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SP-3 — Field SPI Ranking, Stimulation Priority & KRM-6 J Prediction

Calculate the Specific Productivity Index (SPI = J/h) for all five Karama Field wells. Rank them by reservoir quality. Build the J vs h cross-plot to distinguish damaged wells from genuinely inferior reservoir. Construct the stimulation priority matrix. Predict J for proposed KRM-6.

Topic 2.3 · Specific Productivity Index
~40 min
Bloom: Analyse & Evaluate
Sub-Problem 3 of 4 · Topic 2.3

Context & Learning Goals

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Engineering Task: Field Benchmarking & Drilling Decision
Which wells are in good rock? Which are just damaged? Where should KRM-6 be drilled?

SP-2 showed KRM-4 has skin S=+5 and FE=60% — a damaged well in average-quality rock. But is its rock quality representative of the field, or is it in a low-permeability zone? To answer this, you need the Specific Productivity Index: normalising J by net pay thickness isolates rock quality from the thickness effect, enabling genuine well-to-well comparison. This is the foundation of the drilling decision: where to place KRM-6.

Learning Goals for SP-3

  1. Calculate SPImeasured = Jmeas/h and SPIideal = Jideal/h for all five KRM wells
  2. Build the J vs h cross-plot with SPI isoline grid and locate all five wells on it
  3. Distinguish wells below the average SPI line due to damage (recoverable by stimulation) from wells with genuinely lower SPIideal (rock limitation)
  4. Construct the stimulation priority matrix combining SPI rank and FE
  5. Predict J for KRM-6 at h = 115 ft (±15 ft) using the field-average SPI with P10/P50/P90 range
Knowledge Library Link
Review Topic 2.3 — Specific Productivity Index, specifically Sections 3 (J vs h cross-plot) and 4 (field ranking and prediction). Return here to apply to the full KRM field dataset.
Data Slice

SP-3 — Full Karama Field Well Data

Wellh (ft)k (mD)SJmeasJidealSPImeas=J/hSPIidealFE
KRM-111022+21.181.35calculatecalculatecalculate
KRM-29518+140.410.926calculatecalculatecalculate
KRM-49518+50.600.926calculatecalculatecalculate
KRM-313025−11.751.68calculatecalculatecalculate
KRM-57512+90.180.575calculatecalculatecalculate
Carry-Forward from SP-1 & SP-2
KRM-4: Jideal = 0.926, Jmeas = 0.600, S = +5, FE = 0.60. Jideal values for other wells use the same formula with their respective k and h — all computed using μ = 1.4, B = 1.25, re/rw = 3,729 (same field).

Note: KRM-3 has S=−1 (slight stimulation), giving Jmeas > Jideal at S=0 baseline is unusual — account for this in FE interpretation.
Before You Calculate

KWL Planner — SP-3 Specific

K — Know
  • SPI = J/h removes thickness effect
  • High SPI = good rock (high k per ft)
  • Damaged well: SPImeas < SPIideal
  • J vs h cross-plot: SPI = slope of line from origin
W — Want to know
  • Which KRM well is in the best rock quality?
  • Is KRM-4’s SPIideal above or below field average?
  • How confident can we be in a KRM-6 J prediction?
  • Which wells should be stimulated first?
L — Will learn
  • SPIideal ranking: ___ > ___ > ___
  • Field-average SPI = ___ STB/d/psi/ft
  • KRM-6 Jpredicted = ___ STB/d/psi
  • Priority 1 stimulation candidate = ___
Guided Calculations

Task Sequence — Field SPI Analysis

Task 1: Compute SPI for All Five Wells

  1. SPImeasured: For each well, divide Jmeas by h. Fill in the table above column by column.
  2. SPIideal: Divide Jideal by h for each well. This is the rock quality metric, skin-corrected.
  3. FE for each well: FE = Jmeas/Jideal. Note that KRM-3 (S=−1) will give FE slightly above 1.0 — this means the stimulation has pushed it above its undamaged Darcy baseline.
  4. Rank by SPIideal from highest to lowest. This is the true reservoir quality ranking.

Task 2: Field Average SPI and Cross-Plot Interpretation

  1. Field-average SPIideal: Compute the arithmetic mean of SPIideal for all five wells. Record this as your field reference line.
  2. J vs h cross-plot: On graph paper or using the Topic 2.3 simulator, plot Jmeas vs h for each well. Draw the field-average SPI line from the origin.
  3. Identify outliers: Wells plotting above the average SPI line: good rock or stimulation. Wells plotting below: damaged or poor rock. Which category does each KRM well fall into?
  4. Critical distinction: KRM-2 and KRM-4 both plot below the average SPI line using Jmeas. But their SPIideal values are identical. What does this tell you about the nature of their underperformance?

Task 3: Stimulation Priority Matrix

Combine SPIideal (rock quality) and FE (damage fraction) to build a two-axis stimulation priority matrix:

WellSPIidealFE (%)Stimulation ValuePriorityRecommended Action
KRM-2your calcyour calcVery high — good rock + severe damage1stAcid stimulate immediately
KRM-5your calcyour calcHigh — poor rock + severe damage2ndStimulate; manage expectations
KRM-4your calcyour calcModerate — average rock + moderate damage3rdAcid stimulate after KRM-2
KRM-1your calcyour calcLow — good rock + mild damage4thMonitor; consider light acid wash
KRM-3your calcyour calcNone — already stimulated, best rock—Use as model well & drilling guide

Task 4: KRM-6 J Prediction

Seismic interpretation gives hKRM-6 = 115 ± 15 ft at the proposed KRM-6 drill location.

  1. P50 prediction: JKRM-6,P50 = SPIideal,avg × hP50 = SPIavg × 115.
  2. Uncertainty from SPI variability: JP10 = SPIP10 × 115 (use lowest SPIideal in field). JP90 = SPIP90 × 115 (use highest SPIideal).
  3. Uncertainty from h variability: Jh-low = SPIavg × 100. Jh-high = SPIavg × 130.
  4. Which source of uncertainty dominates? Compare the SPI range with the h range. Whichever drives more J uncertainty is the priority for data acquisition (better core/log for h, or more wells for SPI calibration).
  5. Rate prediction at Pwf = 3,100 psia: QP50 = JP50 × (4,850 − 3,100). Does KRM-6 meet the 1,200 STB/day target with P50 SPI?

SP-3 Deliverable

Record: completed SPI table (both measured and ideal for all five wells); field-average SPIideal; stimulation priority ranking; KRM-6 J prediction (P10/P50/P90); and the identified dominant uncertainty source. These carry forward to SP-4 where Jideal = 0.926 (KRM-4’s value at S=0) is used as the linear PI for composite IPR construction.

Just-in-Time Resources

Just-in-Time Resources

Targeted Module 02 assets for this sub-problem. Use them to refresh the method, watch the relevant lecture, and check your own numbers.

Study Course topic page — Topic 2.3 — The Specific Productivity Index (normalisation by net pay and rock-quality comparison).
Watch Lecture 2.3 — The Specific Productivity Index
Lecture 2.3C — Case Study: SPI Ranking & the KRM-6 Prediction (1:1 with this sub-problem).
Self-check spi_toolkit.py — verified calculator: reproduce your numbers.
Assessment

Knowledge Check — SP-3

Question 1

SPImeasured for KRM-4 (Jmeas=0.600, h=95 ft) is:

A. 0.00632 STB/day/psi/ft
B. 0.00974 STB/day/psi/ft
C. 0.00975 STB/day/psi/ft
D. 57.0 STB/day/psi/ft
✓ SPImeas = 0.600/95 = 0.00632 STB/day/psi/ft. Note the very small absolute value — typical for chalk reservoirs. SPIideal = 0.926/95 = 0.00975 STB/day/psi/ft (Option B/C). The difference (0.00975 vs 0.00632) reflects the damage. If you compute SPI by dividing J by something other than h, check your algebra — SPI is always J/h in STB/day/psi/ft.

Question 2

KRM-2 (Jmeas=0.41, h=95 ft) and KRM-4 (Jmeas=0.60, h=95 ft) have different measured SPIs. However, their SPIideal values are identical. What is the correct interpretation?

A. KRM-2 is in inferior rock because its measured SPI is lower
B. Both wells are in the same reservoir quality (k=18 mD, same Jideal); KRM-2’s lower measured SPI is entirely explained by its heavier damage (S=+14 vs S=+5)
C. KRM-4 must have been stimulated, explaining its higher measured SPI
D. The SPI comparison is invalid because both wells are too similar
✓ Same reservoir, same Jideal = 0.926 STB/day/psi, same h = 95 ft, same SPIideal = 0.00975 STB/day/psi/ft. The entire measured SPI difference (0.00432 vs 0.00632) is damage. This is exactly the diagnostic power of SPI: it reveals that KRM-2 and KRM-4 are in equivalent rock, so KRM-2’s poor performance is entirely fixable with stimulation — not a reservoir quality issue.

Question 3

Which well has the highest SPIideal (best reservoir quality per foot of pay)?

A. KRM-1 (highest measured J at 1.18 STB/d/psi)
B. KRM-3 (k=25 mD, h=130 ft, SPIideal ≈ 0.01292 STB/d/psi/ft)
C. KRM-4 (our reference well)
D. KRM-2 (same reservoir as KRM-4 but less information)
✓ KRM-3 has k=25 mD vs KRM-1’s 22 mD, giving higher SPIideal. Verify: KRM-3 SPIideal = 1.68/130 = 0.01292; KRM-1 SPIideal = 1.35/110 = 0.01227. KRM-3 is the field sweet spot — its location should guide the KRM-6 target area if geologically possible. Option A is the classic trap: KRM-1 has a higher absolute J because h=110 ft > KRM-3’s assumption. Always normalise by h before concluding which area has better rock quality.

Question 4

Field-average SPIideal ≈ 0.01047 STB/d/psi/ft. KRM-6 is proposed with h = 115 ft (P50 seismic). Predicted JP50 is approximately:

A. 0.826 STB/d/psi
B. 1.204 STB/d/psi
C. 1.486 STB/d/psi
D. 0.972 STB/d/psi
✓ JP50 = SPIavg × h = 0.01047 × 115 = 1.204 STB/d/psi. QP50 at Pwf=3,100 = 1.204 × 1,750 = 2,107 STB/day — well above the 1,200 target. Option A uses SPIP10 (KRM-5’s SPI): 0.00767×115 = 0.882. Option C uses SPIP90 (KRM-3’s SPI): 0.01292×115 = 1.486. The P10 still gives 0.882×1,750 = 1,544 STB/day — above target. KRM-6 is a strong drilling candidate.

Question 5

A field study changes the net pay cut-off from φ > 10% to φ > 15% for all wells. This would cause SPIideal for all wells to:

A. Decrease because removing pay reduces J
B. Increase because the same J is divided by a smaller h (lower-quality rock removed from h)
C. Stay the same because SPI is normalised
D. Become meaningless because the cut-off has changed
✓ Tightening the φ cut-off removes lower-porosity intervals from net pay, reducing h. J from the well test is unchanged (it reflects all contributing rock, including some below-cutoff). Therefore SPI = J/h increases as h decreases. This is a critical quality-control point: SPI comparisons across wells (or studies) are only valid when consistent petrophysical cut-offs are applied. A study mixing φ>10% and φ>15% cut-offs would give artificially high SPI for the second group.
Output

Carry-Forward Values — Record Before SP-4

Field avg SPIideal
0.01047
STB/d/psi/ft
KRM-6 J (P50)
1.204
STB/d/psi
Stim Priority 1
KRM-2
S=+14, FE=44%
Best Rock Well
KRM-3
SPIideal=0.01292
Interpretation
The SPI analysis reveals: (1) KRM-2 is the priority stimulation target — same rock as KRM-4 but with 3× more damage; (2) KRM-3 defines the field sweet spot and should anchor KRM-6 targeting; (3) KRM-6 is a strong well candidate with P50 J = 1.204 > 0.926 (KRM-4 ideal). In SP-4, use Jideal = 0.926 STB/d/psi (KRM-4, S=0) for the composite IPR linear segment.