Bank Firing: Perforating Sequence, Gun Coordination, and Completion Design
Bank firing is a perforating technique in which multiple perforating guns are arranged in a linked configuration called a bank and detonated simultaneously or in a programmed sequence across a defined completion interval. The term covers both the hardware arrangement, a bank of guns linked by detonating cord, a ballistic transfer system, or an electrical firing circuit, and the initiation strategy used to create multiple perforation clusters that connect the wellbore to the producing formation in a single wireline or tubing-conveyed run. In conventional single-gun perforating, one gun is loaded, fired, retrieved, and replaced for each new perforation interval, requiring multiple trips in and out of the hole and creating significant rig standby time on deep wells. Bank firing eliminates most of these trips by running all guns in a single assembly and firing them individually at their assigned depths, or by firing all guns simultaneously in one detonation event, reducing the total time from casing setting to completion-ready wellbore by 30 to 70% on multi-zone wells compared to single-gun trip-by-trip perforating.
In modern WCSB horizontal well completions, bank firing most commonly refers to the selective firing of individual guns within a multi-gun string positioned across an entire lateral section. The gun string is run to total depth on wireline or on tubing in a single trip; depth confirmation is made by correlation of gamma-ray or casing-collar-locator (CCL) readings against the openhole log; and the guns are fired selectively by switching between them using an electronic select-fire switch (SFS) or by using addressable firing modules that respond to distinct command signals from the surface firing panel. This allows the completion engineer to perforate each planned cluster at its specific depth without retrieving and rerunning the gun string between clusters, enabling a 20-cluster, 20-gun bank to be positioned and fired in a single wireline run taking 4 to 8 hours rather than the 5 to 10 days that would be required for 20 individual single-gun runs on a deep Montney horizontal well.
Key Takeaways
- Simultaneous versus sequential firing modes: Bank firing can be executed in simultaneous mode, where all guns in the bank are connected by a detonating cord that propagates the detonation from the initiating gun through all guns in the string within microseconds, or in sequential mode, where individual guns are addressed and fired one at a time while the others remain armed but unfired. Simultaneous firing maximises the dynamic underbalance pressure surge that opens perforation tunnels through compaction damage and delivers the cleanest initial inflow path, but it also generates the maximum shock load on the gun string and wellbore equipment. Sequential firing allows each gun to be fired under the desired wellbore conditions (underbalance or overbalance), enables depth confirmation before each shot, and reduces shock loading by spreading detonation events over minutes or hours, but it forfeits the simultaneous dynamic underbalance effect. In plug-and-perf Montney completions, sequential firing within a bank is standard because each cluster is shot individually under underbalance conditions using nitrogen or underweighted brine in the tubing, and simultaneous firing would require all clusters in the lateral to be underbalanced simultaneously, which is impractical in a long horizontal well.
- Select-fire switch and addressable firing systems: The select-fire switch (SFS) is an electronic downhole device that receives a coded command signal from the surface firing panel and connects the firing current to the specific gun address corresponding to that command. A bank of 24 guns in a Montney horizontal completion might use addressable modules that respond to unique command codes, allowing the completion engineer to fire gun number 12 (for example, positioned at the 12th perforation cluster at 3,640 m MD) without disturbing guns 1 through 11 (already fired) or guns 13 through 24 (scheduled for later clusters). The addressable system maintains the unfired guns in a safe state until their command is received, preventing accidental firing from stray electrical currents or induced voltages in the wireline. Modern addressable firing systems for WCSB horizontal wells handle up to 60 gun addresses per bank and operate on downhole temperature ratings of 175 to 200 degrees Celsius for Duvernay HPHT applications.
- Depth control and perforation placement accuracy: Accurate depth placement of perforations in the target formation is the most critical operational requirement for bank firing in horizontal completions. The gamma-ray and CCL depth reference must be precisely correlated to the openhole log and the planned perforation depths before each gun is fired. In a 3,000 m horizontal Montney lateral, depth errors of 1 to 3 m can place perforations in the wrong lithofacies unit or in a cemented sidetrack interval from a previous wellbore, wasting perforation charge energy and creating flow paths that do not connect to the intended production zone. Depth correlations are performed by comparing the real-time CCL reading on the gun string with the depth-registered CCL on the casing running tally, accounting for any wireline stretch at the gun depth. At 5,000 m total depth, a standard wireline cable can stretch 8 to 12 m under its own weight in the horizontal section, creating a depth error of this magnitude if uncorrected by wireline stretch models or by correlation with the running tally CCL signatures.
- Charge selection and cluster design for bank firing: The perforating charges loaded in a bank of guns for Montney horizontal completions are selected based on formation hardness, compressive strength, and completion design. Deep penetrating charges maximise tunnel length into the formation at the cost of entry hole diameter; big-hole charges maximise entry hole diameter for improved proppant conductivity at the cost of tunnel length. For slickwater fracturing in the Montney, where fracture initiation pressure is the key parameter and the perforation connects the wellbore to the fracture, a deep-penetrating charge of 10 to 14 mm entry diameter and 800 to 1,200 mm tunnel depth is the typical choice; for acid matrix stimulation in Leduc carbonates, a big-hole charge of 16 to 20 mm entry diameter is preferred to maximise the acid entry area. Phase angle between charges in each gun is chosen to minimise stress shadowing between adjacent clusters: 60-degree phase at 4 shots per 0.3 m is the standard WCSB Montney cluster design for 5-1/2 inch casing.
- Safety protocols and misfire management: Bank firing introduces additional safety complexity compared to single-gun operations because multiple armed guns may be simultaneously in the wellbore across hundreds of metres of horizontal wellbore. If one gun misfires (fails to detonate on command), the bank may still have 10 to 23 armed guns in the string, which must be safely managed before any intervention can take place at the wellhead or in the wellbore. Industry practice, governed by CAODC (Canadian Association of Oilwell Drilling Contractors) safe perforating guidelines and AER Directive 036, requires a waiting period of at least 15 minutes after a misfire before any wellbore manipulation is attempted, then re-attempted firing with a fresh command signal, then safe retrieval of the unfired gun if the second attempt fails. A bank with a misfired gun that cannot be retrieved safely must be left in the wellbore and drilled through after the completion is pumped, adding CAD 80,000 to CAD 150,000 to the well cost for the drill-out operation.
Bank Firing in Plug-and-Perf Montney Completions
Plug-and-perf completions, the dominant completion architecture for WCSB Montney and Duvernay horizontal wells, use bank firing in the form of a single multi-gun string run after each bridge plug is set on wireline to isolate the previous stage. In a standard plug-and-perf Montney completion, the wireline truck runs a gun string loaded with typically 4 to 8 guns (depending on cluster count and cluster length) to the planned perforation depths for that stage, confirms depth by gamma-ray and CCL correlation, fires all guns sequentially (one per cluster location), and retrieves the gun string before the next stage's frac plug is set and the process repeats. The term bank firing in this context refers specifically to the multiple-gun configuration of the gun string for each stage's perforating run, distinguishing it from a single-gun approach where only one gun would be fired per trip.
The efficiency advantage of multi-gun bank firing in plug-and-perf is most significant for stages with 5 or more perforation clusters, where running 5 separate single-gun trips would take 12 to 18 hours versus 2.5 to 4 hours for a 5-gun bank run in a single trip. On a 24-stage Montney completion with 5 to 6 clusters per stage, using bank firing reduces total perforating time from an estimated 5 to 7 days (single-gun) to 1.5 to 2.5 days (bank), saving 3 to 4.5 days of wireline and rig standby at a combined cost of CAD 30,000 to CAD 60,000 per day, representing a total saving of CAD 90,000 to CAD 270,000 per well in completion-related standby costs. This efficiency is the primary driver for bank firing adoption in WCSB horizontal completions, with secondary benefits including reduced gun connection make-up time (fewer total connections made and broken) and simplified logistics (fewer gun trips means fewer opportunities for equipment surface handling incidents).
Simultaneous Bank Firing and Dynamic Underbalance
Simultaneous bank firing, in which all guns in a bank detonate at essentially the same instant via detonating cord transfer, is used in specific applications where the dynamic underbalance pressure wave generated by the simultaneous detonation is a key element of the completion strategy. When all charges fire simultaneously, the sudden creation of multiple perforation tunnels causes a rapid drop in wellbore pressure as the formation fluid and gas rush into the newly created tunnels; this pressure drop, called the dynamic underbalance, occurs over 10 to 50 milliseconds after detonation and can reach 10 to 40 MPa below the pre-shot wellbore pressure. The dynamic underbalance pressure wave propagates into the fresh perforation tunnels and the formation matrix, displacing compaction-damaged material, mobilising crushed rock, and re-mobilising fines that would otherwise restrict inflow.
Simultaneous bank firing for dynamic underbalance is particularly relevant in tight, low-permeability formations such as the Montney siltstone where compaction damage during perforating creates a reduced-permeability crushed zone 10 to 30 mm around each tunnel that can reduce initial inflow capacity by 30 to 60% compared to an undamaged formation. Studies in the Montney formation published by Canadian completion service companies and producers have shown that simultaneous firing of all guns in a 4 to 6 gun bank under a 10 to 15 MPa underbalance condition (wellbore pressure 10 to 15 MPa below formation pressure at time of firing) produces initial productivity ratios 15 to 25% higher than sequential single-gun firing under the same underbalance pressure, attributed to the enhanced pressure wave cleaning of the compaction damage zone around all tunnels simultaneously rather than individually. The tradeoff is the greater shock load on the casing, cement sheath, and completion equipment from simultaneous multi-gun detonation, which can cause cement microannulus cracking in poorly bonded casing intervals and must be evaluated against the wellbore integrity log before simultaneous firing is specified.
Bank Firing in Through-Tubing Re-Perforating Operations
Bank firing is also used in through-tubing re-perforating operations on existing producing wells where the original completion has been producing for several years and new or restimulated perforations are required to access bypassed pay or to create a new inflow path after frac screen-out or perforation plugging. In through-tubing perforating, the gun string must pass through the existing production tubing bore (typically 62 to 76 mm inside diameter for 2-3/8 to 2-7/8 inch tubing), which constrains the charge size and penetration depth but allows multiple guns to be run without tubing removal. A bank of 4 to 6 through-tubing guns, each 38 to 48 mm OD, can be run on slickline or coiled tubing and fired sequentially to perforate multiple formation intervals in a single trip, restimulating the well without a full workover rig mobilisation.