The oil recovery levels typical in subsea oilfields are not as high as they should be. Statoil has now set itself an ambitious goal for increasing oil recovery from subsea fields. Its ambition is to achieve 55% oil recovery by the year 2008. Currently, the average oil recovery level is 45% for Statoil-operated subsea fields.
Statoil believes that the construction of low-cost drainage points in subsea fields will play a major role in achieving its subsea field recovery ambition. Statoil's experience from its platform fields is that oil recovery increases significantly as the number of drainage points constructed and well interventions performed rises. However, the same effect has not so far been achieved in subsea fields.
Due to the high cost of subsea operations, Statoil has looked at novel and more cost-effective ways of constructing drainage points. One of these methods is through tubing rotary drilling (TTRD). The potential of TTRD in subsea operations appears to be significant in terms of cost per drainage point.
In addition to achieving the purely technical goal of producing a highly efficient commercial TTRD solution, Statoil believes that another key to success will be the establishment of a team specialised in this type of operation, which is expected to increase the effectiveness of operations and cut costs significantly. The ultimate target is to construct new drainage points utilising TTRD technology for $10 or less each.
The idea behind TTRD is to utilise as much of the existing well path as possible, thus saving a significant amount of time and cost. A considerable share of the subsea well construction cost is related to drilling down to the top of the reservoir and to the cost of subsea equipment on the seabed.
The cost for a new well can be in the order of $40–50m, when the subsea well slot is readily available, rising to $70m when a new well slot must be installed subsea. Utilising TTRD technology, Statoil envisages that new drainage points to increase the level of oil recovery will typically cost in the order of $10m.
Reducing the cost of a drainage point to this level will help optimise the recovery of oil, but other factors will also be important, including:
- Deployment of an efficient well-control solution – TTRD technology, workover control system
- Efficient rig operations –dynamically positioned rig, minimal moon pool operations over open sea, efficient handling
- Trained and competent staff, onshore and offshore – established team working to continuously improve the learning curve, efficient planning and operation
The through tubing sidetrack is performed using a high-pressure workover riser and a split blowout-preventer (BOP) function. The high-pressure riser facilitates the split BOP function as well as the operation of an efficient intervention system (typically a lower riser package solution) in wells with a horizontal 'Christmas tree'.
For vertical or conventional Christmas trees, a lower riser package (LRP) solution is already in normal use. The main components of the TTRD concept are, from the bottom up:
- Christmas tree interface adapter
- Lower riser package – XT adapter interface
- Lower riser package
- Emergency disconnect package
- Workover riser string
- 'Surface' BOP – located in splash zone or below sea level
- Drilling telescopic joint interface
- High-pressure heave arrestor
- Surface flow tree
- Workover control syste
- Kill system
- Handling and running tools
THE CHALLENGES AHEAD
There are many challenges associated with the application of a totally new technology such as TTRD in subsea wells. The main challenges are to:
- Establish a design basis and functional specifications which incorporate all the relevant requirements
- Develop new BOP products, including LRP cutting capacities
- Perform small operations from a moving vessel
- Interface with the horizontal Christmas tree and tubing hanger
In the project so far, Statoil has not identified anything that would prevent the use of a lightweight BOP system in TTRD operations. However, there are several challenges that will have to be kept in continuous focus, as preparations for the first subsea TTRD operation progress.
Currently, Statoil has identified three TTRD operations that will be assigned to a new lightweight BOP system. Assuming the first operations are successful, Statoil foresees that many more of these operations will be carried out in the future.