Semilog Pressure Transient Skin Calculator
Estimate semilog skin from the fitted one-hour pressure change and positive psi-per-cycle slope. Review the rounded constants and omitted Horner correction.
Estimate semilog skin from the fitted one-hour pressure change and positive psi-per-cycle slope. Review the rounded constants and omitted Horner correction.
Published by PetroCalcHub Team. Linked workflow tools and revision log.
Calculation context and source notes
Estimate liquid-well skin from a selected radial-flow semilog line using a fitted one-hour pressure intercept, slope magnitude and compatible rock/fluid properties. This implements the historical rounded MDH/log-time approximation, not a raw first-hour gauge-reading calculation or a full finite-preflow Horner solution.
Obtain the one-hour pressure by extending the selected radial-flow straight line and subtract the flowing pressure immediately before shut-in. Enter the positive base-10 slope magnitude in psi per cycle. Check wellbore storage, flow regime and the preflow-time approximation before interpreting skin.
Solves for
s (dimensionless)
Best next page
Formula, variables, and sourceUse a fitted one-hour intercept
With fitted pressure change 500 psi, slope 80 psi/cycle, k = 50 mD, porosity 0.20, viscosity 2 cP, total compressibility 0.00001 per psi and wellbore radius 0.25 ft, the rounded expression returns skin 1.356994. The pressure change is a difference from the fitted one-hour line intercept, not an instruction to use the first recorded hour.
The logarithm is base 10. Its empirical field-unit combination assumes permeability in mD, viscosity in cP, compressibility per psi and radius in ft. A slope defined per natural-log interval must be converted to the per-decade basis before entry; porosity must be a fraction, not an unconverted percentage.
Know which buildup approximation is being used
The MDH/log-time approximation shown in the verified source omits the Horner preflow-time term. The full buildup expression includes an additional -log10[t_p/(t_p + 1 hour)] inside the skin bracket. It becomes small only when the preflow duration is sufficiently long relative to one hour; this interface has no t_p input and cannot calculate that correction.
Keeping the original rounded constants preserves existing saved calculations and inverse solutions. Compared with source constants 1.1513 and 3.2275, the default differs by about 0.002525 in skin. Input uncertainty and an incorrect line selection may matter much more, but rounding is still documented rather than presented as exact agreement. Negative skin can represent stimulation and is not clamped to zero.
Formula, variables, and default calculation
Reproducible default result
With the default values shown below, the calculation returns s = 1.356994 dimensionless. Defaults demonstrate the implementation; they are not recommended field values.
500
80
50
0.2
2
0.00001
0.25
Input definitions
Delta_p_1hr
psiFitted 1-Hour Pressure minus Pre-Shut-In Flowing Pressure
m
psi/cycleSemilog Straight-Line Slope
k
mDEffective Permeability
phi
fractionPorosity
mu
cPFluid Viscosity
c_t
1/psiTotal Compressibility
r_w
ftWellbore Radius
Output definitions
s
Skin Factor
Delta_p_1hr
Fitted 1-Hour Pressure minus Pre-Shut-In Flowing Pressure
m
Semilog Straight-Line Slope
k
Effective Permeability
Engineering use, assumptions, and limits
Obtain the one-hour pressure by extending the selected radial-flow straight line and subtract the flowing pressure immediately before shut-in. Enter the positive base-10 slope magnitude in psi per cycle. Check wellbore storage, flow regime and the preflow-time approximation before interpreting skin.
Assumptions
- A defensible infinite-acting radial-flow straight line has been selected, with compatible permeability and fluid/rock properties.
- The input is the fitted one-hour pressure minus pre-shut-in flowing pressure; slope is a positive base-10 magnitude and the MDH/long-preflow approximation is justified.
Limitations
- No test-data regression, derivative diagnostic or finite-production-time Horner correction is performed.
- The constants 1.151 and 3.23 are rounded; the verified source gives 1.1513 and 3.2275, so this page does not claim full-source precision.
Common mistakes
- Using the actual first-hour gauge reading while wellbore storage still dominates instead of the extrapolated radial-flow line.
- Using a negative plotting slope or a natural-log slope directly as positive psi per log10 cycle.
- Treating a negative skin estimate as automatically invalid, or omitting a significant finite-preflow correction without checking it.
Frequently asked questions
What inputs does the Skin Factor from Pressure Transient Semilog Analysis calculator need?
The calculation uses Fitted 1-Hour Pressure minus Pre-Shut-In Flowing Pressure (Delta_p_1hr), Semilog Straight-Line Slope (m), Effective Permeability (k), Porosity (phi), Fluid Viscosity (mu), Total Compressibility (c_t), Wellbore Radius (r_w). Enter values on the units shown beside each field.
What result does this calculator return?
The primary result is skin factor in dimensionless. The formula section shows the declared relationship, variables, and a reproducible default calculation.
What should I check before using the result?
Confirm the unit basis, source applicability, and input quality. No test-data regression, derivative diagnostic or finite-production-time Horner correction is performed. Using the actual first-hour gauge reading while wellbore storage still dominates instead of the extrapolated radial-flow line.
Source and record status
Source metadata identifies the relationship implemented by the calculator. Check the cited edition, unit basis, and scope against the engineering case before operational use.
Formation Evaluation and Analysis of Reservoir Performance: Governing Equations for Buildup, T. A. Blasingame (2012)
T. A. Blasingame, Texas A&M Petroleum Engineering 663 assignments (2012), printed page 5 / PDF page 6, MDH semilog skin relation. Rounded constants retained here; finite-production-time Horner correction is not included.
Open source reference