Viscous Fluids Characterize the Crushed Zone

Abstract

Summary The crashed zone surrounding a perforation tunnel, which contributes to the overall skin effect, is well documented. Many authors have proposed equations to predict the pressure drop caused by both laminar and high-velocity flow through the crushed zone. However, determining the thickness of the crushed zone and its permeability always has been a major problem. We obtained cylindrical single-shot core samples perforated under API conditions. We evaluated the damage surrounding the perforation tunnels in the laboratory using a new technique. The core is saturated with a low-viscosity refined oil. The low-viscosity oil is flushed at a constant flow rate by another refined oil, which has a much higher viscosity. The differential pressure is monitored as the miscible front moves radially outwards. The rate of change of the differential pressure is affected as the high-viscosity oil invades zones with different permeabilities. Our interpretation of the pressure differential was done analytically. We obtained results in terms of effective crushed-zone thickness and crushed-zone permeability alteration. Such results can be used to calculate the contribution of the crushed zone to the skin effect. The success of this nondestructive laboratory technique leads us to evaluate the possibility of injecting a viscous fluid into a real well to investigate in-situ formation damage.