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Geophysical Well Log Analysis (GWLA®)

Well logs need to be carefully conditioned or pre processed prior to their use in a modeling workflow. We term this step Geophysical Well Log Analysis or GWLA

The basic Geophysical Well Log Analysis (GWLA) workflow consists of:

• Collect and organize input data
• Perform geophysical log interpretation for volume minerals, porosity, and fluids
• Edit logs and perform mud filtrate invasion correction (as needed)
• Generate missing curves
• Determine fluid properties (oil API, brine salinity, etc.) and reservoir pressure-temperature
• Perturb reservoir properties using rock physics effective medium models (pseudo-well modeling)
• Compute synthetic seismic traces
• Generate trend curves and crossplots
• Create graphics and digital output files

The specifics of each project or job can be varied to suit the needs of the client and the characteristics of the data available.

a representative GWLA display

Well log analysis for geophysics differs in several important ways from standard log analysis. In most cases well logs are obtained for the purpose of estimating recoverable hydrocarbon volumes. Therefore the zone of interest is mainly the producing interval(s). For geophysics, well logs form the basis for relating seismic properties to the reservoir. While we are still concerned about producing intervals, we also need good information about all of the rock through which the seismic waves have passed. Therefore our zone of interest is much larger and encompasses basically everything from the surface to TD. This means we have to take great care to correctly treat the log data through shales, across drilling breaks, casing points, and washouts.

In all cases the log data will require some editing, normalization, and interpretation before they can be used in a reservoir study. Several specific analysis steps will be followed:

• De-spike and filter to remove or correct anomalous data points
• Normalize logs from all of the selected wells to determine the appropriate ranges and cutoffs for porosity, clay content, water resistivity, etc.
• Compute the volumetric curves such as total porosity, Vclay, and Sw
• Calibrate the volumetric curves to core data if available
• Correct sonic and density logs for mud filtrate invasion if needed
• Compute Vshear on all wells

In wells where important well log curves are missing, we will reconstruct those curves synthetically. There are two ways this is done. The first is through application of modern rock physics principles. For example, several deterministic methods exist for obtaining density from sonic logs or sonic logs from resistivity. The other approach is to use neural network technology. This is often required when no direct physical relationship is available.