Perforation Penetration

Key Takeaways

• Shaped charge design governs the trade-off between perforation penetration depth and entrance hole diameter, with deep-penetrating charges achieving maximum tunnel length at the expense of entrance hole diameter and big-hole charges maximizing entrance hole diameter at the expense of penetration depth: a deep-penetrating charge (DPC) typically produces 20 to 40 inches of API concrete penetration with a 0.2 to 0.4 inch entrance hole diameter; a big-hole charge (BHC) produces 10 to 20 inches of API concrete penetration with a 0.5 to 0.9 inch entrance hole diameter; the choice between DPC and BHC depends on the completion application: DPC is preferred for openhole or cased-hole completions where maximum reservoir contact depth is needed (to bypass the damaged zone around the wellbore, to access natural fractures beyond the near-wellbore region, and to maximize the total productive area of the perforation tunnel network), while BHC is preferred for gravel pack completions (where maximum entrance hole diameter is needed for gravel placement) and for high-rate oil or gas producers in soft formations where the perforation tunnel is not expected to be the flow restriction (and the large diameter of the perforation holes at the casing entry point reduces the pressure drop through the perforations at high flow rates).
• Crushed zone damage around the perforation tunnel is the primary flow restriction in most cased-hole perforated completions and the primary target of perforation cleanup operations: the shaped charge jet creates the perforation tunnel by a combination of penetration (the jet tip penetrates the steel casing and formation at hypervelocity approaching 5,000 to 10,000 meters per second) and blasting (the detonation shock wave compacts and fractures the formation around the jet path), leaving a zone of highly compacted, low-permeability material (the crushed zone or compaction zone) surrounding the tunnel that has permeability 10 to 30 times lower than the undamaged formation beyond the crushed zone; the crushed zone thickness depends on the formation rock type (harder, more brittle formations have thicker crushed zones than softer, more ductile formations), the charge design (higher detonation energy creates thicker crushed zones), and the overbalance or underbalance at the time of detonation; underbalanced perforating (where the wellbore pressure at the time of detonation is lower than the formation pressure) causes immediate back-surge of formation fluid into the wellbore through the newly created perforations, partially cleaning the crushed zone debris from the tunnel and improving the effective connectivity between the formation and the wellbore compared to overbalanced perforating (where the mud hydrostatic exceeds the formation pressure at the time of detonation, driving debris back into the crushed zone and further reducing effective penetration).
• Effective penetration depth (also called the flow-productive penetration) accounts for both the physical perforation tunnel length and the crushed zone damage to give the length over which reservoir fluid can flow into the wellbore at near-matrix-permeability conditions: the effective penetration is typically calculated from the API concrete penetration using an empirical correction factor that accounts for the in-situ compressive strength ratio (the ratio of formation unconfined compressive strength under confining stress to the concrete target's unconfined compressive strength), with the resulting effective penetration ranging from 35 to 60 percent of the API concrete penetration; for a 30-inch API concrete penetration, the effective formation penetration might be 10 to 18 inches, with the innermost 0.5 to 1 inch of that penetration further restricted by the crushed zone; in tight formations (less than 0.1 millidarcy matrix permeability), even the effective penetration past the crushed zone provides minimal incremental productivity because the formation permeability itself limits production, and hydraulic fracturing (which creates a high-conductivity channel far exceeding the perforation tunnel length) is required to achieve economic production rates.
• Shot phasing and shot density interact with perforation penetration to determine the total inflow area and flow efficiency of the completion: a perforation gun with 4 shots per foot at 90-degree phasing creates 4 perforation tunnels per foot of perforated interval, each pointing in a perpendicular direction to adjacent shots, providing maximum azimuthal coverage of the formation around the wellbore; the productivity improvement from increasing shot density (from 2 to 4 to 12 shots per foot) diminishes as shot density increases because adjacent perforations at high density begin to interact (the stress fields and flow fields of adjacent perforations overlap), and the optimal shot density for a given formation and completion type balances the incremental productivity gain from additional shot density against the incremental cost of the perforating services; for hydraulic fracturing completions where the objective of perforating is to create entry points for fracture initiation rather than to provide the primary production flow path, the shot phasing (all shots oriented in the plane perpendicular to the minimum horizontal stress, which is the preferred fracture propagation plane) and limited entry perforation design (few perforations per cluster to create high perforation friction that distributes fluid among multiple clusters) become more important than maximum penetration depth.
• API RP 19B testing protocol provides the standardized target and test conditions for comparing perforating charges from different manufacturers and for specifying perforating job requirements, with Section 1 testing in concrete targets (simulating medium-hard formations) and Section 4 testing in stressed Berea sandstone cores (simulating actual formation conditions more realistically): the API concrete target test is the most commonly reported penetration value in charge data sheets because it is fast, inexpensive, and reproducible, but petroleum engineers selecting perforating systems for specific formations use the Section 4 stressed rock test data (where available) as a better predictor of the penetration achievable in the actual reservoir; the Section 4 test applies a confining stress to the core (simulating the in-situ effective overburden stress on the formation) and measures penetration in Berea sandstone cores with permeability similar to a tight sandstone formation, giving penetration values 40 to 70 percent lower than the Section 1 concrete target penetration for the same charge; the API RP 19B reporting format requires disclosure of the charge explosive type and weight, liner material, standoff distance, casing weight and grade, and test conditions, allowing meaningful comparison between test results from different testing facilities and different charge designs.