Mapping of permeability damage around perforation tunnels

We have investigated porosity and permeability damage around perforations u sing a combination of transient analysis and X-ray CT The method applied al lowed us to perform the entire experiments on samples under simulated in-si tu stress conditions and to map variations in permeability along the length of the core as well as with radial distance front the perforation. Berea ( 10.2-cm, (4-in.) dia) cores saturated with low-viscosity silicone oil were perforated using conventional-shaped charges (6-g HMX) and API RP43 procedu res by using 6.88-MPa (1000-psi) effective stress and 5.16-MPa (750-psi) an d 2.61-MPa (350-psi) underbalance. Low-permeability Torrey Buff Sandstone w as also perforated using 5.16-MPa (750-psi) underbalance. After sufficientl y flowing the perforations, higher-viscosity silicone oil was injected. The movement of fluids was tracked using X-ray CT to measure the local velocit y of the viscous fluid front at different locations along the perforation. Results of these tests were compared in terms of permeability and porosity damage. Quantitative analysis on Berea cores show, for the specific charge and test conditions used, that damage extends approximately 2 cm (0.78 in.) from the center of the perforation. Comparison of tests performed with 2.4 1-MPa (350-psi) and 5.16-MPa (750-psi) underbalance show a clear increase i n permeability near the tunnel wall at the higher underbalance. A zone of s omewhat-reduced permeability exists at approximately 1.7 cm from the perfor ation center in the latter case. Porosity profiles calculated show that por osity is almost uniform out from the tunnel and there is no compacted zone near the tunnel wall in liquid-saturated cores. However, there is a high-po rosity zone from the tunnel wall out about 2 mm. This may be due to a regio n of circumferential partings and small cracks that lead to high porosity o r due to the possible artifacts discussed in the paper Qualitative results have also been obtained for a tight sandstone for which under-balance was i nsufficient to remove debris fi-om. the perforation tunnel. CT images revea l that the plugged tunnel acts as a conduit for fluid flow, showing that th e plugging material has significantly higher permeability than the surround ing rock.