Reduce Exploration Cost with Rigless Well Abandonment

During the drilling of an exploration well, only a fraction of the time spent by the rig is for actual drilling. For subsea wells, NCA's vision is to move as much of the rig activities that are not spent on physical drilling to less expensive work platforms like monohull vessels. These activities include prerig activities (Tophole Drilling) and post rig activities (Well Abandonment). The rig time saved may be used for drilling new wells, thus enabling more wells per year to be drilled from each rig. The potential cost saving is also significant.

NCA has attacked the subsea well abandonment from the back end; since the Subsea Wellhead Picker was introduced in 2007, more and more of the subsea exploration wellheads have been left behind by the rig for later recovery from a monohull vessel. This concept is referred to as Rig Chase, where the rig is "chased" by a monohull subsea constructor support vessel that cost effectively recovers the wellheads in batches. The Rig Chase is now widely offered as a Program in the North Sea, where multiwell, multiclient Rig Chase campaigns are held regulary.

The Subsea Wellhead Picker operation
After arriving on location, the wells are surveyed, any net protection and corrosion caps are removed, the wellbore cleaned and drifted to cutting depth in order to secure access for the Internal Multistring Cutting Tool (IMCT). The Subsea Wellhead Picker is then deployed using the vessel's crane and stabbed into the well assisted by a work class ROV. When the IMCT is at cutting depth a lifting connector is activated, attaching the crane to the wellhead. After successful load testing the cutting process is started.

NCA's patented IMCT is based on the highly effective Abrasive Waterjet Cutting method, which uses high pressure water mixed with abrasives to cut through the combined steel and cement compound that you will find just below the subsea wellhead. A typical casing configuration is 30” conductor, 20” or 18 5/8” surface casing, 13 3/8” casing and 10 3/8” or 9 5/8” casing, which results in a 4-string cemented multistring conductor.

The IMCT enables cutting through all the casing strings from the inside in one go. This is now proven technology with a track record of more than 300 multistring conductors cut, ranging from wells on 100 feet water depth to 1200 feet water depth. The cutting process is typically completed in 2-4 hours with a roundtrip time for the Subsea Wellhead Picker of only 8-12 hours from it leaves the deck of the vessel until the wellhead is safely landed on deck.

The advantages compared to using a costly drilling rig should be obvious; the rig rates for semis are typically 4-5 times higher than a construction support vessel, and the Subsea Wellhead Picker is in general more efficient than even mechanical cutting run on drill string. The IMCT can also go inside casing as small as 7 inch casing and still cut all the way through to a 36 inches conductor, an operation which would require several drill pipe runs and days to perform with a drilling rig.

The alternative cutting method if you use a vessel is explosives cutting which has proven to have a very negative impact on marine life in the area, and not least for the integrity of downhole cement plugs. In addition, the explosive cutting method creates a flared cut which can make it hard to recover the conductor string, and also complicates the deck handling when it is recovered to surface. 

The Subsea Wellhead Picked offers a robust method that allows operations to be carried out throughout the year, with a working envelop up to significant wave height (Hs) of 2.5m.

Releasing more rig days
So far vessels are used for removing the subsea wellheads only, however NCA is working actively to shave off more days from the exploration drilling program. Through the newly established company Subsea P&A AS, which is jointly owned by NCA and Island Offshore Subsea AS (IOSS), the downhole part of the well abandoning is attacked. This increases the potential number of days saved per well from 2 to 10.

In the future NCA will continue to develop innovative abandonment techniques that reduce cost and improve HSE in decommissioning. As shown in this article, these methods can also in many cases be used to increase efficiency and save cost in the exploration and development phases of a field.