c	0.00	0.00	0.00	Brons–Marting / Topic 3.4
Deviation skin	S_c″	−0.005	−0.005	0.00	Cinco-Ley / Topic 3.4
Non-Darcy skin	D·q	+0.001	+0.001	0.00	Katz β / Topic 3.2
Flow efficiency	FE	0.333	0.862	1.000	Standing / Topic 3.5
Productivity index J	stb/d/psi	0.460	1.190	1.380	Darcy equation
Production rate @ p_wf = 2,500 psi	q	782 stb/d	2,022 stb/d	2,346 stb/d	J×Δp
AOF (p_wf = 0)	q_max	1,932 stb/d	4,994 stb/d	5,796 stb/d	J×p̄_R
Damage radius (single zone)	r_s	3.77 ft (k_s/k = 0.145)		r_w = 0.35 ft	Hawkins inverse / Topic 3.3
Critical drawdown (S_w = 0.22)	CDP	62 psi — current drawdown 27× CDP: sand control mandatory			Topic 3.6
Gravel mesh (Saucier, D_50s=215 µm)	D_50g	12/20 mesh (1,075–1,290 µm, ratio 5.9×)			Saucier rule / Topic 3.6
Production uplift Δq	Δq	—	+1,240 stb/d (+159%)	+1,564 stb/d (+200%)	FE framework
Payback period	T_payback	—	< 5 days at $75/bbl ($1,050,000 treatment)		Economics / Topic 3.5
24-month NPV (risk-adjusted)	NPV	—	> $7M (including sand-out avoidance)		SP-5 + SP-6

Final deliverable — Well Performance Report

The Module 03 PBL culminates in a structured Well Performance Report for GK-22, integrating the answers from all six sub-problems into a document that could be submitted to an asset leadership team for a go/no-go investment decision.

Section 1 — Executive summary (1 page)

Current well state (q, FE, J, % of ideal), root cause of S′ = +14, recommended intervention, production uplift, cost and payback, and a go/no-go recommendation with confidence level.

Section 2 — Skin audit (SP-1 to SP-4)

Complete decomposition table, Hawkins damage anatomy (k_s/k, r_s), pseudo-skin verification (S_c, S_c″, Dq), and confirmation that S_d = S′ = +14.

Section 3 — Treatment design (SP-3 & SP-6)

Acid chemistry and volumes (HCl preflush, HF mud acid, overflush), TCP reperforation (8 spf), ICHGP design (gravel mesh, screen type), and the step-by-step treatment sequence with rationale.

Section 4 — Production forecast (SP-5)

IPR table and plot for three conditions (pre / post-acid / post-ICHGP), production uplift at operating p_wf, AOF comparison, and FE before and after treatment.

Section 5 — Economic justification (SP-5 & SP-6)

Daily incremental revenue, payback period, 24-month NPV at base ($75/bbl) and sensitivity ($50, $100/bbl), break-even oil price, and risk-adjusted NPV including sand-out avoidance.

Section 6 — Risk & uncertainties

Post-acid S_d range (+1 to +5), gravel-pack degradation, CDP sensitivity to water breakthrough, production forecast range (P10/P50/P90), and the surveillance programme.

Topic reference library

Topic 3.1 — Skin Factor Concept

Darcy equation, skin definition, PI ratio, pressure wasted to skin, IPR basics. GK-22: J_ideal = 1.380 stb/d/psi, Δp_skin = 1,038 psi.

Topic 3.2 — Total Skin S′ = S + Dq

Forchheimer equation, Katz β correlation, D calculation, FK-7 two-rate separation: D = 0.72, S = +5.44.

Topic 3.3 — Formation Damage Skin

Hawkins’ formula, damage anatomy (k_s/k = 0.145, r_s = 3.77 ft), two-zone model, mud-acid design.

Topic 3.4 — Pseudo-Skin Components

Brons–Marting S_c (= 0 for GK-22), Cinco-Ley S_c″ (≈ 0), Jones–Watts h/h_p multiplier.

Topic 3.5 — Flow Efficiency (FE)

FE = 7/(7+S), Vogel-FE extension, IPR construction, production uplift, NPV framework. FE: 0.333 → 0.862.

Topic 3.6 — Sand Control Skin

CDP assessment, Saucier sizing, S_g = 0.12 (ICHGP clean), gravel degradation, ICHGP vs SAS vs OHGP.

Assessment criteria

The Module 03 PBL is assessed on six equally weighted criteria

Accuracy of skin decomposition — all component calculations correct (S_d, S_g, S_c, S_c″, Dq) with appropriate formula selection and complete working.
Physical reasoning — can you explain why each component is the size it is? Does the Hawkins anatomy fit the drilling history? Is sand control justified by the CDP calculation rather than chosen by default?
Correct use of the FE framework — IPR construction complete and consistent across all three conditions; uplift correct; economics derive directly from the skin audit, not reverse-engineered.
Treatment-design coherence — acid chemistry matches mineralogy; gravel mesh follows Saucier; TCP sequence in the right order; ICHGP parameters consistent with the S_g calculation.
Economic-justification quality — NPV correct; break-even calculated; risk-adjusted NPV incorporates sand-out probability; a sensitivity range, not a point estimate.
Professional communication — the executive summary is decision-ready: one paragraph a non-technical asset manager could act on, with a clear, quantified recommendation and confidence level.

Common mistakes to avoid

Watch for these

Attributing Dq to formation damage. For GK-22, Dq ≈ 0 — do not include it in the treatable skin budget.

Forgetting the h/h_p Jones–Watts multiplier. Only relevant when partial penetration exists (it does not for GK-22, but it matters for the SP-4 hypothetical).

Using S′ directly in the FE formula without first subtracting pseudo-skins. FE applies to the full S′, but only S_d responds to acid; FE_post-acid uses S_d,post + S_g + other non-treatable components.

Specifying 20/40 mesh gravel for GK-22. With D_50,sand = 215 µm, the Saucier target is 5×215 = 1,075 µm → 12/20 mesh. 20/40 (ratio 2.95×) is under-sized and degrades faster.

Forgetting to sequence acid before the gravel pack. HF acid dissolves silicate gravel, so acid must be fully displaced before the gravel slurry is pumped.

Using a fixed p_wf for the economics. At fixed p_wf = 2,500 psi the uplift is +1,240 stb/d; with nodal analysis (TPC interaction) the real uplift is higher as p_wf drops to a new operating point. Report both and use the nodal result for NPV.

Reference

Reservoir & fluid: k_o = 85 md · h = 42 ft · μ_o = 1.8 cp · B_o = 1.32 rb/stb · p̄_R = 4,200 psi · r_w = 0.35 ft · r_e = 1,650 ft
Well test: q_measured = 782 stb/d · p_wf = 2,500 psi · S′_measured = +14 · build-up k = 85 md (confirmed)
Geomechanics: UCS = 0.8 MPa · CDP = 62 psi at S_w = 0.22 · Agbada Fm, Miocene, Niger Delta
Completion: vertical (θ < 5°) · h_p = 42 ft (full pay) · 4 spf overbalanced · WBM filtrate k_s/k = 0.145 (core flood) · clay: kaolinite 8–12%, smectite 3–5%
Economics: oil price $75/bbl · opex $5/bbl · discount rate 10%/yr

Required

Modules 01–02: Darcy radial flow, the PSS deliverability equation, productivity index and the skin–PI relationship are assumed before starting here.

Required

Mathematics: natural logarithm (ln), algebra and rearrangement (Hawkins inverse), and unit handling (psi, md, cp, rb/stb).

Helpful

Completion & stimulation background: familiarity with matrix acidizing and gravel-pack completions helps with SP-3 and SP-6, but both are developed from first principles in the topics.