SPE Drilling & Completion
Volume 24, Number 4, December 2009, pp. 678-685

Summary

For natural completions, well productivity is proportional to the depth of the perforation tunnels extending beyond the drilling damage (all other things being equal). 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 (σ_c) and pore fluid pressure (P_p) 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 (σ_eff). However, the data suggest a new definition of effective stress. Historically, the perforating community has defined σ_eff = σ_c – P_p, but a new treatment (σ_eff = σ_c – aP_p; 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.

© 2009. Society of Petroleum Engineers

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History

• Original manuscript received: 16 November 2007
• Meeting paper published: 13 February 2008
• Revised manuscript received: 21 January 2009
• Manuscript approved: 10 February 2009
• Published online: 17 September 2009
• Version of record: 23 December 2009