reservoir · inflow performance
Pressure Buildup — Horner Analysis
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Inputs
hr
STB/d
cp
RB/STB
ft
Paste one row per line — separate columns with a comma or tab: Shut-in Time (Δt) (hr), Shut-in Pressure (psi)
⚠ Needs at least 3 valid rows to calculate — 0 so far.
Description
Analyzes a pressure buildup test using the Horner plot method: shut-in pressure (Pws) against the log of the Horner time ratio (tp+Δt)/Δt forms a straight line during radial, infinite-acting flow, whose slope gives permeability and whose intercept (at a Horner ratio of 1, i.e. Δt→∞) gives the extrapolated static pressure P* — closing the gap Skin Factor's own documentation flagged, where k and the semilog slope m were previously required as pre-solved inputs rather than derived from raw buildup data.
Variables
| Symbol | Unit | Description |
|---|---|---|
| k | mD | Permeability |
| P* | psi | Extrapolated Pressure (P*) |
| tp | hr | Cumulative producing time before the well was shut in for the buildup test — used to form the Horner time ratio (tp+Δt)/Δt for each shut-in reading. A common approximation is cumulative production divided by the most recent stabilized rate. |
| q | STB/d | Stabilized production rate immediately before shut-in — the rate used in both the Horner time ratio's tp and the permeability formula. |
| μ | cp | Viscosity of the flowing fluid at reservoir conditions. |
| Bo | RB/STB | Oil formation volume factor at reservoir conditions. |
| h | ft | Net (flowing) reservoir thickness open to the wellbore. |
| Δt, Pws | — | One row per pressure reading during the buildup, from the middle-time (radial flow) portion of the test only — exclude early points still affected by wellbore storage and late points showing boundary effects. At least 3 points are needed for a meaningful semilog straight-line fit. |
Assumptions
- The pasted Δt/Pws points fall in the middle-time, radial, infinite-acting flow region — not still dominated by wellbore storage (early time) or already showing reservoir-boundary effects (late time)
- The well produced at an approximately constant rate q for the full producing time tp before shut-in — the classic Horner approximation for a more complex real rate history
- q, μ, Bo, and h are representative constants for the tested interval, applicable uniformly across all buildup readings
Limitations
- Requires visual or algorithmic identification of the genuine semilog straight-line region before pasting data — including wellbore-storage-affected or boundary-affected points will bias both k and P*, flagged only indirectly via a degraded R²
- The constant-rate-before-shut-in (Horner) approximation degrades for wells with a highly variable production history; a full superposition-time analysis would be needed for a more rigorous treatment
- Does not compute skin factor — pair this calculator's k and the semilog slope (m = 162.6·q·μ·Bo/(k·h), recoverable from this calculator's own k output) with the Skin Factor calculator's ΔP1hr input for a complete pressure transient analysis
Use Cases
- → Permeability from a buildup test: Derive reservoir permeability directly from raw shut-in pressure data, replacing an assumed or externally-sourced k value used elsewhere on this platform (e.g. in Skin Factor or Darcy-based inflow calculators).
- → Extrapolated static pressure (P*): Estimate the average or initial reservoir pressure P* for use in a material balance or productivity index calculation, without needing to fully shut in the well for an extended stabilization period.
- → Skin factor workflow completion: Feed this calculator's fitted k and semilog slope into the Skin Factor calculator's k and m inputs, completing a full pressure-transient workflow from raw buildup data through to a skin estimate.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring buildup tests often show a long wellbore-storage-dominated early period after multi-stage fracturing — identifying the true middle-time radial flow region (or a linear-flow regime instead) requires careful diagnostic (log-log derivative) plotting before selecting points for this calculator.
Gulf of Mexico
Conventional, high-permeability Miocene/Pliocene sandstone buildups typically reach the radial flow regime quickly, giving a well-defined Horner straight line over a wide Δt range.
North Sea
Brent Group buildup tests are commonly affected by nearby-boundary or fault effects at late time — restrict the pasted points to the confirmed middle-time region rather than the full test duration.
Global
Always cross-check P* against a separately-estimated average reservoir pressure (e.g. from a material balance) — a P* far outside the expected range often indicates the Horner straight line was picked from the wrong flow regime.
References
Primary source
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