Deep Penetrating Charge

Key Takeaways

• Deep penetrating charge performance optimization involves the metallic liner geometry, explosive type, and explosive loading density that collectively determine the jet tip velocity, jet length, and jet coherence time that control the penetration depth into rock targets: the liner (typically manufactured from electrolytic tough-pitch copper, though tungsten, lead, and tantalum are also used for specific applications) is formed into a precise conical shape with a controlled apex angle, wall thickness gradient, and surface roughness, because small deviations in liner geometry significantly affect the jet formation dynamics and the resulting penetration depth; when the explosive behind the liner detonates, the detonation wave collapses the liner from apex to base, accelerating liner material to jet velocities of 6 to 9 kilometers per second at the jet tip and 1 to 3 kilometers per second at the jet tail, with the tip-to-tail velocity gradient (the velocity gradient along the jet) governing the jet's ability to penetrate deeply before the slower jet tail catches up to the faster tip and the jet loses coherence; deep penetrating charges use liner materials and geometries that maximize this velocity gradient and delay jet breakup, allowing the jet to penetrate further before becoming incoherent and losing its hydrodynamic penetrating capability; API RP 19B Section 1 testing (the standardized API test protocol for shaped charge performance evaluation) measures the penetration depth in concrete targets of defined strength and porosity at standard standoff distances, providing the penetration depth performance specification that DPC manufacturers report in their product data sheets.
• Crushed zone bypass requirement for effective perforation in low-permeability reservoirs is the fundamental reason that deep penetrating charges are specified in tight gas and tight oil completions: when a shaped charge jet perforates cased and cemented production casing, it creates a tunnel of compacted formation rock around the perforation axis where the radial stress of the jet impact has broken the grain bonds and rearranged the grains into a tighter packing than the original undisturbed formation, reducing the local permeability to 5 to 50 percent of the undamaged matrix permeability; this crushed zone extends radially from the perforation tunnel axis to a radius of approximately 0.5 to 1.5 times the perforation tunnel diameter (typically 0.1 to 0.5 inches around the tunnel), creating a high-flow-resistance region that imposes a positive skin on the well; in a high-permeability formation (greater than 100 millidarcies), the crushed zone skin is a small fraction of the total pressure drop and the perforation length is less critical because the undamaged formation beyond the crushed zone is easily accessed; in a tight formation (less than 1 millidarcy), the crushed zone skin can equal or exceed the reservoir skin from partial penetration, and the only way to minimize the total completion skin is to extend the perforation tunnel beyond the crushed zone into undamaged rock, which is precisely what the deep penetrating charge accomplishes.
• Trade-off between penetration depth and perforation diameter in shaped charge design requires the operator to select the charge type that best matches the specific completion objective, because the same gun OD and explosive weight can be configured to optimize either penetration depth or tunnel entrance diameter but not both simultaneously: a deep penetrating charge of a given gun OD achieves its greater penetration depth by using more explosive energy and a longer liner to form a longer jet, leaving less energy available for forming a large-diameter entrance hole, so the DPC perforation entrance diameter is typically 0.2 to 0.4 inches (5 to 10 mm) compared to the 0.5 to 0.8 inches (13 to 20 mm) of a big hole charge of the same gun size; in high-permeability unconsolidated sandstone reservoirs where sand control screens or gravel packs are placed across the perforations, the entrance diameter is critical because it must be large enough to allow the gravel pack slurry to enter the perforation tunnel and pack it with gravel without bridging at the entrance, making big hole charges the preferred choice; in hydraulic fracturing completions in low-permeability formations, the entrance diameter has less significance because the fracture treatment bypasses the perforation entirely and the perforations serve only as the initial fracture initiation points, making deep penetrating charges preferable for the pre-fracturing connectivity they provide between the wellbore and the formation and for their ability to initiate fractures from deeper within the formation where the stress state is less influenced by the wellbore stress concentration.
• API RP 19B qualification testing and the reporting of deep penetrating charge performance must be interpreted carefully by the completion engineer because the standard test conditions (concrete target of defined strength, defined standoff, ambient temperature and pressure) differ from the downhole conditions in which the charge actually performs in a production well: the concrete target used in API RP 19B Section 1 testing has a compressive strength of approximately 7,000 psi, which is significantly weaker than the hard formations (limestones, tight sandstones, dolomites) with compressive strengths of 10,000 to 30,000 psi where deep penetrating charges are most commonly used, so the actual penetration depth in hard rock is lower than the API test penetration depth; the standoff in API testing (the distance between the charge face and the target face) is held constant and specified by the test protocol, but in a real wellbore the standoff varies with the gun centralization, the gun OD, and the casing and liner ID, with larger standoffs generally producing less penetration depth than the minimum standoff that brings the charge face closer to the casing wall; the in-situ stress at depth (confining pressure on the formation) compacts the formation and further reduces penetration depth relative to the unstressed API test target, with a typical depth correction factor of 0.6 to 0.8 applied to the API concrete test penetration to estimate the in-situ penetration depth in a pressured formation at depth.
• Deep penetrating charge selection in naturally fractured reservoirs reflects the probabilistic nature of fracture intersection: each additional inch of penetration depth increases the probability that the perforation tunnel will intersect at least one natural fracture, and the optimal completion in a fractured reservoir has perforations long enough that every perforation gun shot has a high probability of connecting the wellbore to the natural fracture network at the scale of the average fracture spacing; if the average natural fracture spacing in a carbonate reservoir is 20 inches (50 cm), a perforation penetrating only 10 inches (25 cm) has a probability of less than 50 percent of intersecting a fracture, while a perforation penetrating 40 inches (100 cm) has a much higher probability of intersecting multiple fractures; in this application the deep penetrating charge's superior penetration depth directly translates to a higher probability of fracture intersection per perforation shot, and the smaller tunnel diameter that is the trade-off for greater penetration is of minor consequence because the natural fracture provides the high-permeability flow path once it is intersected by even a small-diameter perforation tunnel.