Mechanisms of Perforation-Induced Damage in Carbonates and the Effect on Injection Flow Performance

摘要

Fluid diversion in heterogeneous carbonate formations is critical to the design of successful stimulation treatments. In cased andperforated wells, microstructural damage around the perforated tunnels represents a significant challenge for the analysis of flow distribution along the wellbore. While this damaged or"crushed" zone has been a subject of numerous studies involving sandstones, the specific mechanisms of perforation damage in carbonates and theeffect on flow efficiency are not well understood. We examine the carbonate crushed zone at a fundamental level by interpreting the results of flow experiments on perforated cores using direct observation and analysis of the crushed zone at the porescale. Injectivity index was recorded in Indiana limestone (IL) cores perforated using a gas-filled wellbore, an approach that suppresses wellbore dynamics and produces worst-case damage conditions. After the flow test, continuum modelsof the perforated coreswere constructed fromcomputerized tomography (CT) scans and used to calculate core flow efficiency (CFE) andaverage crushed zone permeability. Novel image analysis algorithms extracted the radial variation of pore-scale quantities across the crushed zone from high- resolution scans of thin sections at multiple axial locations along the tunnel. A correlation wasthen developed which estimates the radial variation of permeability in the near-tunnel region from each thin section. The permeability model is calibrated using analytical solutions that connect the crushed zone and virgin rock permeability profiles to experimental measurements made in the core test. The calibration approach makes use of a new approximate analytical solution for the flow field around a perforation with nonuniform tunnel geometry and crushed zone damage. In the flow experiments, we observe a persistent drop in CFE for multiple charge types and test fluids as the initial core permeability is increased over two orders of magnitude. This is found to coincide with a decreasing ratio of average crushed-zone to virgin-rock permeability. Thin section analysis reveals that crushed zone damage is dominated by pore compaction near the tunnel edge, which has a characteristic signature in terms of the radial variation of porosity and pore-perimeter to surface-area ratio. Analysis of the spatial distribution of permeability suggests that flow efficiency is controlled by a low-porosity zone near the tunnel edge in which the damage is dominated by porecompaction.