Understanding formation geomechanics

An operator in the Llanos basin, Colombia, wanted to include geomechanics data in designing the perforating program for an exploration well. The unconfined compressive strength (UCS) and stress regime of the reservoir significantly influence the success of perforating operations. An analysis based solely on conventional petrophysical data would not be able to fully assess near-wellbore alteration for specifying a gun system configuration and shaped charges to reliably penetrate into unaltered formation and minimize skin.

Acoustically characterizing the reservoir in three dimensions

The Sonic Scanner acoustic scanning platform measures the formation axially, azimuthally, and radially to deliver a fully 3D acoustic characterization that addresses both intrinsic and drilling-induced anisotropy. The platform's multiple monopole and dipole transmitters produce compressional, shear, and Stoneley waveforms of unprecedented quality for advanced processing of slowness values that delivers the most comprehensive geomechanical understanding.

The shear radial profile indicated a 6-in depth of alteration whereas the monopole radial profile showed 8 to 10 in. The more extensive monopole-based depth of alteration was conservatively employed for conducting SPAN Rock stressed-rock perforating analysis to specify the perforating gun system and select shaped charges that would penetrate beyond the alteration zone while minimizing skin.

Reducing skin by more than 60%

Petrophysical evaluation determined that the sand body had 10% to 15% porosity and 5% to 10% clay volume. The predicted UCS was 13,000 psi. These data and the geomechanics information from the Sonic Scanner platform were used to evaluate PowerJet Nova extradeep penetrating charges in comparison with conventional charges at 12 shots per foot (spf). The predicted skin factor for the conventional charges was 2.

Following the perforating operation using the specified PowerJet Nova charges, the perforated zone was tested with the drillstring still in the well. The log-log plot of the first derivative matched to the test parameters determined that the resulting skin was only 0.766, which is a more than 60% reduction from the modeled conventional perforations to a value with practically no adverse effect on flow performance. Perforation penetration beyond the alteration zone and the reduced skin confirmed the benefits of including Sonic Scanner platform's geomechanical data in the perforating design.