We have performed perforation tests on three large rock specimens and mapped indentation hardness around the perforation. We find reduced hardness up to 1.5 inches from the center of the perforation. Maximum reductions as high as 75% occur near the wall of the perforation. In Berea Sandstone, there is a clear trend towards increasing damage near the entrance hole and reduced damage near the perforation tip. On the assumption that the indentation hardness represents more fundamental mechanical properties, a trial elastic model has been developed that illustrates the potential effect of the observed damage on stress distribution, and thus on fracture breakdown pressure. Based on the experimental results, this model predicts incipient fracture initiation at lower pressures, but also predicts high compressive stresses near the boundary of the damaged and undamaged material. This may be interpreted as representing a barrier to further crack extension, thus increasing the effective breakdown pressure. Preliminary, but more detailed computer calculations including wellbore and perforation flow suggest that the damage zone effect can be substantial in restricting ultimate breakdown. It is noted that the actual mechanical model should include brittle and plastic failure to more accurately predict breakdown behavior.

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