"Drilling with a Straw"
Very often for production purposes, a small diameter pipe is run into the well from surface to just above the rock formation that is producing hydrocarbons, to enable flow or to increase the recovery rate. It is generally advantageous to keep this tubing in the well once in place, as it can be costly and time consuming to remove or to re-run this tubing in the well. If the tubing is to remain in the well, however, it can greatly restrict the size of anything that can be put into the hole that may be utilised to drill out from the existing casing, at the depth required, to access any additional oil or gas reserves.
Conventional jointed pipe has external upsets as the connections that increase the diameter of the pipe and the surface equipment required to convey this pipe into and out of the well (tripping) can be limiting. Coiled Tubing consists of a length of pipe made from a steel composite that is wrapped onto a large drum. The steel is sufficiently flexible to be able to be spooled in this way, and stiff enough to be able to be pushed into the well. The material of the steel prevents it from breaking or cracking, while being exposed to the bending forces involved. The tubing, itself, is run into and out of a well by using two opposing chains that grip the tubing through the use of friction. This technology provides the means for accessing wells where the existing production tubing is quite small. These wells would be difficult to enter using conventional techniques.
Advantages
- Connections are eliminated as the correct length of tubing required can be specified at the manufacturing stage
- Continuous lengths of tubing up to 7km can be custom made
- A seal around the pipe at the top of the well prevents wellbore fluids from escaping while running the tubing into or out of a well under pressure
- The tubing can be conveyed to the required depth with great accuracy
- An electrical cable can be run through the tubing to allow communication from surface to the downhole end of the coil in 'real-time'
Disadvantages
- The tubing cannot be rotated as the tubing is wrapped on a drum
- Various 'tools' must be attached to the bottom end of the tubing to provide the means for achieving any required operation
- Drilling mud must be pumped through a motor attached to the bottom of the CT to provide the force and speed required by a drill bit
Sometimes there is a requirement to drill a new hole out of the existing casing. In this instance the method used to drill through the casing generally incorporates the use of a whipstock. This is essentially a metal wedge that is fixed at the required depth in the well. A motor and drill bit are then run in on the coiled tubing to the whipstock.
When additional weight is set down on the bit, the face of the whipstock, or wedge, forces the bit against the wall of the casing, which is then slowly milled away. This hole provides a 'window' that subsequent runs use to gain access to the formation, allowing the new hole to be drilled.
The direction of the hole can be steered from surface to achieve the required profile that will access the desired hydrocarbon bearing rock formations. Once the required depth has been reached, this hole can either be cased with pipe or, if the formation is sufficiently strong enough, left as it is. This is described as a 'barefoot' completion.
