The Valhall reservoir – a problem child
It was initially thought that the field would need no less than three separate drilling platforms to achieve an effective utilisation of its resources.[REMOVE]Fotnote: Ali, Nazir, and Alock, Tim, “Valhall Field, Norway – The First Ten Years”, North Sea Oil and Gas Reservoirs III, 25–40. Five further exploration wells were certainly required to define the total size of Valhall as roughly 1.5 million barrels in two separate formations. That made it large enough for a declaration of commerciality.
A series of appraisal wells in 1975-79 clearly indicated that problems were lurking. The unusual properties of the rock formations often meant that early production tests ended with a flow of dissolved chalk and the collapse of the well casing. Although large quantities of oil were present in Valhall, the question which had to be answered was whether they could be recovered.
This is also a story of making haste slowly. It was not simply a case of drilling and producing at full blast – wells could collapse if they were too long or the pressure was too high. When a production well is opened, securing the best possible control of its development involves a process which can easily last more than a year. Everything must be continuously monitored, and flow tailored to the properties of the chalk reservoir. The same applies for water injection, which is important for improving the recovery factor (getting out more of the oil) and reducing subsidence.
Valhall came on stream in October 1982, but the initial results were far below expectations and were much disrupted by the intrusion of solids such as sand and stones into the wellstream.
However, it had been seen that wells in firmer rock formations were less vulnerable to collapse when attempts were made to stimulate them with the aid of fracturing (fracking).
This led to the development of a completion method known as “up and under”, which involves perforating the upper formation and then fracking upwards in areas of high-porosity chalk. But the first two years showed a depressing trend. Daily production fell from a peak of about 63 000 barrels to roughly 37 000.
The major problems posed by the chalk had been underestimated, and these results prompted pessimism about future output. The Norwegian Petroleum Directorate (NPD) downgraded its estimate of Valhall reserves by about two-thirds.[REMOVE]Fotnote: Ministry of Petroleum and Energy, Facts , 1982–2001.
Development of the field in the second half for the 1980s was influenced by the problems with drilling and completion, where the up-and-under technique was the norm in almost vertical wells until a massive well collapse in 1985. This approach was replaced by fracking combined with filling the cracks formed in this way with sand – known as propped fracture stimulation – and by gravel packing. Particular use was made of the latter, which contributed to a substantial increase in daily production. This reached 85 000 barrels in 1989.
The NPD also recovered its confidence in the field and restored its estimate of the Valhall reserve to the original level in 1990. But difficulties persisted, with a number of problems related to gravel packing identified in the same year:
- An additional pressure drop occurred in the uppermost part of the field, reducing recoverability
- Completion costs increased rapidly, to roughly USD 11.3 million in the upper formation alone
- Problems with well workovers as a result of the packing
- Repeated well collapses even with the packing.
A two-pronged initiative was launched to overcome these challenges, with direct fracturing in the upper formation and then backfilling with small resin-coated ceramic spheres without gravel packing. In addition, stricter control of choking the wellheads was imposed to encourage a dampening effect on the gravel pack.
Production benefited, but wells continued to collapse with a fair degree of regularity. Delays in completing the Hod platform (see separate article) made it possible to introduce six new well slots on the main field. That in turn allowed more wells to be drilled in order to gain a better understanding of the reservoir. But they had little effect on output.
Production from the flanks
The first horizontal well was drilled in 1990-91. That proved the start of a new era, because it became possible to reach the flanks of the field and exploit their high production potential. This technology made it possible to counter the reduced effect of gravel packing.
Confirmation that the flanks contained much producible oil and the demonstration that these reserves could be recovered led to a decision to conduct the first three-dimensional seismic survey of the area.
Until then, attention had concentrated primarily on the topmost zones in the reservoir. Although the seismic data did not provide particularly good information on available reserves because of the Tertiary gas in the overlying strata, this was where the expectations had been highest.
However, the 3D seismic data and an extra well identified additional reserves which could be recovered with a new 19-slot wellhead platform (WP) installed in 1996 (see separate article). To exploit hydrocarbons in the outer parts of the field fully, the decision was also taken to build two new platforms. These were approved by the Storting (parliament) in 2001 and came on stream in 2003 and 2004.
Opportunities for improving recovery from Valhall had been assessed and studied ever since the field came on stream in 1982. A trial was conducted in 1990-93 with a special well to test the potential for and risk of water injection.
The pilot project concluded that, under certain circumstances, it would be commercially beneficial to adopt the technique to boost the field’s recovery factor. Proposals for the construction and installation of an injection platform (IP) were submitted to the government in 1999, and a structure with 24 well slots was ready five years later. This installation supported wells which were specialised for either injection and production, or which could be used for both purposes.
For the first few years, this gave indications that recovery from the reservoir could be improved. But Valhall’s eastern flank delivered yet another surprise in 2007.
A well was then drilled into an area which proved to be thinner than originally thought. That led in turn to a fresh reduction in estimated reserves.
At 31 December 2012, estimates for total recoverable volumes in Valhall and Hod were:
oil: 200 million standard cubic metres (scm), with 158 million produced
gas: 36 billion scm, with 29 billion produced
condensate: eight million tonnes, with six million produced.[REMOVE]Fotnote: Norwegian Petroleum Directorate, Facts , 2013.
Ali, Nazir, and Alock, Tim (1994), “Valhall Field, Norway – The First Ten Years”, North Sea Oil and Gas Reservoirs III, Norwegian Institute of Technology (NTH), 25-40.
Barkved, Olav, Heavey, Pete, Kjeldstadli, Roar, Kleppan, Tore, and Kristiansen, Tron Golder (all BP) (2003), “Valhall Field – Still on Plateau After 20 years of Production”, SPE 83957, presented at Offshore Europe, Aberdeen, UK, 2-5 September 2003.
Tjetland, G, Kristensen, T G, and Buer, K (all BP)), “Reservoir Management Aspects of Early Waterflood Response After 25 Years of Depletion in the Valhall Field”, IPTC 11276, presented at the International Petroleum Technology Conference, Dubai, UAE 4-6 December, 2007.