The Elastic properties wizard calculates elastic and acoustic properties from a well's sonic curves and, where a property requires it, bulk density. With Calculate all cleared, a run produces one curve for the selected method. With it checked, one run calculates the selected properties from shared inputs.

It runs at the selected well or, with Batch calculate, at a group of wells, and it shares the well selection, depth range, unit handling and run reporting common to the calculators of the Petrophysics bar.

Launching the wizard

Open the Petrophysics ribbon bar and click Elastic properties in the GeoMech section; on the button the caption is shortened to Elastic. Select the well and, if needed, limit the interval on the MD range, TVD range, TVDSS range, Markers range or Zones range tab. For one output, leave Calculate all clear and choose a method. For several outputs, check Calculate all and configure the property table and shared inputs described below, then click Run. The figure shows Calculate all mode.

The dialog carries two dividers: one between the depth-range tabs and the Method group, and one between the left pane and the quality-control panel on the right. Drag either of them to give more room to the part that is being worked on.

Calculate all

Check Calculate all to replace the single Method choice and Output curve field with a property table. Check each property to calculate in the Property column, or use Select all and Clear all. Every property listed below starts checked except Predicted shear slowness, Acoustic impedance (P) and Shear impedance (S); these three remain available to select.

The Inputs column identifies what each property needs. Choose the required curves and units in the shared input rows; the shear source, lithology and coefficients apply where needed. Highlight a property row to see its equation and, after a run, its output track. The checkboxes determine which properties are calculated. Edit each selected property's Output curve name and Unit separately.

Enter an Output group name for the results folder; the default is Elastic properties. The same selection can be run for several wells with Batch calculate. Select at least one property before running or saving the setup to a workflow.

Methods

With Calculate all cleared, the Method list is grouped by property family, and the group headers are labels that cannot be selected. Choosing a method rebuilds the input rows, the equation shown under them and the Output curve name and unit list, while the curves and units already assigned are kept — so one input assignment can be run through one method after another without being set up again. The same properties are available as rows in the Calculate all table.

Velocities

• P-wave velocity (Vp) — the compressional slowness converted to velocity. Output curve Elastic - Vp, in m/s or ft/s.
• S-wave velocity (Vs) — the shear slowness converted to velocity. Output curve Elastic - Vs, in m/s or ft/s.
• Vp/Vs ratio — the ratio of the two velocities, taken directly from the slownesses, so it needs no density curve. Output curve Elastic - Vp over Vs, without a unit.
• Predicted shear slowness — writes the shear slowness predicted from the compressional log as an ordinary curve, so it can be inspected, plotted and reused. Output curve Elastic - DTS predicted, in µs/ft or µs/m.

Impedances

• Acoustic impedance (P) — density multiplied by the compressional velocity. Output curve Elastic - Acoustic impedance, in (g/cc)*(m/s) or (kg/m3)*(m/s).
• Shear impedance (S) — density multiplied by the shear velocity. Output curve Elastic - Shear impedance, in the same units.

Elastic moduli (dynamic)

• Poisson's ratio — ν = (Vp² − 2·Vs²) / (2 · (Vp² − Vs²)). It is a velocity ratio, so it needs no density curve. Output curve Elastic - Poisson's ratio, without a unit.
• Young's modulus — E = 2 · μ · (1 + ν), with μ = ρ · Vs². Output curve Elastic - Young's modulus, in GPa, MPa or Mpsi.
• Shear modulus (μ) — μ = ρ · Vs². Output curve Elastic - Shear modulus, in GPa, MPa or Mpsi.
• Bulk modulus (K) — K = ρ · (Vp² − 4/3 · Vs²). Output curve Elastic - Bulk modulus, in GPa, MPa or Mpsi.
• Lamé's first parameter (λ) — λ = ρ · (Vp² − 2·Vs²) = K − 2/3 · μ. Output curve Elastic - Lambda, in GPa, MPa or Mpsi.
• P-wave modulus (M) — M = ρ · Vp² = K + 4/3 · μ. Output curve Elastic - P-wave modulus, in GPa, MPa or Mpsi.
• Bulk compressibility — Cb = 1 / K. Output curve Elastic - Bulk compressibility, in 1/GPa or 1/MPa.

AVO attributes

• Lambda-Rho (λρ) — λρ = AI² − 2 · SI². Output curve Elastic - Lambda-Rho, in GPa*g/cc.
• Mu-Rho (μρ) — μρ = SI². Output curve Elastic - Mu-Rho, in GPa*g/cc.

Both AVO attributes are built from the impedances, so they agree with the impedance curves that the same input assignment produces.

Input curves

With Calculate all cleared, only the rows that the selected method uses are shown. With it checked, the rows show the inputs needed by any checked property. Each curve list contains all curves of the well and carries its own unit list; on first opening, a row is filled from the well's standard mnemonics and the unit follows the one recorded on the chosen curve.

• Compressional slowness — the sonic log, in µs/m or µs/ft, preselected from DT, DTCO, DTC or DTP. It is shown for every method except Shear impedance (S) and Mu-Rho (μρ), which read the shear velocity only — and it appears there as well whenever the shear velocity is derived from the compressional log.
• Shear source — where the shear velocity comes from: DTS curve adds a Shear slowness row with its own unit list, preselected from DTS, DTSM or DTSH; Predicted from Vp derives it from the compressional log; and Vp/Vs constant applies a single typed Vp/Vs ratio, 1.90 by default. The row opens on DTS curve when the well carries a recognised shear log and on Predicted from Vp when it does not, so the wizard opens in a state that produces a result. It is hidden for the methods that do not use the shear velocity — P-wave velocity (Vp), Acoustic impedance (P) and P-wave modulus (M) — and for Predicted shear slowness, which always predicts.
• Bulk density — the density log, in g/cc, kg/m3 or lb/ft³, preselected from RHOB, RHOZ or DEN. It is shown only for the methods that read density: both impedances, all moduli except Poisson's ratio, and both AVO attributes.

Supply each curve in the unit it was logged in and let the wizard convert it; the unit lists follow the project measurement system where the curve itself carries no unit.

Predicting the shear velocity

With Predicted from Vp the shear velocity is calculated from the compressional velocity through Vs = c · Vp² + a · Vp + b, with both velocities in km/s. The Lithology list fills the Coefficients from a published relation, and they stay editable afterwards, so a locally derived relation can be entered instead:

• Mudrock line (Castagna) — a = 0.8621, b = −1.1724, c = 0
• Sandstone — a = 0.80416, b = −0.85588, c = 0
• Shale — a = 0.76969, b = −0.86735, c = 0
• Dolomite — a = 0.58321, b = −0.07775, c = 0
• Limestone — a = 1.01677, b = −1.03049, c = −0.05508

A predicted shear velocity is always marked: the line Shear velocity is predicted, not measured. appears under the coefficients, and the preset and coefficients used are stored with the output curve. Choosing the Predicted shear slowness method instead writes the prediction out as a curve of its own.

Output curve

With Calculate all cleared, the Output curve field carries the default name of the selected method, listed above, and can be edited before the run. The unit list beside it offers the units that suit the property. The calculation is evaluated in SI — velocities in m/s, density in kg/m³, moduli in Pa — and converted once into the selected output unit, which the note under the equation repeats; the same note states that these are dynamic properties, obtained from sonic-frequency measurements and not from laboratory loading.

The method, the input curves with their units, the shear source — including the preset and coefficients where the shear velocity was predicted — and the calculated interval are stored together with the curve. The Show elastic properties settings command on that curve in the Data manager reopens the wizard with those values.

Running and saving

The wizard has the standard row of buttons along the bottom of the dialog:

• Save to workflow — adds the current setup to the project Workflow as a task that can be re-run later. In Calculate all mode, it retains the selected properties, their output names and units, and the output group.
• Run — calculates the selected method or all checked properties for the chosen well or batch. The preview shows the highlighted property's output in Calculate all mode.
• Save — stores the calculated curve for a single method. In Calculate all mode, it places only the selected results in the named folder under each well's Logs in the Data manager, rather than as ungrouped curves. If an output name already exists in the destination, a Confirm prompt asks before replacing it; if nothing has been calculated yet, the wizard reports Nothing to save.
• Close — closes the wizard.

Changing the method, an input curve or unit, a coefficient or the zone selection discards the previous result. In Calculate all mode, changing a property's checkbox, output name or unit also clears the result; run the calculation again before saving it.

Results

After Run, the wizard lists the wells for which the calculation completed. In Calculate all mode, it also reports how many properties were selected and shows sample counts by cause for each property that produced null values. In either mode, the reported causes are:

• a required input that is null or not positive;
• Vp ≤ Vs, where the shear arrival outruns the compressional one, which points at bad data or a mispicked curve;
• Vp ≤ √2 · Vs, the boundary below which Poisson's ratio is no longer positive, so every method that reads the shear velocity nulls the sample;
• a bulk modulus of zero, for Bulk compressibility;
• a result that is not a finite number.

A large count is usually the sign of a mispicked curve or a wrong unit assignment rather than of the rock, which is why the causes are counted separately. Wells that were skipped or only partly calculated are reported after the run summary.

Quality control

The panel on the right of the dialog carries two tabs:

• Input/Output — carries the name of the selected well in its title and plots the required input curves beside the calculated Output track. In Calculate all mode, the output track follows the highlighted property row.
• Crossplot — plots one of the pairs offered by the Pair list: ν vs Vp/Vs, AI vs Vp/Vs or λρ vs μρ. This is where a wrong unit assignment becomes visible: curves that look plausible on their own land in an impossible place on the plot. The caption states how many points are plotted and from which curves, and, where the shear velocity was not logged, whether it came from the Vp/Vs constant or from the prediction, with the preset and coefficients used.

See Also

• Petrophysics
• Base properties
• Pore pressure
• Workflows