Detonating Cord

Key Takeaways

• PETN (pentaerythritol tetranitrate) is the primary explosive filler used in most oilfield detonating cord because of its combination of high detonation velocity (6,900 to 7,200 m/s for the pure compound), good thermal stability (decomposes slowly below 150 degrees Celsius, with a melting point of 141 degrees Celsius that limits its use in HPHT wells without special formulations), chemical stability (resistant to water, acids, and most oilfield fluids at ambient temperatures), and sensitivity that allows reliable initiation by standard oilfield detonators while remaining insensitive to accidental initiation from friction, impact, or electrostatic discharge under normal handling conditions; for high-temperature applications (HPHT wells above 150 degrees Celsius where PETN would melt and exude from the cord, creating a reliability hazard), alternative high-temperature explosive fillers such as HNS (hexanitrostilbene, thermally stable to 318 degrees Celsius), PYX (2,6-bis(picrylamino)-3,5-dinitropyridine), or octanitrocubane-based formulations are used in heat-resistant detonating cord that maintains reliable initiation performance at elevated temperatures; the linear loading density (grams of explosive per meter of cord) is selected for the specific application, with low-density cord (1 to 5 g/m) used in perforating gun systems where the cord detonation must not damage the gun carrier or the casing, and higher-density cord (15 to 40 g/m) used in applications requiring higher initiation energy or in surface blasting where no wellbore hardware is at risk from cord detonation.
• Detonating cord in perforating gun systems performs three functions simultaneously: it initiates each shaped charge in the gun string, it provides a redundant firing path (ensuring that even if the lead wires or the detonator circuit is partially severed by perforation debris or well conditions, the detonation wave continues propagating through the cord to initiate subsequent charges), and it provides the timing synchronization (all charges detonate within microseconds of each other because the detonation velocity of the cord is essentially constant) that ensures simultaneous penetration of all perforations and maximizes perforation entry hole symmetry and tunnel length uniformity; the detonating cord is threaded through the center of each shaped charge carrier (a metal block containing 4 to 8 shaped charges oriented radially, spaced along the gun carrier at 2 to 12 shots per foot depending on the perforation density specification) and is connected at the top of the gun to the detonator assembly (the primary initiator, triggered electrically, optically, or electronically by signals from the surface or by a downhole-set time delay or pressure-actuated initiator); reliability of the detonating cord initiation system is verified before deployment by a dud check procedure (confirming that each individual charge fires when tested individually in a test fixture) and by quality control of the cord splicing (the method of joining two lengths of cord end-to-end without interrupting the explosive column, which must be done correctly to ensure detonation wave transfer across the splice).
• Handling and transportation of detonating cord is regulated under the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) in the United States, the Explosives Regulations in Canada, and equivalent national regulations in all countries, classifying detonating cord as a Class 1 explosive (UN Hazard Division 1.1D for high-detonating-velocity PETN cord and UN 1.4S for low-sensitivity signal cord) requiring licensed storage in compliant explosives magazines, licensed transportation in approved vehicles by licensed drivers, and authorized handling only by trained and licensed blasters or perforation engineers; accidental initiation of detonating cord by electrostatic discharge (ESD), radio frequency energy (RF hazard), or stray electrical current is a documented risk in both surface and downhole oilfield operations, managed by specific procedures including bonding of equipment to equalize static charges before gun assembly and deployment, radio blackout zones (no radio transmissions within 50 feet of assembled perforating guns above surface), and the use of low-impedance shunts across the detonator bridgewire until the gun is deployed in the wellbore below a minimum depth (typically 30 feet below the surface to prevent surface detonation if inadvertently fired); dual-fire systems (where the detonating cord can only be initiated when two independent signals are received simultaneously) are used in some high-consequence perforating environments to provide an additional layer of protection against accidental detonation.
• Detonating cord end-to-end splices (used when connecting multiple gun sections in a long perforating gun string) must maintain continuous explosive contact without gaps or offset that would prevent detonation transfer: the standard splice method (the PETN-to-PETN overlap splice) places the two cord ends parallel and in contact over a length of approximately 10 to 15 centimeters, securing them with tape or a preformed splice connector; the detonation wave from the incoming cord initiates the outgoing cord over the contact length through friction, shock, and sympathetic detonation; a properly made overlap splice transfers the detonation wave reliably at any firing orientation (the cord need not be aligned with the detonation direction); alternative splices include the inline booster (a small shaped charge or PETN pellet inserted at the splice point to boost the initiation energy, used when the cord detonation energy is insufficient to reliably initiate across a physical barrier such as a debris plug or a bulkhead adapter fitting); quality inspection of all cord splices before gun assembly (visual inspection for gaps, inadequate overlap, or mechanical damage) is a mandatory step in perforating gun assembly procedures and is required by most oil company perforation specifications to prevent the costly consequence of a partial detonation (some charges firing and some not, which can leave unexploded ordnance downhole and require complex and hazardous wellbore intervention).
• Detonating cord used in deep HPHT perforating must maintain its explosive properties at elevated temperature and pressure for the duration of the run-in and firing cycle: a typical deep HPHT perforation run (in a well at 7,000 meters depth with 200 degrees Celsius bottomhole temperature) may expose the gun string and its detonating cord to 200 degrees Celsius for 4 to 8 hours during the run-in period before firing; standard PETN-loaded cord would soften and partially melt at this temperature, potentially causing the PETN to migrate within the textile braid and create gaps in the explosive column that would prevent detonation transfer; the high-temperature cord for HPHT applications uses HNS or PYX explosive fillers with higher melting points and thermal stability, contained in a sealed jacket that prevents exudation; pressure tolerance (the ability of the cord to function after exposure to wellbore hydrostatic pressure of 100 to 200 MPa in ultra-deep HPHT wells) is also a design consideration, since the hydrostatic compression of the cord reduces the effective volume of the explosive column and can cause mechanical damage to the textile braid if the jacket is not designed for deep pressure exposure; HPHT detonating cord is tested to the specific temperature-pressure-time envelope of the planned well before deployment, following API RP 19B (Recommended Practice for Evaluation of Well Perforators) test protocols for high-temperature perforating equipment.