Optimum Underbalance for the Removal of Perforation Damage

Abstract

Abstract It has been estimated that the permeability of the damaged zone around a perforation tunnel is reduced to approximately 20% of the original rock permeability. Since the perforation tunnel is the conduit for all produced fluids into the wellbore, removal of some or all of this damaged material is critical to the success of a perforated completion. Underbalanced perforating is regarded as the most efficient method of removing this damage. The degree of underbalance required to generate efficient perforations is still subject to much debate. Several guidelines have been produced in recent years, all of which calculate the optimum underbalance in terms of the permeability of the matrix (and fluid and tunnel parameters). All agree that the optimum underbalance should fall with increased permeability, but there is a great discrepancy in the specific relationship proposed. Most of the data on which the correlations are based and the models are validated comes from experiments on rocks with permeabilities in the 100–200 md range; extrapolation to lower and higher permeabilities is dangerous. In contrast to published models, we propose in this paper that underbalanced perforating promotes productive perforations by initiating mechanical failure of the damaged zone. A method is developed which predicts the optimum underbalance in terms of the strength of the matrix, the strength and extent of the damaged zone and the imposed stress conditions. Comparison with the results of single-shot perforating experiments lends support to the theory.