Increased recovery from subsea wells
17/09/2004 Increased recovery from subsea wells - that is the main challenge for subsea technology on the shelf. This was one of the messages Director General Gunnar Berge gave when he held the opening at a large international conference concerning subsea technology in Bergen today.
Approximately 40 % of the production from the shelf derives from subsea completed wells. This percentage is increasing. In a few years time, more than half of the production on the shelf will come from this type of wells.
There has been a marked increase in the use of subsea completed wells since the first were put in production in the early 80's. Approximately 380 subsea wells are now in operation. More are underway. Both Snøhvit and Kristin will be developed using subsea wells. The PDO for Ormen Lange is also based on subsea well completions.
The latest estimates from the NPD show that the total resources on the Norwegian continental shelf are 12,9 billion Sm3 oil equivalents (o.e.).
- 29 % has been produced,
- 32 % remains in fields as reserves,
- 10 % is considered as contingent resources in both fields and discoveries,
- 3 % is related to possible future measures for increased recovery and
- 26 % is in undiscovered resources.
A large portion of the remaining, undeveloped discoveries lie close to existing infrastructure. Several of these will possibly be developed as subsea satellites. Actual distance and water depth make for the use of known technology. The challenge ahead consists of continuously making this technology more cost-efficient.
Approximately 65 percent of the undiscovered resources are thought to lie in "prospects" that have not yet been drilled. A prospect is a mapped structure that has a potential of containing oil and gas. The remaining 35 percent are in "plays" - delimited areas where a specific set of geological factors exist and where one assumes there are prospects. The potential prospects in a play are not mature enough to be a target for exploratorion drilling.
A study of the nearly 1000 prospects that are registered in the NPD prospect database shows that 65 percent of these lie in less than 400 meter water depth and are less than 50 km from existing infrastructure or land. This implies that the possibility of tying back possible discoveries to existing infrastructure, using state of the art technology, is high. The technology exists. The challenge consists of developing this technology to be more cost-efficient. Subsea development of the remaining prospects relies on development of multiphase flow and subsea separation technology.
Many of the discoveries and prospects that can be developed with subsea solutions lie in the "mature areas" of the continental shelf. These are areas that have sufficient infrastructure and where the geological aspects are well known. The potential of making more finds is large. But exploration activity has to be increased.
The recovery factor represents a challenge for subsea completed wells. A study performed by Statoil, Hydro and the NPD shows that the recovery factor from subsea wells is 15-20 percent lower than from wells with direct platform access. There are several reasons for this. The accessibility to subsea completed wells is more difficult and represents large costs than wells drilled from a fixed installation. Even for minor jobs a vessel or mobile rig has to be used. The number of mobile units available for this work in the North Sea is limited. This can lead to lack of well maintenance thus reducing the recovery from the well. A huge effort has therefore been made in the last years to have mobile units available. In cases where the subsea wells are far from the processing unit, keeping necessary pressure large enough over time to maintain tail production, can be difficult. It is also important to collect data from these wells to optimize reservoir management. This implies a technical challenge - but also a potential gain. The NPD is therefore glad to see Statoils ambitious goal of achieving an oil recovery factor of 55 percent from subsea completed wells within 2008.
It is the NPDs opinion that the technology to build light intervention vessels or rigs is available and under constant development. But up to now the development risk has been carried mainly by the suppliers. To succeed in building these mobile units, the operating companies must take further responsibility and work hand-in-hand with the industry.
The Subsea conference in Bergen is held bi-annually. It is the largest of its kind in Europe. The conference organizer, Underwater Technology Foundation, consists of large industrial and financial players together with research institutions, the Norwegian Petroleum Society and the municipality of Bergen. This year more than 350 delegates from Norway and abroad have signed up for the conference.