Tubing conveyed perforating

Tubing Conveyed Perforating, often abbreviated as TCP, refers to the use of tubing, drillpipe or coiled tubing to convey perforating guns to the required depth. The technique was initially developed as a means for conveying the gun string on the production tubing, with the guns remaining in the well until they are removed during the first workover (integrated TCP). The other technique, shoot and pull, requires the TCP strings be retrieved after the guns have fired and well perforated.

Benefits

The benefits of using TCP techniques include the following: it is the only means of gaining access to the perforating depth in highly deviated and horizontal wells; TCP techniques enable perforating very long intervals in one run. For example, some TCP strings have exceeded 8,000 ft (2,440 m) in length. TCP also facilitates running large and heavy guns and using high underbalance. When TCP is deployed in conjunction with drillstem test (DST) tools, well fluids can be easily controlled.

Risks of mis-fire and mitigation measures

With TCP operations we should always think about the "what ifs" and the risks should be fully explored at the design phase of a TCP operation. If the operation is shoot and pull, the back up in case of mis-fire could be to remove the guns from the well. If using a drop bar firing head, the bar MUST be removed first before retrieving the string. When types of firing head including pressure activated and electronic are used, ensure that the mechanism cannot be accidentally activated if need to pull the guns. Usually firing heads are placed at the top of a string so that they can be removed before the guns are above the rig floor. There is usually a safety-spacer for the same reason. Once the firing head is removed, the guns can then be removed and rigged down if required. In case of a misfire, there are a lot of other precautions that must be taken. There are rare events such as hang-fire that can occur. There are explosives that become more sensitive with temperature that need to be handled carefully. There is potential for trapped pressure.

When designing TCP operations with permanent completions, back-up firing mechanisms are often built in to the design to minimise the risk: - Redundant fire heads: to use more than one firing head and firing head type for redundancy, access to the firing head so that they can be removed independently. In the deepwater arena, drop-bars are now rare and the norm tends to be dual-hydraulic or an electronic firing head. Some companies offered to run a redundant hydraulic head on the base of the guns, but this exposes the crew to the additional risk of working around what are arguably armed guns as they make up the carrier segments above that head. While there are ways to reduce this risk, this configuration has become rarer. Usually its two heads on top of the guns, with a safety spacer to ensure they're below the floor before arming.

Ability to drop the guns so that wireline guns can be run through the completion if required. If a bar was dropped, it must be recovered before pull out of hole (POOH) to ensure the gun movement doesn't rattle a stuck bar and allow it to drop and initiate the firing process. Time must be spent before POOH to minimise the risk of a hang-fire finally initiating when off-depth. Some firing heads can be recovered and re-run with slickline, giving another chance to get the gun to fire while also enhancing safety when POOH.

Good success has been achieved with the relatively simple dual-hydraulic firing heads, initiating with applied tubing pressure. Because the tubing is open-ended, the TCP packer must be set before the required absolute pressure can be generated which is a nice safety feature.

The TCP misruns can be due to: - Control line damage on an annulus pressure firing head, where a slim control line runs from the annulus at the TCP packer to the guns, in situations where firing pressure cannot be applied to the well below the packer, ie, existing open perfs. This specific hook-up is rare and not always reliable. - A low-order detonation, where the firing head worked properly but the detonation cord burned rather than exploded and failed to ignite the inter-carrier connector charges.

In the above cases the crew waited, repeated the firing process, waited, called the beach to inform them (adding to the wait time) and then POOH. The area around the moon pool was cleared of non-essentials when the guns neared the floor and the guns were rendered (relatively) safe by removing the firing head ASAP. There's always risk and worry about hang-fire, but this cannot to eliminated entirely.