Flow laboratories have been used for six decades to evaluate and optimize perforating systems for downhole environments. A key experimental variable is the pressure/flow boundary condition on the target core. Two standard configurations are axial flow, and radial flow. The implications of each configuration have been addressed somewhat in the literature in the context of steady flow with identical but prescribed tunnels. There has never been a careful comparison of the influence of boundary conditions on the actual tunnels.
In this paper we summarize the state of understanding regarding these two flow regimes by primarily focusing on the sensitivity of each to perforation damage and cleanup. In addition to addressing the differences under steady flow conditions, we also experimentally investigated the influence of target boundary conditions on the transient flow regime (i.e. during dynamic underbalance perforation cleanup). This was primarily an experimental investigation, supported by numerical simulations to determine baseline flow performance.
We find that radial flow provides the most meaningful measure of perforation damage (as indicated by post-shot steady flow). Indeed, radial flow is indispensable to parameterize skin and productivity models as they are currently formulated.
Axial flow yields steady production flow which is most sensitive to penetration depth, and relatively insensitive to perforation damage.
Finally, and perhaps most significantly, the transient perforation cleanup process is different for radial vs. axial boundaries. The intrinsic inflow characteristic of the tunnel is itself a function of the boundary conditions at shot time.
Proper selection of laboratory flow geometry is essential to yield meaningful measurements of perforation damage. In addition to this diagnostic purpose, target boundary conditions play a critical role in the removal of perforation damage, evolution of open tunnel length and diameter, and the resulting perforation flow efficiency.