Kill-pills and completion fluids form internal and external filter cakes in perforation tunnels. A laboratory study on the cleanup of water-based kill-pills in lab-simulated perforation tunnels is presented. Flow initiation pressures (FIP) and return permeabilities are measured at constant pressure during flowback.

A sized calcium carbonate kill-pill and a bentonite mud are tested on rocks ranging in permeability from 25 md to 1500 md with various lab-simulated perforation tunnel dimensions. Both single-phase and two-phase experiments are conducted to better understand the factors affecting the cleanup of the perforation tunnels.

The FIP values are found to range from 2 to 15 psi depending upon the perforation tunnel dimensions. The FIP values are small (< 4 psi) in large diameter perforations and the external filter cake is formed only along the walls of the tunnel. These small FIP values in large diameter perforations are comparable to the FIP values obtained for open-hole completions with no perforations. However, in small diameter perforations, large FIP values are obtained, and the external filter cake is found to completely plug the tunnels. Both internal and external filter cakes determine the FIP in perforations with small diameter; however, the FIP in large diameter perforations is mostly determined by the internal filter cake. Pressure gradients up to 20 psi / inch and flowback rates up to 0.3 bbl/day/perforation are found to result in approximate skin factors < 2 for all simulated perforations which had an overbalance of 100 psi.