The Fluid substitution wizard models how replacing the pore fluid changes elastic well-log curves. It estimates the dry rock frame from the input elastic logs and their original fluid state, inserts a target fluid, and prepares substituted curves without changing the input logs.

The calculation uses isotropic, low-frequency Gassmann substitution with homogeneous Wood fluid mixing. Its results are modelled, not measured.

Launching the wizard

Open the Petrophysics ribbon bar and click Fluid substitution in the Rock Physics section; the button caption is shortened to Fluid sub. Choose one Well, or select Batch calculate and choose All wells, a well filter, a Data Manager well folder, or a well section. Limit the interval on the MD range, TVD range, TVDSS range, Markers range, or Zones range tab, complete the three parameter tabs, and click Run.

Drag the horizontal and vertical dividers to give the interval, parameters, preview, or QC area more room. Their positions are restored the next time the wizard opens.

Setting up the substitution

On the Inputs tab, every curve list offers all curves of the selected well and preselects a recognized curve type where one is available. The unit follows the selected curve where possible.

• Compressional velocity — select the P-wave velocity or slowness curve and set its unit to m/s, ft/s, µs/ft, or µs/m.
• Shear velocity — select the S-wave velocity or slowness curve in the same units. This wizard does not predict a missing shear curve.
• Bulk density — select the density curve and set its unit to g/cc or kg/m3.
• Porosity source — choose Curve, Constant, or Derived from density. The density-derived value is φ = (ρgrain − ρbulk) / (ρgrain − ρfluid). Total and effective porosity are separate curve types, so select the one that represents the rock model.
• Matrix modulus source — choose a Constant Km, a Curve in GPa or MPa, or Mineral mixing (VRH). Mineral mixing uses matching Mineral Volume Estimation curves from one LAS set and the editable bulk-modulus values in Mineral table.... A mineral with no bulk modulus is excluded and reported rather than treated as zero.
• Porosity cut-off — samples below this value are left null with LOW_POROSITY. The default is 0.02.

Curves written by Elastic properties are recognized by their curve types. If a required shear curve is missing, create it there with Predicted from Vp, then reopen Fluid substitution so the new curve can be selected. Mineral mixing (VRH) similarly consumes curves written by Mineral Volume Estimation.

The line below Shear velocity reports the selected curve's provenance:

• measured — the Elastic properties settings identify a measured DTS source;
• predicted (Greenberg-Castagna) — the substituted Vs inherits a prediction from Vp;
• constant Vp/Vs — the shear modulus and substitution depend on the constant ratio;
• unknown — the curve carries no Elastic properties run settings, so the wizard cannot establish whether it was measured or modelled.

The prediction and provenance warnings are informative; they do not block the calculation.

Defining the fluids

The Original fluid tab describes the fluid corresponding to the input logs. The Target fluid tab describes the fluid to insert into the dry frame. Each definition has fixed Brine, Oil, and Gas columns with a saturation, bulk modulus in GPa, and density in g/cc.

• Constant — applies the values shown on the fluid tab throughout the selected interval.
• Per zone — uses a separate fixed-column row for each selected zone. Select the zones on the interval selector first, then choose Per zone or click Define per zone.... Fill every value in a row, or leave the whole row empty to use the constants; the completion report names every zone that fell back.

For each fluid, Sw, So, and Sg must be between 0 and 1 and must sum to 1. The wizard displays the sum and does not normalize it. It calculates the homogeneous Wood mixture as Kf = 1 / (Sw/Kbrine + So/Koil + Sg/Kgas) and ρf = Sw·ρbrine + So·ρoil + Sg·ρgas.

Enter one Scenario name per run. While Output name suffix is left at its default, it follows the scenario name automatically.

Calculation and assumptions

For each valid sample, the original saturated bulk modulus and shear modulus are Ksat original = ρ1·(Vp1² − 4/3·Vs1²) and μ = ρ1·Vs1². The inverse Gassmann step removes the original fluid to obtain Kdry; the forward step inserts the target fluid to obtain Ksat substituted. The target density is ρ2 = ρ1 + φ·(ρf target − ρf original), and the wizard derives the substituted Vp and Vs from the new density and moduli.

The shear modulus is unchanged by the fluid, but Vs can still change because the density changes. The calculation does not clamp non-physical moduli, porosity, or saturations. Null input gaps and values outside an input curve's depth range remain null.

The Assumptions area states the model scope: isotropic, low-frequency Gassmann behavior; homogeneous Wood fluid mixing; unchanged rock frame, shear modulus, and porosity; and equilibrated pore-fluid pressure.

Output curves

Select the outputs in the Output curves table. Every selected row has an editable curve name and a unit list.

• Selected by default — Vp, Vs, Density, AI, SI, and VpVs.
• Optional — Kdry, Ksat original, Ksat substituted, Shear modulus, Kf original, Kf target, Fluid density original, Fluid density target, and Poisson's ratio.

Vp and Vs can be written as velocity or slowness in m/s, ft/s, µs/ft, or µs/m. Density outputs use g/cc or kg/m3; AI and SI use (g/cc)*(m/s) or (kg/m3)*(m/s); moduli use GPa or MPa; VpVs and Poisson's ratio are unitless. Poisson's ratio remains null where the substituted Vp is not greater than √2·Vs.

The default suffix is FS, followed by the scenario when one is entered. For example, scenario Gas_20pct produces names such as Vp - FS Gas_20pct. Editing the shared suffix updates names that have not been customized individually.

The saved results are ordinary typed well-log curves. Substituted Vp or slowness together with substituted Density can be selected by name in Well-Tie; AI and SI are not Well-Tie inputs. The curves can also be displayed in well-log views, used in cross-plots, or exported. The AVO tab uses prestack seismic rather than well logs and does not consume these outputs.

Running, quality control, and saving

• Save to workflow — adds the current wells or batch scope, interval, inputs and units, porosity and matrix sources, both fluid definitions, scenario, outputs, names, units, and QC threshold to the project Workflow for later replay.
• Run — calculates the selected curves and updates the Input/Output preview and QC flags report without writing curves to the well.
• Save — stores all calculated outputs. If a destination name already exists, one confirmation covers the affected output set; if no current result exists, the wizard reports Nothing to save.
• Close — closes the wizard.

Changing an input, unit, parameter, fluid value, output, or zone selection clears the previous result so an obsolete preview cannot be saved.

The report counts every evaluated sample as OK or under one of these QC flags:

• MISSING_INPUT — a required curve value, porosity, matrix modulus, or mineral volume is unavailable.
• LOW_POROSITY and INVALID_POROSITY — porosity is below the cut-off or outside the physical range from 0 inclusive to 1 exclusive.
• INVALID_SATURATION, INVALID_FLUID_MODULUS, and INVALID_DENSITY — a fluid definition or density is not physically valid.
• INVALID_VELOCITY, INVALID_KSAT, and KSAT_EXCEEDS_MATRIX — the input velocity relationship or original saturated modulus is non-physical, including an original Ksat at or above Km.
• INVALID_KDRY, INVALID_TARGET_KSAT, and NUMERIC_SINGULARITY — the inverse or forward substitution cannot produce a finite physical modulus.
• UNIT_ERROR — the selected units put velocity or density outside a plausible rock range.
• MINERAL_MODULUS_MISSING — mineral mixing has no usable bulk-modulus value for the available mineral volumes.

The completion report gives totals for all wells and a separate QC breakdown for each well. A batch continues when one well is missing a required curve; the well is skipped and the reason is reported. It also names zones that used constant fluid values and minerals excluded from a VRH mix.

Scope and limitations

Each run has one target scenario and the fixed brine, oil, and gas components. The wizard does not calculate pressure-, temperature-, salinity-, API-, or GOR-dependent fluid properties, and it does not apply patchy-saturation, anisotropic, invasion, pressure-dependent frame, or dynamic-to-static corrections. It never silently normalizes saturations, clamps porosity or moduli, fills gaps, or modifies the original logs.

See Also

• Petrophysics
• Elastic properties
• Mineral Volume Estimation
• Workflows
• Well-Tie