Skin Factor Calculator

Fast engineering tool for diagnosing near-wellbore performance today. Choose your method, units, and advanced options. Get clear interpretation and shareable outputs in seconds easily.

Calculator Inputs
Pick a method, fill required fields, then submit.
Downloads run once
Choose the model that matches your data.
This selector applies to steady-state only.
Use if high-rate turbulence is expected.
Units: 1/(rate). Rate uses q you enter.
Steady-state radial flow inputs
stb/d
cp
rb/stb
mD
ft
psi
psi
ft
ft
Tip: keep radii in the same length units (both ft or both m).
Transient semilog straight-line inputs
Uses infinite-acting radial flow straight-line analysis with standard oilfield constants.
stb/d
cp
rb/stb
ft
psi/log-cycle
ft
psi
psi
Pick a point on the straight line.
hr
Enter as fraction, not percent.
1/psi
What you get
  • Estimated permeability from slope
  • Skin factor from straight-line point
  • Extra pressure drop due to skin
Damage-zone (altered permeability) inputs
Use consistent length units for rd and rw.
mD
mD
Any consistent length unit.
Only needed when turbulence is enabled.
Use case
Good when you know an altered zone radius and permeability.
How to use
How to use this calculator
  1. Select a method that matches your dataset.
  2. Enter inputs using the shown units and formats.
  3. Optionally enable turbulence and enter D.
  4. Press Calculate to view results above the form.
  5. Use CSV or PDF buttons to download the report.
Formula used
1) Steady-state radial flow
Rearranged from radial Darcy flow pressure drop with a skin term.
Oilfield:   s = (k·h·(pe−pwf)) / (141.2·q·B·μ) − ln(re/rw)
SI:        s = (2π·k·h·Δp) / (q·μ·B) − ln(re/rw)
Extra pressure drop due to skin is computed as Δpskin = multiplier · s.
2) Transient semilog straight-line
Uses permeability from semilog slope and a straight-line point at time t.
k = (162.6·q·B·μ)/(m·h)
X = (0.000264·k·t)/(φ·μ·ct·rw2)
s = (pi − p(t))/m − log10(X)
3) Damage-zone (altered permeability)
Mechanical skin due to an altered zone around the wellbore.
s = (k/kd − 1) · ln(rd/rw)
Advanced option: turbulence (non-Darcy) skin
stotal = s + D·q
Enable only when you have a credible D estimate.
Example data table
Method Key inputs (example) Example skin
Steady-state q=500 stb/d, μ=1.2 cp, B=1.1, k=50 mD, h=50 ft, pe=3000 psi, pwf=2500 psi, re=1000 ft, rw=0.33 ft 5.397
Transient q=500 stb/d, μ=1.2 cp, B=1.1, h=50 ft, m=30 psi/log-cycle, pi=3000 psi, p(t)=2700 psi, t=10 hr, φ=0.20, ct=1e−5 1/psi, rw=0.33 ft 4.141
Damage-zone k=50 mD, kd=20 mD, rd=2.0, rw=0.33 (same units) 2.705
Examples are illustrative; use your project-specific inputs.

What Skin Factor Represents

Skin factor is a dimensionless measure of extra near-wellbore resistance relative to an ideal completion. A positive value indicates additional pressure loss caused by damage, partial penetration, fines migration, or completion geometry. A negative value suggests stimulation, cleanup, or improved connectivity. Because it is dimensionless, skin lets you compare wells and interventions across reservoirs and operating rates.

Choosing the Right Method

The steady-state option uses a single pressure drawdown and a drainage radius assumption. It works best when boundary effects are established and pressures are stabilized. The transient semilog option is preferred when you have a valid straight-line segment on a semilog plot. It estimates permeability from slope and then back-calculates skin using a representative point at time t.

Use the same reference wellbore radius used in test interpretation, not casing OD. If partial completion or perforated interval differs from net thickness, adjust h to the effective flow height. Sensitivity runs with plausible re values can show whether uncertainty is dominated by boundaries or near-wellbore effects during early screening.

Interpreting Results and Pressure Loss

The calculator reports skin and an equivalent pressure-drop contribution due to skin. That pressure increment helps translate a dimensionless number into operational impact, such as increased drawdown for the same rate or reduced deliverability at a fixed flowing pressure. If turbulence is enabled, a non-Darcy term is added as D·q, producing a total skin that grows with rate.

Data Quality and Unit Consistency

Accurate skin requires consistent radii and reliable pressure measurements. Keep re and rw in the same length units, confirm re is larger than rw, and ensure viscosity and volume factor match the produced phase. For transient analysis, choose points within the infinite-acting window and avoid early-time wellbore storage or late-time boundary influence.

Reporting and Workflow Integration

Engineering reviews often require traceable inputs, assumptions, and calculated outputs. The export buttons create a compact CSV or PDF report from the latest run, including method, units, skin, and the key inputs used. This supports quick comparisons between test interpretations, completion candidates, and post-stimulation evaluations. Store the files with test reports to simplify audits and future re-interpretations.

FAQs

1) Is skin factor always caused by formation damage?

No. Skin aggregates many near-wellbore effects, including perforation efficiency, partial penetration, well deviation, completion geometry, and altered permeability. Damage is common, but skin is a lumped indicator, not a unique diagnosis.

2) What does a negative skin mean operationally?

Negative skin indicates reduced near-wellbore resistance compared to the ideal model. It often reflects stimulation, effective perforations, or cleanup. Operationally, it usually means higher productivity for the same drawdown or lower drawdown at a target rate.

3) When should I enable the turbulence term D·q?

Enable it for high-rate wells where non-Darcy flow near the wellbore is expected and you have a defensible D estimate. If D is uncertain, compare results with and without the term to understand sensitivity.

4) Why do steady-state and transient skins differ?

They rely on different assumptions and data windows. Steady-state depends on drainage radius and stabilized pressures. Transient analysis uses an infinite-acting radial segment and a semilog straight line. Boundary effects, storage, or poor line selection can shift the estimate.

5) Can I mix units between inputs?

Avoid mixing. Use the shown units and keep radii consistent. For steady-state, select the unit system and enter all related inputs accordingly. Mixing units can distort ln(re/rw) and pressure multipliers, producing misleading skin values.

6) What if I only know damage radius and altered permeability?

Use the damage-zone method. Enter k, kd, rd, and rw in consistent units to compute mechanical skin from altered permeability. Optionally add D·q if you want to include high-rate turbulence effects.

Notes & checks
  • Positive skin generally indicates damage or restriction.
  • Negative skin often indicates stimulation or cleanup.
  • Keep radii consistent and ensure re > rw.
  • For transient analysis, use a straight-line semilog segment.
  • If φ is entered as percent, divide by 100.
Want to store results long-term? Use the CSV/PDF buttons.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.