North Sea veteran Thistle was discovered nearly 40 years ago, in 1973, and was declared commercial before completion of the first two appraisal wells. The extension of the field into block 211/19 was confirmed by well 211/19-1, drilled in 1974, and the field was subsequently unitised.
The field comprises a Middle Jurassic Brent Group oil-bearing reservoir sequence. The structure is an easterly dipping, rotated-fault block divided into five main compartments by north-south normal faults and east-west cross-faults.
A 60-slot drilling and production platform was installed in 1976 and production began in February 1978, with oil exported by pipeline to the Sullom Voe terminal in Shetland.
During BP’s tenure as operator, and due to only limited aquifer support, injection wells were drilled only on the east flank of the field in order to maintain reservoir pressure. In addition, several wells in partly isolated fault blocks were produced by gas-lift completions.
By the end of 1988, cumulative production was 334million barrels from 34 production wells, with support from 13 injection wells.
That tally had reached almost 400million barrels by the end of 2000.
By the early-2000s, the field was regarded by BP to be in late-stage field life and close to achieving a 50% recovery factor based on the, by then, stock tank estimate of 824million barrels.
The millennium challenge for BP, then, was to continue economic production and further increase reserves recovery. Basically, however, BP wanted rid of the asset, and had done so since the mid-1990s, when it was one of four assets grouped together and groomed for sale. In the event, Talisman bought the other three, but not Thistle.
In 2003, BP entered negotiations with Lundin Petroleum, a deal was struck and the transaction was completed in February 2004. Lundin believed it could get a lot more out of Thistle and extend its life until at least 2018.
Thistle has remaining proven and probable recoverable reserves of 17.4million barrels of oil, plus contingent resources of 18.9million barrels. Core to getting at that prize was to drill. But the Thistle drilling capability had been out of service for years.
Rather than resort to selective use of mobile rigs to achieve at least a portion of that objective, the decision was made to restore drilling to the platform – basically a rebuild of its remaining, dilapidated drilling package (the platform started life with two derricks).
Today, the UK North Sea Thistle field is run by EnQuest, a joint venture between Lundin Petroleum and Petrofac, which was announced early-March this year. However, the decision to restore a drilling capability back to the field’s production platform predates the decision to invest in what turned out to be a significantly more expensive task than originally envisaged.
KCA Deutag played a key role in the transformation, with Alisdair Young working hand in glove with Lundin (now Enquest) drilling manager Simon Richards.
Richards confirmed that the Thistle drilling package overhaul was not cheap. Indeed, it swallowed around £40million, compared with £150million for the considerable amount of work carried out on the platform itself.
Rather than shutting down the platform, Lundin sought to keep it going while the redevelopment work in terms of the rig and wider platform was going on, including systems integrity and all-round reliability.
The rig overhaul was essentially completed by January, as was related subsurface work, so clearing the decks for running in the drilling package, bringing crew up to speed and preparing for what will hopefully become an extended drilling programme in three phases, with the first due to start about now (mid-2010).
“Prior to that, we first of all wanted a safe start-up,” Richards told Energy.
“It was critical for everybody that we had a safe and seamless start-up of a rig that hasn’t, until now, seen any activity for 20 years.
“Reintroduction of drill crews to a platform that hasn’t seen such activity for so long was a major event in its own right. And, of course, a crew that’s new to a platform needs a bit of time to acclimatise.
“So, apart from some deliberate time in the programme … more than a month was devoted just to getting the people up to speed … to get them comfortable with the platform … to get the platform feeling comfortable with them. It was important to become fully familiar with the equipment and competent to the point where they could operate safely. In all, that was a five-week period.
“Then we wanted an activity that the crew could undertake without taking on too much risk. So we found a workover operation that was attractive economically and was about the simplest and most straightforward job that we could conceive of for the crew to start on.
“I’m pleased to be able to say that we started that early-February and it was completed below budget and within time. The objective was to boost Thistle production.”
There are 55 well-slots on Thistle, of which almost 50 have been utilised. A core element of the revamp is to reclaim redundant slots for reuse when drilling further new wells.
Slots that have never been used are apparently “compromised” in some way and are not available for use.
One aspect of early post-reconstruction work also on the list of things to do is ensuring the integrity of some four wells that are no longer in use. Abandonment activities such as this remain the responsibility of former Thistle owners BP and ConocoPhillips as they retained decommissioning liabilities when Thistle was sold to Lundin.
As stated earlier, Lundin (or rather EnQuest) sees three phases to its Thistle drilling programme, which is about combing the field for unexploited pockets, of which a number have been identified.
Richards: “There are a considerable number of subsurface opportunities on Thistle and if the phase one programme delivers what is anticipated then there will be a lot of work following on behind that in terms of phases two and three. And if all of that turns up, it will have been money well spent on the rig to put it back into action.”
So does that mean it was still cheaper to rebuild the rig rather than replacing it, given the cost?
“If the full programme comes off, I suspect the answer is yes,” says Richards.
“If it turns into a limited programme then there might have been other ways of going about that. Time will tell.
“Phase one will not inform any near-field appraisal. Phase one is about exploring remaining targets within the existing field and opening up opportunities within that.”
CNR’s Murchison field is seen as an analogue of Thistle.
“There has been an extensive infill drilling programme … Thistle has seen none of that infill drilling. CNR have produced a hell of a lot of oil from Murchison and had a very active rig over an extended period. It’s a very good analogue. Thistle has been producing some 4,500 barrels per day of late. The hope with phase one infill drilling is to double that to around 10,000.
“If we’re going from 4,500 to 10,000bpd with three wells; we’re not going to get wells capable of 20,000 barrels per day. It’s going to be limited … perhaps 4,000-5,000bpd initially but dropping off fairly quickly thereafter.”