With over 70% of the world’s oil and gas production being produced from mature fields, it’s not surprising that operators are looking at ways in which they can increase a well or field’s recovery beyond its decline natural curve. It’s becoming more and more of a focus each year.

There are many ways in which an operator can improve recovery such as artificial lift, drilling and completing infill wells or gas or chemical injection but for this blog, we are going to focus on the applications for Coiled Tubing Drilling.

The primary reason to use Coiled Tubing Drilling is its ability to re-enter depleted wells quickly and cost-effectively but there are some parallel reasons that sometimes get overlooked.

Underbalanced drilling (UBD)

True underbalanced drilling or even managed pressure drilling is only possible with Coiled Tubing Drilling. Unlike conventional drilling, there is no need to make connections and therefore breaking the pressure equilibrium or underbalance conditions, Coiled Tubing offers a closed system to control the pressure downhole creating the perfect conditions to achieve at balance or underbalance.  Underbalanced drilling also allows the well to produce while drilling, making the payback period much shorter for an operator.

Thru-tubing

It may not be possible to pull the completion before re-entering the well. Coiled Tubing Drilling is capable of being run through tubing and exiting a completion with a whipstock or a cement plug. The former is generally costlier but often more reliable than a cement plug and therefore the costs may be recouped quickly in efficiency of operation.

Lost circulation zones

Lost circulation is usually one of the main reasons for an operator wanting to perform managed pressure drilling (MPD) or underbalanced drilling and given that CTD is the only way in which you are able to truly create these types of downhole conditions, it is a great technology for this type of problem. Whether you use MPD or UBD will depend on what you expect the reservoir to produce while you are drilling. MPD may be more appropriate if the equipment is not able to cope with large volumes but the great thing with CTD is that you are able to make adjustments on the fly if you aren’t able to predict this ahead of time.

Unconventional gas shales

Tight shales do not produce naturally and the ‘go-to’ method of extraction is often fracturing. However, there have been many cases where the need to fracture these tight shales has been completely removed if underbalanced drilling is performed instead, reducing the cost per well substantially. By decreasing the pressure in the wellbore, it allows the gas to flow from the surrounding fractures more easily and avoids any skin damage that may be caused by drilling in overbalanced conditions where the pressure is forcing drilling fluids, fines, clays or cuttings into the rock matrix resulting in the oil and gas flow being impeded.

Accessing bypassed reserves and multi-laterals

It used to only be conceivable to produce from vertical wells, but in 2015, according to the EIA 77% of wells producing more than 400bopd and 42% of wells producing between 15 and 400bopd were drilled horizontally. Directional Coiled Tubing Drilling not only allows you to drill horizontally, it also allows you to drill many laterals all accessing different zones of the reservoir that were previously bypassed when the well was drilled vertically or even horizontally.

High pressure zones  

Because Coiled Tubing Drilling is an enclosed pressure system that can be controlled, if you were to drill through a highly pressurised zone, expectedly or unexpectedly, any change in downhole pressure conditions would be safely controlled by adjusting the choke at surface and circulating out. Additionally, with e-line running down the centre of the CT, any changes sensed by the CTD BHA will be immediately reported back to surface giving you warning of the changes. These are two of the advantages over conventional drilling, making the operation much safer for people on the rig floor.  

Coal bed methane extraction (CBM extraction)

CBM extraction is a method used for extracting methane gas from a coal deposit. The methane can be extracted by drilling wells into the coal seam. With Coiled Tubing Drilling, the bottom hole circulating pressure can be controlled from surface allowing you to decrease the pressure in the well bore and can avoid the risk of fracturing the coal seam at the same time. The objective is to decrease the water pressure by pumping water from the well and the decrease in pressure allows the methane to be unloaded from the coal and flow as a gas up the well to surface.

Underground coal gasification or in seam gasification

UCG is the conversion of coal into synthetic gas (Syngas) using controlled combustion in situ underground. By using directional CTD, the combustion process can be controlled much more precisely, making the process much more cost-effective and safer.

While CTD may not be suitable for every drilling situation, is it a very attractive application for the above scenarios.

Author: Clare Miszewska

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