reservoir · inflow performance
Skin Factor
S = 1.151 × [(ΔP1hr)/m − log10(k/(φ·μ·ct·rw²)) + 3.2275]
click formula to derive ↑
Inputs
mD
fraction
cp
psi⁻¹
ft
psi/cycle
psi
Description
Calculates skin factor from pressure transient (drawdown or buildup) test data, quantifying near-wellbore damage or stimulation relative to ideal (undamaged) reservoir flow. Also derives the effective wellbore radius and an approximate flow efficiency.
Variables
| Symbol | Unit | Description |
|---|---|---|
| S | — | Skin Factor |
| rw,eff | ft | Effective Wellbore Radius |
| FE | — | Flow Efficiency |
| k | mD | Reservoir permeability from the pressure transient test analysis. |
| φ | fraction | Effective porosity of the tested interval. |
| μ | cp | Viscosity of the flowing fluid at reservoir conditions. |
| ct | psi⁻¹ | Total system compressibility (rock + fluids), from the pore volume compressibility calculator or a representative field value. |
| rw | ft | Wellbore radius — typically 0.3-0.4 ft (≈0.1 m) for standard completions. |
| m | psi/cycle | Slope of the straight-line portion of the semilog (Horner or MDH) pressure transient plot, in pressure units per log cycle. |
| ΔP1hr | psi | Pressure change from the start of the test to 1 hour, read from the semilog straight line (extrapolated if needed). |
Assumptions
- The pressure transient data has a clearly identifiable semilog straight line representing radial, infinite-acting flow (not still in wellbore storage or already showing boundary effects)
- k, φ, μ, ct, and rw are all representative average values for the tested interval
- Flow efficiency uses an approximate ln(re/rw)≈7 in the absence of a directly measured or estimated drainage radius — treat FE as indicative, not exact
Limitations
- Skin derived from a single drawdown/buildup test reflects conditions at the time of the test — damage or stimulation effects can change over a well's life (e.g., scale buildup, frac decline)
- The flow efficiency approximation (ln(re/rw)≈7) introduces error if the well's actual drainage geometry is significantly different — use the exact ln(re/rw) when the drainage radius is known
- Requires a clean semilog straight line; wellbore storage, fractures, or layered reservoirs can distort the early-time data used to pick ΔP1hr
Use Cases
- → Stimulation candidate screening: Identify wells with high positive skin as workover or re-stimulation candidates to recover lost productivity.
- → Post-frac effectiveness check: Compare skin factor before and after a hydraulic fracture treatment to quantify the stimulation's effectiveness (large negative skin indicates an effective fracture).
- → Productivity index correction: Use the effective wellbore radius to correct a productivity index or IPR calculation for the actual (damaged or stimulated) near-wellbore flow condition.
Related Calculations
Region Notes
Permian Basin
Successfully stimulated Wolfcamp/Bone Spring horizontal wells commonly show S of -4 to -6 per stage, consistent with effective multi-stage hydraulic fracturing.
Gulf of Mexico
Sand control completions (gravel pack, frac-pack) in unconsolidated Miocene sands often show small positive skin (S=2-5) from completion-related restriction even on otherwise undamaged wells.
North Sea
Acid-stimulated Brent Group completions typically target S of -2 to -4; positive skin above +5 often triggers a re-stimulation evaluation.
Global
Always cross-check a skin factor derived from a short test against longer-term well performance — early-time storage effects can bias S if misidentified as the radial flow regime.
References
Primary source
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