Shear Log Modeling Module

Purpose

The Shear Log Modeling module allows users to generate synthetic shear sonic logs that can be used for analyzing geomechanical properties.

Primary Outputs

The following curves are the primary interpretations made in this module.

Screenshot showing synthetic shear logs from various methodologies in the Shear Log Modeling module.

Discussion

The Shear Log Modelling module is intended to help users generate a calibrated shear sonic log for wells in which only a compressional sonic log was measured, as is often the case. The module uses the Greenberg and Castagna methodologies for calculating a synthetic shear log that can then be used in other calculations, such as in the geomechanics module or the fluid substitution module. A common workflow may be to:

1. Identify a well that has a shear sonic log
2. Interpret that well through each module through the water saturation
3. Navigate to the shear log modelling module
4. Identify the parameters that create a good fit between the observed and modelled shear sonic (DTS) curves
5. Apply that calibration by using those parameters in the key well (if the present well is not the key well)
6. Use those curves in the geomechanics, fluid substitution or other modules.

The Vs Model option allows you to select the model that best fits these observed data. Options include:

• Greenberg & Castagna Sandstone
• Greenberg & Castagna Limestone
• Greenberg & Castagna Dolomite
• Greenberg & Castagna Shale
• Blended Lithology model
• Mudrock Line model
• Carrol model
• Brocher model
• Castagna Limestone
• Castagna Dolomite
• Eskandari models

The Greenberg & Castagna models use the given end-member lithology to reconstruct the shear wave, while the blended lithology model uses the mineral proportions from the mineral inversion module in combination with published mineral properties to determine the shear log response. The Mudrock line model from Castagna et al. to calculate the shear log.

The Scale Factor parameter is a multiplier to help you stretch or squeeze results to obtain a better fit between the observed and calculated DTS curves. The Linear Shift parameter is a simple addition/subtraction from the calculated DTS curve, also designed to help obtain a better fit between observed and calculated results.

Calibrating and QC’ing the Model

The log tracks displayed show the results from each of the Greenberg & Castagna end-member models and the Blended Lithology model against the observed DTS log. Red shading indicates the observed DTS is greater than the calculated DTS, while blue shading indicates the opposite. In an ideal match there is very little difference between the curves. The user should evaluate each of the models on a zone-by-zone basis and select the model that gives the best fit in each zone, accounting for their understanding of the lithology. The scaling and shifting parameters can be subsequently applied, however, best practice is to keep the scaling factor as close to 1 as possible and the shifting factor as close to 0 as possible.

There are additional tracks for model QC. The observed vs. modelled Poisson’s ratio track can highlight areas of impossibility (e.g., PR > 0.5 or < 0) as well as highlight areas of severe misfit once the model has been set and calibrated for each zone. A second track shows the modelled and observed shear modulus (G). Once again, after all calibration has been finalized a substantial mismatch between these two would suggest that the model is likely not ideal.

Remember that very few wells in your data set likely have a DTS curve, so it can be useful to use the Filters and set the required logs to “Custom” and enter “dts” in the search box. However, remember to remove this filter once you are ready to move along to other workflows that you want to apply to the broader set of wells.

An ideal approach, if you have two wells with DTS curves, is to set the parameters on one, and use the other for a blind test. Although this can work at scale and across large numbers of wells, if you need precise results you will need to test your calibration across several wells to gain confidence that it is delivering the desired results.