GeoServices

Well Log Processing

One of the principal sources of information about rock properties is the sonic log. The compressional and shear wave velocities, together with density, allows the calculation of dynamic elastic properties. By the application of suitable physical models, these can be related to static properties and rock strength.

The measurement of the shear wave velocity is usually indirect: The dipole sonic observes the borehole flexural wave which travels at shear wave speed. In practice, detection of the flexural wave arrival is not an entirely automatic process and often requires some intervention by the interpreter.

In a stressed rock, the shearwave will be birefringent. The dipole sonic is usually oriented in the same way as an image log, and the direction of the fast shear wave can be measured by comparing arrivals at orthogonal sets of receivers.

The Stoneley (tube) wave is reflected by fractures at a high angle to the borehole, as is often the case in horizontal wells. It therefore provides an independent observation of extensive open fractures. The Stoneley wave can also be used to estimate rock permeability.

Eriksfiord performs sonic processing as a standalone service, or as part of a geomechanical evaluation. In any case, if sufficient background information is available, the result is presented as a mechanical properties log.

Borehole image logs of all types are processed by Eriksfiord - acoustic, electrical (water or oil base mud), azimuthal density. The "speed correction" to eliminate the effect of friction is the most critical stage of pre-processing, and some interaction is usually required to obtain the best result. The correction is based on integration of acceleration along the sonde axis.

After converting curves to a rectangular image format, image processing allows compensation for artifacts and the enhancement of geological features of interest.

The borehole image is used for the basic geological characterization of the rock structure (facies) down to cm scale, the orientation of planar structures (bedding, faults, fractures), the detection and typing of fractures, and features on the borehole wall arising from stress instability (breakout and induced fracturing).

Eriksfiord uses PC based software for log processing (LogIC, cfr. www.luk.net). We can process data in the base office, when traveling or on location, a flexibility which facilitates dialogue with the client. Output is easily moved across to software for report writing, presentation or post-processing with other systems.