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Imaging a Fault in Valhall Field

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Background

Here we present an integrated characterisation of a fault zone in mudstones (from the Valhall Field, Central N Sea) using log and core data from a single well. The data was collected to estimate the petrophysical properties of the mudstone - a few years after drilling the well, more advanced formation evaluation data and core was evaluated for fault characterisation

Observations made

The following observations were made at a similar depth are interpreted as a fault or fault zone.

  • Fast shear azimuth - (on right hand most track in Fig. 1) is a useful proxy for the direction of Shmax. Across this logged section it is consistently NW-SE apart from at the red hooped interval in Fig. 1. This suggests a perturbation in the anisotropy orientations and magnitudes - which can also be seen as a change in the character of the calliper curve (track 1).

  • Density log (track 6, fig. 2) - with resolution enhancement - shows a zone of bedding disturbance at approx. same depth interval as above.

  • Shear slowness vs. distance from borehole (track 7, fig. 2 with borehole in centre and track edges represents 2 ft from borehole axis - right hand image is variation in slow shear whereas left hand image is fast shear) - note the change at approx same interval as the logs above.

  • Near wellbore reflection image (track 9 fig. 2) - picking up reflections from planar features in the formation - note the stronger reflections at the interval of interest from the above logs.

  • Rubble zone (fig. 3) consisting of pods of mudstones that were originally surrounded by black hairline fractures - the pods were dislodged during coring.

Fig. 1. Azimuthal anisotropy from Sonic LWD data

Fig. 1. Azimuthal anisotropy from Sonic LWD data

Fig. 2. Integration of log data showing location of fault.

Fig. 2. Integration of log data showing location of fault.

Fig. 3 Photo of mudstone core from a rubble zone

Fig. 3 Photo of mudstone core from a rubble zone

Full article…

Bradley et al (2019). Overcoming coring challenges in a New Unconventional Play Offshore by Integration of Formation Evaluation Data. SPE paper presented at the SPE Annual Technical Conference and Exhibition held in Calgary, Alberta, Canada, 30 Sept - 2 Oct, 2019.