Abstract
The process of creating perforation tunnels is a critical aspect in well completion because it has a direct influence on the efficiency of relevant stimulation treatments and the overall productivity of the well. An overwhelming majority of perforating jobs in the industry are performed with shaped charges or jet perforators. A shaped charge consists of a liner, a main explosive load and primer charge all compressed within a metal case that is typically made out of steel or zinc. The zinc case charge is the low-debris option as it yields smaller (powdered) debris which can easily be flowed back, and is acid soluble, avoiding significant operational issues that often result from the larger debris of steel cases. In recent years, there have been operational concerns with zinc-case charges related to their impact on formation damage, increased detonation energy that may be detrimental to downhole equipment and most importantly, the compatibility of exotic wellbore fluids with the burn characteristics of zinc-case shaped charges.
As part of a study to develop a perforated completion design and execution strategy for deepwater subsea wells offshore Africa, the perforation flow laboratory was utilized to evaluate the flow performance and tunnel clean-up characteristics of zinc-case shaped charges in Buff Berea sandstone. A wide range of downhole scenarios including static underbalance and/or dynamic underbalance and the interaction between them, as well as overbalance conditions were considered in this study. Measured data demonstrated that the critical factor that drives perforation clean-up is the overall underbalance, not just the static or dynamic underbalance. Further, the testing program was extended to investigate compatibility issues (formation of precipitates) at elevated temperature in various wellbore fluids such as sodium bromide and calcium chloride. Advanced interpretation techniques including core analysis, computerized tomography, scanning electron microscopy, x-ray diffraction (XRD) analysis and x-ray fluorescence (XRF) were also used to provide insight into the physical state and composition of the debris in the perforation tunnel.
The results of this study indicate that the benefits of zinc-case shaped charges can be fully realized when optimal magnitude of dynamic underbalance is achieved. Optimal dynamic underbalance ensures maximum clean-up of the finely powdered debris from the perforation tunnel and enhances productivity. Correspondingly, it was also observed that insufficient dynamic underbalance leads to tunnel plugging by debris reducing the productivity of the tunnel. The mineralogical analysis of the tunnel debris also showed there were no detrimental effects of the reaction between sodium bromide and zinc material (less than 5% of the total debris consisted of precipitates).
This study demonstrates an engineering approach to evaluate and understand perforating strategies with zinc-case shaped charges using the capabilities of the flow laboratory and advanced analytical techniques. The results from this study provide the insight for a completion engineer to better understand the design and optimization of a perforating job when using low-debris zinc-case charges to benefit from reduced operational risks.
