Wells - Technology
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 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, encompassing 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.
Rock Physics Modeling (iMOSS®)
Rock Physics Diagnostics (RPD) is the process of finding a rock physics model that is consistent with the well and core data available, and can help us understand the behavior of the reservoir and non-reservoir zones and correct for some of the problems encountered in well log data. For example, we may find that some zones in the well are closely fitted with an unconsolidated sand model while other zones follow critical porosity or elliptical crack models. These models may have adjustable parameters such as pore aspect ratio or critical porosity that can be determined empirically from the local data. Similarly some Vs prediction methods are best calibrated to local conditions if core Vp and Vs data or dipole shear wave logs are available.
Rock physics calibrations can also aid in selecting a fluid mixture model such as homogeneous or patchy distribution. Well log data can also be compared to available lab data and to theoretical limits such as Hashin-Shtrikman bounds.
The purpose of RPD is to allow reliable modeling and perturbation of seismic properties with changes in reservoir properties. For example, the data above shows P-wave impedance plotted versus total porosity. Superimposed on the data is a set of rock physics models with different clay fractions. For the data in question, there is a definite link between clay content and porosity. Therefore, if we wish to change porosity, then clay content must also be changed. The rock physics model derived allows prediction of seismic properties away from the wellbore.
Rock Physics Diagnostics in iMOSS