The creation of perforation tunnels through the use of shaped charges is often found to leave a zone of compacted rock of low permeability around the tunnel. The low permeability of this compacted zone is a result of grain crushing and fines accumulation. Laboratory experiments reported earlier indicate that the distribution of fines and the permeability around the perforation depend on several factors including underbalance pressure, rock strength and fluid properties.

In the present work, a model has been developed to understand the factors controlling the permeability distribution around a perforation tunnel. During underbalance perforating, the permeability of the region around the perforation is controlled by fines generated by the shock wave and by fines entrained during surge flowback into the wellbore. Surge flow around the perforation tunnel determines the radius of the tunnel formed as well as the cleanup of fine-grained material near the face of the perforation. Fines are mobilized around the perforation tunnel wherever the surge velocities exceed a critical velocity. Some of the mobilized fines are trapped at pore throats near the tunnel giving rise to the crushed zone while those very close to the tunnel ( 0.5 cm) are produced. The crushed zone thickness is found to increase with rock tensile strength, fines content and fluid viscosity. The model also clearly shows why a lower optimum underbalance pressure is required for higher permeability rock. Results from the model are compared with experimental data on fines content and permeability around a single perforation. Good agreement is obtained with lab and field observations as the key parameters (rock permeability and underbalance pressure) are varied for both gas and oil wells.