The influence of the distance of a perforation gun to the inner casing wall is known to be critical for the correct assessment of the perforation performance by way of entry hole diameter and perforation consistency. Presented in the paper shall be a study on how the perforating gun body to casing wall clearance value will influence the perforating tunnel geometry and ultimately the charge performance on very hard rocks for hydraulic fracturing in an API Section II Test environment that simulates a decentralized gun inside the casing.

Three different shaped charge designs were tested on one type of extreme hard rock core whose UCS is upwards of 30,000 psi. Traditional performance evaluation methods of measuring the Entrance Hole Diameter and Depth of Penetration were undertaken, as well as the evaluation of tip fractures and rock strength influence. The cylindrical rock cores used in this test series were pre-scanned or pre-evaluated to reduce as much test anomalies as possible by employing alternative rock characterization methods such as sonic velocities of the rock and scratch test method for UCS measurement.

The pre-screening process proved itself as an easy method to reduce the variation in test results. Still certain deviations were visible for identical shots, which can be related to differences in the sonic velocities of the target. It becomes also visible, that different charge designs react differently to changes in the rock properties. Some are more resilient or robust than others.

The influence of the gun to casing clearance on the penetration is present, but to smaller extent, than the entrance hole variation. Two types of rocks were chosen as targets, a quartzite and shale cores. The results are generally transferable from the very hard quartzite to shale targets, but the geological anisotropy of the shale, which is not present for quartzite, manifests itself in a reduction in penetration for shots perpendicular to the layering of the shot and in the fracture pattern.

The rock used in the testing is quite representative of some of the more challenging rock formations that are currently perforated for hydraulic fracturing for oil, gas and geothermal applications. Therefore, the knowledge gained from the test series is hopefully of valuable use for the perforating industry going forward.

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