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SP-6: Sand Control Decision, Sg Calculation & Module 03 Final Skin Audit

Assess GK-22 sand production risk (CDP = 62 psi vs 1,700 psi drawdown = 27×). Verify 12/20 mesh gravel sizing (Saucier rule). Calculate ICHGP skin Sg = 0.12. Compile the complete Module 03 skin audit: S′post = +1.12, FE = 0.862, q = 2,023 stb/d. Produce the Module 03 Final Report.

Module 03 · SP-6
Topic 3.6 — Sand Control
Gashaka GK-22
~60 min + Final Report
SP-6 · Topic 3.6 · Sand Control Skin Sg

SP-6: Sand Control Decision, Sg Calculation & Final Audit

Engineering Question for SP-6
Module 03 PBL · Gashaka GK-22 · FINAL SUB-PROBLEM

SP-5 showed that acid treatment delivers ~2,053 stb/d at a payback of 5.1 days. But there is a critical risk: GK-22 Agbada Formation sands are unconsolidated (UCS = 0.8 MPa). The current drawdown of 1,700 psi exceeds the critical drawdown pressure (CDP = 62 psi) by 27 times. Post-acid production at 2,053 stb/d will further stress the formation. Without sand control, catastrophic sand influx is not merely possible — it is near-certain on a 12–24 month timescale.

SP-6 addresses two questions: (1) Is sand control mandatory for GK-22? (2) If an ICHGP (inside cased-hole gravel pack) is installed, what gravel-pack skin Sg does it add, and how does this modify the post-treatment deliverability and economics? SP-6 then assembles all six sub-problem results into the Complete Skin Audit table and the Final Report framework.

Data for SP-6

SP-6 Data Pack — Geomechanics, Gravel Design & Locked SP Outputs

Locked Inputs From SP-1 Through SP-5
Jideal = 1.380  |  Sd,post = +1.0  |  Sc = 0  |  Sc″ ≈ −0.005  |  Dq ≈ 0.001  |  qpost,acid-only ≈ 2,053 stb/d

Geomechanical Parameters

ParameterSymbolValueUnitsSand Control Significance
Unconfined compressive strengthUCS0.8MPaUnconsolidated class — CDP < 100 psi
Current water saturationSw0.22fractionCDP = 100 × UCS × (1−Sw)
Critical drawdown pressureCDP62psi100 × 0.8 × (1−0.22) = 62.4 psi
Current operating drawdownΔp1,700psi27× CDP — sand onset certain without control
D50 formation sand grainD50,sand215μmSaucier sizing: target D50,gravel = 1,075 μm
CDP at water breakthrough (Sw = 0.80)CDPwb16psi100 × 0.8 × 0.20 — effectively zero

ICHGP Design Parameters (12/20 mesh, 8 spf)

ParameterSymbolValueUnitsDesign Basis
Gravel type—12/20 mesh Ottawa sand—Saucier: D50,g = 1,265 μm, ratio = 5.9× ✓
Gravel permeability (clean)kg800,000mdLarge grain, high kg, minimises Sg
Shot density (upgraded for workover)spf8shots/ftDoubled from initial 4 spf → halves Sg
Total perforationsNp336perforations8 spf × 42 ft
Perforation lengthLp0.75ftTCP gun specification
Perforation radiusrp0.021ft (0.25 in)TCP gun specification
Wire-wrapped screen skin (clean)Sscreen0.12—OFA ≈ 12%, clean wire-wrap screen
ICHGP additional cost vs acid-only—600,000USDScreen + gravel + placement, Niger Delta
KWL Table
K — Know
  • CDP = 100 × UCS × (1−Sw) for sand onset threshold
  • Saucier rule: D50,gravel = 5–6 × D50,sand
  • Sg = Sg,Darcy + Sscreen
  • HF acid must NOT contact gravel pack (dissolves quartz)
W — Want to Know
  • How does GK-22 drawdown compare to CDP?
  • What is Sg for 12/20 mesh ICHGP at 8 spf?
  • What is the final S′ with ICHGP added?
  • Is ICHGP NPV-positive vs sand-out risk?
L — Learned
  • CDP = ________ psi; drawdown/CDP = ________×
  • Gravel sizing: ________ mesh correct
  • Sg,Darcy = ________ | Sscreen = ________
  • Sg,total = ________ | S′post = ________
  • FEfinal = ________ | qfinal = ________ stb/d
Just-in-Time Resources

Just-in-Time Resources

Pull these up as you work SP-6. Each maps to the Module 03 topic behind this sub-problem: read the topic page, watch the matching lectures, then reproduce your numbers with the verified calculator.

Study
Topic 3.6 — Sand Control & Gravel-Pack Skin — when to act on sand, gravel-pack skin Sg, and standalone screen vs gravel pack selection.
Topic 3.6 — Sand Control & Gravel-Pack Skin
Watch
Lecture 3.6a — Sand Production Mechanisms: Why Sands Fail and When to Act
Lecture 3.6b — Gravel Pack Skin Sg: Theory, Calculation & Design Impact
Lecture 3.6c — Sand Control Selection: Standalone Screen vs Gravel Pack vs OHGP vs Frac-Pack
Produced lectures
Self-check
sand_control.py — gravel sizing, gravel-pack skin Sg, and the final post-treatment S′ the Final Report asks you to cite.
Verified calculator — reproduce your numbers
Tasks & Requirements

SP-6 Calculation Tasks

  1. Sand production risk assessment — quantify the hazard
    CDP = 100 × UCS × (1 − Sw) = 100 × 0.8 × (1 − 0.22) → verify = 62.4 psi.
    Current drawdown / CDP ratio = 1,700 / 62.4 = ?
    Post-acid worst-case: if pwf drops to 1,500 psia, drawdown = 2,700 psi → ratio = ?
    CDP at water breakthrough Sw = 0.80 → CDPwb = 100 × 0.8 × 0.20 = ? psi.
    Write a two-sentence engineering statement on sand control necessity for GK-22.
  2. Saucier gravel sizing verification
    Target: D50,gravel = 5 × D50,sand = 5 × 215 = 1,075 μm (centre of 5–6× range).
    12/20 mesh Ottawa sand: D50 ≈ 1,265 μm → ratio = 1,265/215 = 5.9× ✓
    20/40 mesh: D50 ≈ 635 μm → ratio = 635/215 = 2.95× — why is this wrong?
    Explain in one sentence what happens to kg over time if the gravel is under-sized.
  3. Calculate Sg for the GK-22 ICHGP design
    Darcy component:
    Sg,Darcy = 241.1 × q × μo × Bo / (kg × h) × [Lp / (2 × rp × Np)]
    Use q = 782 stb/d, μ = 1.8 cp, B = 1.32 rb/stb, kg = 800,000 md, h = 42 ft, Lp = 0.75 ft, rp = 0.021 ft, Np = 336.
    Total: Sg = Sg,Darcy + Sscreen (= 0.12)
    Expected: Sg,Darcy ≈ 0.0007, Sscreen = 0.12. Sg,total ≈ 0.12. The Darcy component through the gravel is negligible for 12/20 mesh; the screen face resistance dominates. This confirms 12/20 mesh is the correct design choice for GK-22.
  4. Compile the complete post-treatment skin audit with ICHGP
    S′post,ICHGP = Sd,post + Sg + Sc + Sc″ + Dq
    = 1.0 + 0.12 + 0.000 + (−0.005) + 0.001 = ?
    Calculate FEfinal = 7/(7+S′post,ICHGP), Jfinal, and qfinal at pwf = 2,500 psia.
    What is the production difference between acid-only (SP-5) and acid + ICHGP?
    Expected: S′post ≈ +1.116 ≈ +1.12. FE = 7/8.12 = 0.862. Jfinal = 0.862 × 1.380 = 1.190 stb/d/psi. qfinal = 1.190 × 1700 = 2,023 stb/d. Production penalty from ICHGP: 2,053 − 2,023 = 30 stb/d (1.5% of the treatment gain).
  5. ICHGP risk-adjusted economic justification
    ICHGP additional capital cost: $600,000 (above acid-only $450,000; total = $1,050,000).
    Production penalty from ICHGP: 30 stb/d × $70/bbl × 365 days × 2 years = ?
    Sand-out risk assessment (estimate: 60% probability of sand-out within Year 1 at 2,053 stb/d):
      • Workover to clean plugged perforations: $3,500,000
      • Lost production during 90-day workover: 90 × 2,053 × $70 = $12,933,900
      • Total expected cost of sand-out: 0.60 × ($3,500,000 + $12,933,900) = ?
    Calculate: is ICHGP economically justified given this risk?
Theory Reference
Gravel Pack Skin S_g — GK-22 ICHGP (12/20 mesh, 8 spf)S_g = S_g,Darcy + S_screen S_g,Darcy = 241.1 × q × μ × B / (k_g × h) × [L_p / (2 × r_p × N_p)] Numerator group: 241.1 × 782 × 1.8 × 1.32 = 241.1 × 1,857.9 = 447,936 / (800,000 × 42) = / 33,600,000 = 0.013331 Perforation geometry: L_p / (2 × r_p × N_p) = 0.75 / (2 × 0.021 × 336) = 0.75 / 14.112 = 0.05314 S_g,Darcy = 0.013331 × 0.05314 = 0.000709 S_screen = 0.12 (wire-wrapped, OFA ≈ 12%, clean initial state) S_g,total = 0.000709 + 0.12 = 0.1207 ≈ 0.12 Complete Post-Treatment Skin AuditComponent Pre-Treatment Post-Treatment Source S_d (damage) +14.000 +1.000 SP-3 S_g (ICHGP) 0.000 +0.121 SP-6 (this task) S_c (partial) 0.000 0.000 SP-4 (b=1.0) S''_c (deviation) -0.005 -0.005 SP-4 (θ<5°) Dq (turbulence) +0.001 +0.001 SP-2 (negligible) ───────────────────────────────────────────────────────────────── S' (total) +13.996≈+14.00 +1.117≈+1.12 AUDIT COMPLETE FE_post,ICHGP = 7 / (7 + 1.12) = 7 / 8.12 = 0.8621 J_post,ICHGP = 0.8621 × 1.380 = 1.190 stb/d/psi q_post,ICHGP = 1.190 × 1700 = 2,023 stb/d (vs 2,053 acid-only) ICHGP penalty = 2,053 - 2,023 = 30 stb/d (1.5% of treatment gain)
The ICHGP adds only 0.12 skin units — a 1.5% reduction in the treatment production gain. Against a sand-out risk with expected cost ~$9.9M, the ICHGP is unambiguously justified. Its capital cost ($600K) is recovered from the sand-risk avoidance within weeks of the treatment.
Module 03 Complete Skin Audit

All Six Sub-Problems Integrated — Final Answer

Skin ComponentSource SPPre-TreatmentPost-Treatment
(Acid + ICHGP)		Treatable?
Sd — Formation damage (Hawkins; ks/k = 0.145, rs = 3.76 ft) SP-3 +14.000 +1.000 ✓ Yes — 3%HCl/2.5%HF mud acid
Sg — Gravel pack skin (ICHGP; 12/20 mesh, 8 spf, kg = 800,000 md) SP-6 0.000 (no GP) +0.121 Managed by completion design
Sc — Partial completion (Brons–Marting; b = hp/h = 1.0) SP-4 0.000 0.000 N/A — full penetration
Sc″ — Deviation skin (Cinco-Ley; θ′ = 2.83°, hD = 169.7) SP-4 −0.005 −0.005 N/A — near-vertical
D·q — Non-Darcy turbulence (D ≈ 9.4×10^−7; (stb/d)−¹) SP-2 +0.001 +0.001 N/A — analytically negligible
S′ TOTAL All SPs +13.996 ≈ +14.00 +1.117 ≈ +1.12 —
Performance MetricPre-Treatment (Current)Post-Treatment (Acid + ICHGP)Ideal (S = 0)
Flow Efficiency FE0.3330.8621.000
PI J (stb/d/psi)0.4601.1901.380
q at pwf = 2,500 psia782 stb/d2,023 stb/d2,346 stb/d
AOF (pwf = 0)1,932 stb/d4,996 stb/d5,796 stb/d
Production uplift vs pre-treatment—+1,241 stb/d (+159%)+1,564 stb/d (+200%)
Final Recommendation — Module 03 PBL
GK-22 requires: TCP reperforation (8 spf, 1,000 psi UB) + 5% HCl preflush (200 gal/ft) + 3% HCl/2.5% HF mud acid (350 gal/ft ≈ 1× damaged-zone pore volume, ≈ 350 bbl total over h = 42 ft) + KCl overflush (2× wellbore volume below perfs) + ICHGP (12/20 mesh Ottawa sand, wire-wrapped screen, kg = 800,000 md initial). Total cost: $1,050,000. Payback: ~12 days. 24-month NPV at $75/bbl: ~$3.7M. Sand control mandatory: drawdown/CDP = 27× at current conditions, approaching zero at water breakthrough. Risk-adjusted value of ICHGP (vs expected sand-out cost of ~$9.7M expected value): clearly positive.
Final Report Template

2-Page Engineering Recommendation Memo — Structure

Your Final Report must follow this structure. All sections are mandatory. Every number must reference its source sub-problem.

SECTION 1 — Complete Skin Audit Table

  • Reproduce the table from the Final Audit section above with your own calculated values. Annotate each entry with the SP source and key equation or measurement it comes from.

SECTION 2 — Confirmed Diagnosis (1 paragraph)

  • State: what is causing the skin (WBM filtrate + kaolinite/smectite clay response), the physical mechanism, the evidence (core flood ks/k = 0.145; 18 days WBM exposure; rs2 = 8.08 ft filtrate zone), and why all of S′ = +14 is treatable by acid stimulation.

SECTION 3 — Treatment Specification

  • Step 1: TCP reperforation — shot density, underbalance pressure, tool type, justification
  • Step 2: HCl preflush — concentration, volume in gal/ft, purpose
  • Step 3: HF mud acid — concentration (3%HCl/2.5%HF), volume in bbl and gal/ft, soak time, placement method, basis for volume (SP-3 PV calculation)
  • Step 4: KCl overflush — volume, purpose, safety requirement (displace HF before gravel)
  • Step 5: ICHGP — gravel type (12/20 mesh), kg, screen type, OFA, shot density, gravel volume

SECTION 4 — Post-Treatment Production Forecast

  • S′post = +1.12, FE = 0.862, J = 1.190 stb/d/psi, q = 2,023 stb/d at pwf = 2,500 psia. Include annotated dual IPR chart from SP-5. Note the fixed-pwf assumption and that actual nodal production will be higher.

SECTION 5 — Economic Justification

  • Total treatment cost: $1,050,000. Daily revenue gain: ~1,241 stb/d × $70/bbl ≈ $86,870/day. Simple payback: ~12 days. 24-month NPV at $75/bbl: ~$3.7M (programme monthly-annuity convention: monthly CF × annuity factor ÷ 12 − cost). Risk-adjusted NPV including ICHGP sand protection: add expected sand-out cost avoided (~$9.9M) → total >$7M (≈ $13.6M).

SECTION 6 — Risk Register (minimum 2 risks)

  • Risk 1: Acid underperformance — Spost > +3 (Likelihood: Low based on core floods; Consequence: Reduced Δq; Mitigation: Post-job well test to verify Spost, re-stimulate if needed)
  • Risk 2: Gravel pack degradation — Sg increasing from +0.12 toward +5 within 3–5 years (Likelihood: Medium in Agbada kaolinite sands; Consequence: Progressive production decline; Mitigation: Annual pressure build-up tests to track Sg increase, plan workover at S′ > 7)
  • Risk 3 (optional): Water breakthrough collapsing CDP to 16 psi — confirm ICHGP design holds at Sw = 0.80
Final Deliverable

✅ SP-6 & Final Report Checklist

  • Sand risk: CDP = 62 psi, drawdown = 1,700 psi (27×), statement on sand control necessity
  • Gravel sizing: 12/20 mesh verified correct (5.9×), 20/40 mesh shown incorrect (2.95×)
  • Sg calculation: Sg,Darcy = 0.0007, Sscreen = 0.12, Sg = 0.12 with full formula working
  • Complete skin audit table: All 5 components, pre- and post-treatment, with SP sources
  • Final deliverability: FE = 0.862, J = 1.190 stb/d/psi, q = 2,023 stb/d
  • ICHGP economic case: Penalty 30 stb/d vs sand-out expected cost $9.9M → recommendation justified
  • Module 03 Final Report: Complete 2-page memo following the template above, submitted for assessment
Module 03 PBL Complete — Return to Hub for Debrief
All six sub-problems are now complete. The GK-22 skin audit is evidence-based, internally consistent across six independent calculations, and economically justified. Return to the Module 03 PBL Hub to submit your Final Report and access the facilitated debrief materials. The debrief covers the most common errors teams make in each SP and contrasts GK-22's outcome with analogous Niger Delta wells where different damage mechanisms or completion choices produced different results.