Unconventional Reservoirs: Contact Area and Fracture Network Resulting from Perforations in Shale. A Comparative Study of Different Shale Targets and Shaped Charge Designs Optimized for Hydraulic Fracturing

Abstract

After decades of a continuous improvement of the plug and perf technology for horizontal wells and especially the shaped charges employed, operators nowadays have the choice between a variety of shaped charge designs. As a guidance to choose the optimal charge, this snapshot examines the influence of shale rock type and shaped charge design on the tunnel created in the reservoir rock during perforation. Tests were conducted in an API Section II Test environment, simulating in-situ downhole conditions. Specifically, the investigation focused on the characteristics of the contact surface and the induced fracture network resulting from different perforation charges, each with its own distinctive tunnel geometry. Three different shaped charge designs were tested on various shale targets. This included equal entrance hole charges, maximum formation contact, and oriented perforation tailored charges. To assess the impact of the formation rock on the results, test shots were made on Marcellus, Mancos, and Lotharheiler, which is similar to the Haynesville or Eagle Ford, shale cores. The analysis included CT scans to identify tip fractures and to examine the shape of the tunnel as well as conventional core analysis. Additionally, newly formed fractures within the rock and on the surface of the perforation tunnel were identified. The test results indicate that both the charge type and the rock type significantly influence the tunnel geometry and fracture network. Although all charges created roughly the same entrance hole diameter in the casing, variations in tunnel length and contact surface as well as in the newly created fractures were observed. Notably, the shape of the tunnel deviated strongly from the theoretical assumed cylindrical or conical tunnel. Doglegs, as well as cavities were detected at many tunnel tips, which change the overall stress field at the tunnel wall. To determine which rock parameters are relevant, the cores underwent analysis in an external laboratory to assess their petrophysical properties for further correlation analysis. From a practical perspective shale rock proved to be a challenging target rock due to its high anisotropy and significant differences in rock strength between targets of the same formation. Additionally, the target cores were prone to cracking during the rock preparation process. Therefore, this study should be considered as a snapshot and conclusions drawn from this set of tests should be approached cautiously and account for these circumstances. Our study provides insights into the dependency of the perforation result on the type of shale and charge design. Depending on the combination of the perforation technique and the characteristics of the rock formation, distinct fracture networks and tip deviations are formed. This improved understanding will help to identify the best perforation strategy tailored to the specific reservoir rock's unique properties.