geomechanics · elastic properties
Brittleness Index (Rickman)
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Inputs
GPa
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GPa
GPa
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Description
Estimates a shale's relative brittleness for hydraulic-fracture completion design by averaging a normalized Young's modulus and an inverted, normalized Poisson's ratio (higher E and lower ν both push toward more brittle behavior). This implements the widely-cited normalized-average version of "the Rickman brittleness index" — see this calc's limitations for a direct disclosure that the primary paper (SPE-115258) could not be accessed this session, that at least two materially different formulas both circulate under Rickman's name, and that this calc implements the more commonly cited but not primary-confirmed one. This calc was built as a deliberate, owner-directed exception to this project's usual evidence bar (comparable open questions elsewhere — Swab/Surge, Breakout Width, Tensile Strength, both IFT calcs — were deferred or permanently parked rather than shipped); it carries the heaviest disclosure of any calculator in the project as a direct consequence.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Enorm | — | Normalized Young's Modulus |
| νnorm | — | Normalized Poisson's Ratio |
| BI | — | Brittleness Index |
| Edyn | GPa | Dynamic Young's modulus, re-entered from this discipline's own Dynamic Elastic Moduli calculator (same rock, same depth) — not re-derived here from Vp/Vs/ρ directly, matching this platform's established re-entered-scalar convention. |
| νdyn | — | Dynamic Poisson's ratio, re-entered from Geophysics's Vp/Vs calculator's own promoted ν output (same rock, same depth) — a cross-discipline re-entered scalar, the same pattern already used by this calc's own Dynamic Elastic Moduli cross-check note. |
| Emin | GPa | Lower Young's-modulus calibration bound (≈1 Mpsi, the value reported in secondary literature for Rickman et al.'s own Barnett Shale dataset) — a basin-specific calibration constant, NOT a universal physical limit. Rickman's own paper's central thesis is that other shale plays are not clones of the Barnett; using this default outside a Barnett-like play is likely wrong and should be replaced with a locally-calibrated min/max from the play's own core or log population. |
| Emax | GPa | Upper Young's-modulus calibration bound (≈8 Mpsi, the value reported in secondary literature for Rickman et al.'s own Barnett Shale dataset) — a basin-specific calibration constant, NOT a universal physical limit. See Emin's description: recalibrate for any non-Barnett-like formation. |
| νmin | — | Lower Poisson's-ratio calibration bound. Commonly cited in secondary literature alongside Rickman et al.'s method, but this specific value could not be traced to an authoritative primary-source confirmation this session (SPE-115258 itself was paywalled) — treat with LOWER confidence than the E bounds above, and recalibrate locally wherever possible rather than trusting this default. |
| νmax | — | Upper Poisson's-ratio calibration bound. Same lower-confidence caveat as νmin: commonly repeated in secondary literature but not traced to an authoritative primary-source confirmation this session — recalibrate locally wherever possible. |
Assumptions
- E and ν represent the same rock volume, at the same depth, as each other and as the calibration window's own dataset — mixing formations/depths without matched calibration bounds produces a meaningless index.
- Emin/Emax/νmin/νmax are treated as the correct calibration window for the formation being screened; the defaults supplied are Barnett Shale values and are NOT assumed to transfer to any other play without re-derivation.
- The rock is assumed to behave as adequately characterized by these two elastic terms alone — no mineralogy, TOC, natural-fracture density, or in-situ stress state is considered, even though all of these materially affect real frac-ability.
Limitations
- FORMULA ATTRIBUTION IS UNCERTAIN. "Rickman's brittleness index" appears in the secondary literature under at least two materially different formulas: this normalized-average version (BI = (Enorm+νnorm)/2), and a separate linear regression on Rickman et al.'s own Barnett dataset reported elsewhere as B = 7.14·E − 200·ν + 72.9 (static E in Mpsi, static ν). The primary paper (Rickman, R., Mullen, M., Petre, E., Grieser, W., and Kundert, D., 2008, SPE-115258, 'A Practical Use of Shale Petrophysics for Stimulation Design Optimization: All Shale Plays Are Not Clones of the Barnett Shale') is paywalled at OnePetro and could NOT be directly accessed this session (or the geomechanics-brittleness-phase1 research session that preceded this build). This calc implements the more widely-cited normalized-average version, on the strength of multiple independent secondary characterizations converging on it — but this is NOT a confirmed reproduction of the paper's own original method, and the two formulas are not numerically equivalent.
- STATIC/DYNAMIC GAP, STACKED. This calc consumes DYNAMIC E (from Dynamic Elastic Moduli) and DYNAMIC ν (from Vp/Vs) — velocity-derived quantities at sonic/seismic strain rates. Rickman's method is reported in secondary literature to use STATIC E/ν, obtained via a static-dynamic correlation not verified this session. This is a second, independent uncertainty layer stacked on top of Dynamic Elastic Moduli's own already-disclosed dynamic/static gap: even if the normalized-average formula above is exactly right, feeding it dynamic inputs where Rickman's method calls for static ones is a further, unquantified departure from the original method.
- CALIBRATION BOUNDS ARE BASIN-SPECIFIC, NOT UNIVERSAL. Emin/Emax/νmin/νmax are calibration constants from Rickman et al.'s own Barnett Shale dataset (E bounds ≈1-8 Mpsi, reasonably well corroborated across secondary sources; ν bounds ≈0.15-0.4, corroborated with LOWER confidence — no source this session could be traced to an authoritative primary confirmation of the ν bounds specifically). Rickman et al.'s own paper's central argument, per its title, is that other shale plays are 'not clones of the Barnett Shale.' Using these unmodified defaults to screen a different play's brittleness is very likely to produce a misleading result — this is not a theoretical caveat, it restates the cited paper's own thesis.
- This calc was built as an explicit, owner-directed exception to this project's normal evidence bar for shipping a calculator: comparable open provenance/verification questions elsewhere in this project (Swab/Surge, Breakout Width, Tensile Strength, and both Interfacial Tension calcs) were deferred or permanently parked rather than built. That decision was made deliberately and with full knowledge of this departure — see the project's Decision Log for the geomechanics-brittleness-phase2 entry — not as a default or oversight.
- No mineralogy (XRD/spectroscopy), TOC, natural-fracture density, or in-situ stress state is incorporated — this is a purely elastic-moduli-based screen, one of at least two competing families of brittleness index in the literature (the other being mineralogy-based indices, e.g. Jarvie's quartz/(quartz+clay+calcite) or Wang & Gale's dolomite/TOC-extended variant — neither of which this platform currently has the input infrastructure to support).
Use Cases
- → Landing-zone ranking for horizontal completion design: Rank candidate landing zones within a lateral by relative brittleness score to prioritize the most frac-favorable intervals, always cross-checked against locally-calibrated E/ν bounds rather than the Barnett-derived defaults.
- → Offset-well brittleness logging: Compute a continuous brittleness-index log from an offset well's Dynamic Elastic Moduli and Vp/Vs outputs to extend a completion-design brittleness screen laterally, pending core-calibrated static E/ν and locally-derived normalization bounds.
Related Calculations
Region Notes
Barnett Shale (Fort Worth Basin)
The only play this calc's default calibration bounds are actually derived from — use the defaults here with the least additional caution, though still per-well core calibration is preferable to any global default.
Global (all other plays)
Do not use the supplied Emin/Emax/νmin/νmax defaults without first checking them against the target play's own core or log-derived E/ν population — Rickman et al.'s own paper's stated thesis is that shale plays are not interchangeable with the Barnett.
References
Primary source
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