For natural completions, well productivity is proportional to the depth of the perforation tunnels. Perforation depth, in turn, is generally inversely related to the formation effective stress. Accurate productivity modeling, therefore, requires accurate knowledge of the relationship between the downhole stress environment and perforation depth.

A comprehensive experimental effort was recently conducted to evaluate the penetration performance of shaped charges into stressed Berea sandstone cores. Rock confining stress (σc) and pore fluid pressure (Pp) were varied from ambient to 10,000 psi, to simulate a range of downhole stress environments. This current work featured a broader and more systematic investigation of the influence of pore pressure than previous studies.

Our experiments yielded the expected inverse correlation between penetration depth and effective stress (σeff). However, the data suggest a new definition of effective stress. Historically, the perforating community has defined σeff = σc - Pp, but a new treatment (σeff = σc - aPp; a=0.5) better fits present data. Furthermore, this new effective stress law better fits published historical penetration results. Pore pressure's influence on penetration depth is therefore weaker than previously thought; for a given confining stress, increasing pore pressure does increase penetration, but to a lesser extent than conventional models would indicate. The present work suggests that all shaped charges would be similarly affected.

These findings are relevant to penetration modeling, and in turn to well productivity modeling and prediction. Further implications are to laboratory testing, regarding scaling of parameters to accurately simulate field conditions.

This work culminates an initial application of combined penetration mechanics and geomechanics analyses to the investigation of shaped charge penetration into geologic materials. Future work will address different rock types, additional poroelastic quantities, and dynamic effects as they contribute to pressure-induced strengthening of reservoir rock.

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