Coiled tubing is a continuous length of small diameter steel pipe with a diameter of about 0.75 to 4.5 inches. It is coiled or spun in a reel continuously from one end to the other and different from typical production tubing used in production operations. Production tubing is not continuous while coiled tubing is continuous. Drill pipes and production tubing usually come in about 30 ft. in length. During drilling operations, these individual tube lengths are screwed together to make a long joint pipe. Coiled tubing can be as long as 25,000 ft. in length, with some being even longer in length.
Range of Applications
Coiled tubing has gained industry acceptance and is used in a wide range of applications ranging from workover and stimulation operations to drilling operations. Workover and stimulation operations are carried out on a well to restore and increase production as well as carry out repairs. They include perforating the well, squeeze cementing to repair casing and well cleanout. Conventionally, workover rigs are deployed to carry out these operations; however, coiled tubing offers the unique advantage of carrying out repairs and remedial operations even when the well is flowing. This means that there is absolutely no need to kill the well or shut down the production while carrying out repairs. A coiled tubing has a pressure containing unit called a “stripper” and at times a blow out preventer (BOP) is a part of the unit. This added advantage makes coiled tubing technology very attractive as the hydrocarbons produced with the coiled tubing inside the well can be used to offset the cost of the operation. That said, let’s delve into some applications of coiled tubing:
Well Cleanout: As production continues, there is always the possibility of deposition on the wall of the production tubing or casing or in worst case a total blockage of the production tubing or casing by the deposit. This deposit can come from the scale precipitates of the produced water, or asphaltenes from the produced oil, or even from formation sand or proppants due to hydraulic fracturing operations. These tend to reduce the effective internal diameter of the production tubing or casing available for flow or totally restrict the flow. To solve this problem, a coiled tubing unit that houses a bottom hole assembly (BHA) fitted with a nozzle is deployed. This nozzle jets out water, brine or chemical flushes at high pressure to flush out or remove the deposits and return the production tubing or casing to its original internal diameter thereby optimizing recovery and restoring production.
Well Perforation: The bottom hole assembly can be configured as desired depending on the operation to be carried out. In this case, the BHA of the coiled tubing can be configured with a perforating gun to carry out well perforating operations. Normally, in perforations, fluid is left in the hole to keep formation fluid in place. There are basically two kinds of perforations, i.e., overbalanced and underbalanced. In overbalanced perforation, the hydrostatic pressure exerted by this fluid column in the hole is deliberately kept higher than the formation pressure. Overbalanced perforation has the problem of flushing out perforation debris like spent bullet or gun debris from the formation. Coiled tubing allows for underbalanced perforation where the formation has a higher pressure than the fluid in the hole. This is made possible by the pressure containing feature of coiled tubing units. In underbalanced perforation it is easier for the formation to self-cleanse after the perforation operation. Since the formation has a higher pressure than the pressure in the well, the formation fluids immediately rush out after the perforation, flushing out perforation debris. Although, the perforation gun can also be lowered on a wireline, a wireline is simply what it is; a wire. It can be braided but it is still a wire and thus it depends on the weight of any tool attached to its lower end (BHA) to get to the target depth. In vertical wells, this is not a problem but in highly deviated wells or horizontal wells, the force of gravity from the weight of the perforating gun will not be enough and an external push is required so as to get to the target depth. The coiled tubing is actually a steel pipe hence it is rigid enough to get to target depth on its own weight and still flexible enough to twist and turn based on well orientation or deviation.
Coiled Tubing Drilling: Apart from workover and stimulation operations, coiled tubing technology has expanded into drilling. In this case a drilling bit is fitted to the lower end of the coiled tubing units which have pressure containing features (the coiled tubing stripper and any associated BOP), so the pressure can be checked. Everything seems to be in place except a means of rotating the drill bit. In conventional rotary drilling, rotating action is facilitated by the rotary table which is absent in a coiled tubing unit. However, in this case the drilling personnel use mud motors that are positioned just above the bit and they convert hydraulic forces supplied by the drilling fluid to torque which rotates the drill bit. This drilling technique is called slide drilling. It's important to note that conventional rotary drilling also makes use of this technology when required. Rotary steerable systems can be installed in the BHA to help control well path. Besides coiled tubing drilling has the added advantage of requiring zero tripping to make connections due to the fact that it is a continuous pipe. Coiled tubing drilling also generates fewer cuttings due to the small-hole diameters drilled from the surface to the target depth. A combination of these advantages saves rig time and cost and also reduces accident incidents.
The list above is by no means exhaustive. It is simply a pointer to some of the numerous adaptations of coiled tubing. All you need do is change the bottom hole assembly and you have coiled tubing performing a different function. Among the many benefits of coiled tubing technology is the faster rig-up and mobilization compared to workover units or conventional drilling rigs and a smaller environmental footprint.