Calculator
Example Data Table
These points mimic late-time straight-line behavior. You can tick “Use sample dataset” to run a full example instantly.
| t (hr) | pwf (psi) |
|---|---|
| 0.25 | 3320 |
| 0.50 | 3305 |
| 1.00 | 3289 |
| 2.00 | 3273 |
| 4.00 | 3257 |
| 8.00 | 3241 |
| 16.00 | 3226 |
| 24.00 | 3219 |
| Δt (hr) | pws (psi) |
|---|---|
| 0.25 | 3192 |
| 0.50 | 3205 |
| 1.00 | 3217 |
| 2.00 | 3227 |
| 4.00 | 3236 |
| 8.00 | 3244 |
| 16.00 | 3251 |
| 24.00 | 3255 |
Formula Used
Semilog slope to permeability (oilfield internal units):
k (mD) = 162.6 · q · B · μ / (m · h)
Skin factor using the straight-line pressure at 1 hour:
s = 1.1513 · [ (pᵢ − p(1hr))/m − log10( (0.000264·k)/(φ·μ·cₜ·rᵥ²) ) ] (drawdown)
s = 1.1513 · [ (p(1hr) − pᵢ)/m − log10( (0.000264·k)/(φ·μ·cₜ·rᵥ²) ) ] (buildup)
Horner ratio (buildup):
H = (tₚ + Δt) / Δt, then fit p vs log10(H)
Helpful derived metric
rᵢ (ft) = 0.0325 · √(k·t / (φ·μ·cₜ))
Uses representative time t (hours) and computed k (mD).
Constants assume radial flow and standard field-unit correlations. Always verify straight-line time range and data quality before final decisions.
How to Use This Calculator
- Select Drawdown or Buildup, then choose your units.
- Enter reservoir and fluid inputs: q, μ, B, h, φ, cₜ, rᵥ.
- For drawdown, provide pᵢ. For buildup, provide tₚ.
- Pick Regression and paste late-time points, or use Direct if you already have m.
- Press Submit. Results appear above the form.
- Use the download buttons to export the report as CSV or PDF.
FAQs
1) What time range should I use for regression?
Use late-time data where the semilog plot becomes a straight line. Avoid early-time wellbore storage and noisy transitions. If R² is poor, refine the range and remove obvious outliers.
2) What does the semilog slope represent?
The slope is the pressure change per logarithmic cycle of time (or Horner ratio). It is directly related to permeability. A steeper slope generally indicates lower permeability for the same rate and thickness.
3) How is permeability calculated here?
Permeability is computed from the fitted slope using a standard radial-flow relationship in field units. The calculator converts your selected units internally, then outputs k in millidarcies for easy comparison.
4) What does a positive skin factor mean?
Positive skin commonly suggests near-wellbore damage or extra pressure loss. Values around 0–5 are moderate. Very large positive skin can indicate severe impairment, partial completion, or other mechanical restrictions.
5) Can skin be negative?
Yes. Negative skin often reflects stimulation such as acidizing or fracturing, which reduces near-wellbore pressure loss. Strongly negative values should be checked against data quality and confirmed with additional diagnostics.
6) How does the Horner plot estimate initial pressure?
For buildup, the straight-line fit is made versus log10(H). At H=1 (log10=0), the intercept corresponds to the extrapolated reservoir pressure. This is why consistent shut-in behavior matters.
7) Why do I need porosity and compressibility?
They are used to compute the dimensionless group inside the skin term and to estimate radius of investigation. Incorrect φ or cₜ values can distort skin and investigation radius, even if permeability looks reasonable.
8) What do the CSV and PDF downloads include?
CSV contains a summary plus a data snapshot table from the fitted points. PDF provides a compact one-page summary suitable for sharing. For detailed interpretation, keep your full original dataset and plots.