This paper compares the results of two perforation tests performed to investigate porosity and permeability damage due to perforating in gas-saturated versus liquid-saturated conditions. We used a method combining transient analysis and X-ray computed tomography (CT) map variations in permeability and porosity around the perforation tunnel. Experiments were conducted on 10-cm (4-in.)-diameter Berea Sandstone cores saturated with either low-viscosity silicone oil or nitrogen gas. Perforation flow tests were performed according to API RP43 procedures using conventional 6-gram HMX explosive charges, 6.9-Mpa (1000-psi) effective stress, and 5.2-Mpa (750-psi) underbalance. After flushing the perforations to remove explosion gases and loose debris, higher viscosity silicone oil was injected into the tunnels and movement of the front in the core was tracked using X-ray CT to measure the front velocity. Radial permeability was determined by relating differential pressure, front position and velocity. Porosity near the perforations was measured by comparing images taken when the sample was saturated with the two different fluids.
Analysis of the images and the data show that, for the specific charge and test conditions used, the type of fluid saturating the rock results in significantly different perforation geometries and damage under the same operating conditions. In the gas-saturated core, we obtained a shorter total penetration depth and a tunnel largely plugged with charge and rock debris. A zone of compacted rock, about 6 mm thick and with 6–8% less porosity than the average rock, surrounded the perforation tunnel. Near the entrance hole, the compaction was more severe when compared to other locations along the tunnel. The liquid-saturated core had a clean tunnel with a larger diameter and deeper penetration than the gas-saturated core. The liquid-saturated rock had no compacted zone. Permeability damage extended approximately 1.5 cm from the center of the perforation in the liquid-saturated core. The thickness of the damaged zone was a little higher in gas-saturated case when compared to liquid-saturated one.