In cased completions, perforations provide the essential link between the wellbore and the reservoir. Productivity of the completion is promoted by optimizing perforation characteristics such as geometry, phasing and density, but unfortunately it is restricted by the perforation damage zone— a region of low permeability material surrounding the perforation tunnel, and created by the impact of the shaped charge jet on the rock fabric.

Perforating underbalanced has become the primary means of removing perforation damage and maximizing productivity, though the mechanism by which it does so is still not very well understood. Underbalance perforating also serves to remove some or all of the comminuted sand grains that fill the perforation tunnel immediately after penetration of the rock by the shaped charge jet. Predictions of the required undeibalance to remove the damage zone or remove the comminuted fill are at best uncertain.

In this paper we describe the development of mathematical models that predict the pore pressure in the rock surrounding the perforation tunnel as the wellbore pressure drops during, for example, a dynamic underbalance operation. From this we calculate the magnitude and duration of the induced surge flow. The third stage in the analysis investigates the mechanisms through which the damage zone is removed.

As a result of these calculations we are able to predict the surge rate (and associated underbalance) required to remove the damage zone. Moreover we predict the perforation skin that results from incomplete removal of the damaged zone and for the first time we are able to determine how the skin depends on the degree and rate of underbalance.

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