petrophysics · rock physics
Pore Volume Compressibility (Newman, Sandstone/Limestone)
cf = 97.3×10⁻⁶/[1+55.9φ]^1.429 psi⁻¹ (consolidated sandstone) | cf = 0.856/[1+2.48×10⁶φ]^0.930 psi⁻¹ (limestone) [Newman 1973]
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Inputs
fraction
—
Description
Estimates pore volume compressibility (cf) from porosity using Newman's (1973) lithology-specific regression correlations for consolidated sandstone and limestone. Provides a lithology-selectable alternative to Hall's (1953) sandstone-only correlation (see pore_compressibility) for use in material balance calculations quantifying the rock expansion drive term in undersaturated oil reservoirs.
Variables
| Symbol | Unit | Description |
|---|---|---|
| cf | μpsi⁻¹ | Pore volume compressibility |
| φ | fraction | Total porosity of the reservoir rock. Valid calibration range depends on the selected lithology: 2–23% for consolidated sandstone, 2–33% for limestone. Use the porosity representative of the rock being evaluated in material balance. |
| Method | — | 1 = Consolidated sandstone (Newman 1973, valid 2–23% porosity); 2 = Limestone (Newman 1973, valid 2–33% porosity). Select the lithology matching the reservoir rock being evaluated — the two regressions are not interchangeable. |
Assumptions
- The selected lithology (methodCode) accurately reflects the reservoir rock — the consolidated sandstone and limestone regressions are not interchangeable and produce meaningfully different cf at the same porosity.
- Compressibility is evaluated at initial reservoir pressure; neither regression models pressure-dependent cf during depletion.
- Friable sandstone and unconsolidated sand — the other two lithologies Newman's 1973 paper covers — are not implemented in either branch of this calc; do not apply this calc to those rock types (see limitations).
- Pore volume compressibility dominates rock compressibility; grain and bulk compressibility are assumed negligible, the same convention used in pore_compressibility (Hall).
Limitations
- Verification-grade sourcing is partial: these coefficients were verified against a single secondary transcription — Almohammad (University of Basrah, Petroleum Engineering Dept., "Lecture Four: Rock Compressibility" course notes) — not against the original SPE-3835-PA paper directly, and not cross-verified against a second independent source. Treat as probably correct but thinly evidenced, not textbook-consensus-confirmed.
- Friable sandstone and unconsolidated sand coefficients from Newman's original paper are not implemented — no verification-grade source was found for those two lithologies during research; they remain deferred.
- cf is pressure-dependent — it increases as reservoir pressure declines below initial pressure. Both regressions give a single-value estimate at initial conditions only.
- The limestone regression's coefficient structure has a much larger porosity-scaling term (2.48×10⁶) than the sandstone regression (55.9); this asymmetry could not be cross-checked against a second independent source. Validate against core measurements wherever possible before relying on the limestone branch for decisions.
Use Cases
- → Lithology-specific material balance input: Supply a lithology-appropriate rock expansion term (cf × Δp × Vp) for undersaturated material balance when the reservoir is limestone, or clearly consolidated sandstone, rather than defaulting to Hall's sandstone-only correlation.
- → Cross-check against Hall's correlation: Compare the sandstone branch against pore_compressibility (Hall) for the same porosity as a sanity check — large disagreement between the two independently-derived sandstone correlations flags a porosity or lithology-classification issue worth investigating before trusting either result.
- → First-pass carbonate reservoir estimate: Provide a first-pass cf estimate for limestone/carbonate reservoirs, where Hall's sandstone-only correlation is known to be inapplicable, while disclosing the single-source verification caveat to the user.
Related Calculations
Region Notes
Carbonate reservoirs (general)
The limestone branch is intended for carbonate reservoirs where Hall's sandstone-only correlation (pore_compressibility) does not apply. Given the single-source verification status disclosed in limitations, treat results as a first-pass estimate pending a core measurement or a field-specific literature cross-check.
Consolidated sandstone (general)
For sandstones already well-described by Hall's (1953) correlation, prefer pore_compressibility unless there is a specific reason to use Newman's sandstone branch instead — running both as a QC comparison is recommended when in doubt.
References
Primary source
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