Christoph Koch, Matthias Döhler • RWE Dea
The Mittelplate oilfield, operated by RWE Dea, is off the German North Sea coast on the southern boundary of the Wattenmeer Tidelands National Park. Since 1987 the field has produced from four main sandstones within the Dogger formation, namely the Beta, Gamma, Delta, and Epsilon sandstones.
Throughout the field’s history, the Delta and Epsilon sandstones have been the most productive, and have been developed in stages from the Mittelplate offshore drilling and production facility, and via extended reach wells from the Dieksand onshore station.
Until 2006, these high capacity sandstones were developed separately to allow for selective production. However, the next well in the sequence, A20, was designed to intersect both the Dogger Delta and Epsilon formations in the northern part of the field. This well also was the first application of intelligent well completion technology for the Mittelplate development.
For A20, which includes a horizontal deviation of up to 3,600 m (11,811 ft), the reservoir simulation indicated that water breakthrough would occur considerably earlier in Delta than in the Epsilon formation. This was due to the production index (PI) difference and the structural setting, with the Epsilon well section expected in an up-dip position, and the Delta section in a somewhat down-dip position, closer to the oil-water contact.
Completion requirements
Based on the results of the reservoir simulation and field experience, four main requirements were identified for the A20 intelligent well completion (IWC):
1. Due to anticipated early water breakthrough in the Delta formation, the Delta and Epsilon formations needed to be isolated hydraulically
2. Interval flow rates needed to be adjustable in order to manage geological and reservoir uncertainties – this applied not just to the Delta interval, which was a probable candidate for choking, but also to Epsilon
A20 completion.
3. Reservoir pressure, flow rate, and water cut had to be determined for both zones; information on the water cut data would be critical to achieving reasonable control over flow rates
4. Due to the costs and technical difficulties associated with the project, there was a need for a “lifetime” completion which could remain in the well until depletion. This would prove especially challenging given the inevitable application of an electric submersible pump. Other concerns were the potential for carbonate scaling problems, resulting from production of reservoir water in conjunction with fresh injection water, and the fact that some Mittelplatte wells had encountered sand ingress associated with water production.
Completion design
Hydraulic isolation of the zones and well control were achieved using two retrievable packers. The upper one facilitates feed-through of a total of six control lines, while the lower packer requires no feed-through.
Two hydraulically actuated subsurface control/interval control valves (ICVs) were deployed for flow control. They incorporate 10 flow positions and one closed position. Both ICVs are actuated via three control lines, comprising one open line for each valve and a common close line. The three control lines are integrated in a “flatpack” deployed downhole.
Night-time view of the Mittelplate drilling and production “island” off the coast of Schleswig-Holstein, northern Germany.
The ICVs include an Accupulse device to facilitate stepwise opening of the valve and single-step closing, by pressurizing the relevant control line. Design of the valve characteristics was performed according to the estimated PIs, and was identical for both zones.
Determination of reservoir pressures, production rates and water cuts – the latter calculated via a density determination – was achieved through pressure and temperature measurements at five points in the completion. These measurements are transmitted to the surface through one electrical connection.
Reservoir pressures are recorded at the lower gauge carrier (dual gauge carrier) through measurements in the tubing for the lower interval, and in the annulus for the upper interval. The upstream pressures at the ICVs are determined analogously at the upper gauge carrier (triple gauge carrier). Intake pressure of the upper valve is measured in the annulus, while the lower valve’s intake pressure is measured through a snorkel line.
To accommodate the requirement for a lifetime completion, various boundary conditions had to be met. In order to prevent corrosion, all components exposed to reservoir fluids were manufactured from Cr13 steel or alloys of a higher grade. For scale prevention inside the completion, an outlet was designed to facilitate injection of required chemicals.
The two injection lines are integrated in a second flat pack, along with the electrical connection. The flat pack is routed to the chemical injection sub and the gauge carriers, respectively. For the contingent case of sand production, the lower ICV is protected against plugging or excessive wear by means of a wire-wrapped screen.
The electric submersible pumps in the Mittelplate wells have a limited operating life of a few years. To achieve the goal of a lifetime completion, the IWC needed to be designed in a way that would restrict the impact of sporadic ESP changes to a minimum. A completion-wise de-coupling from the IWC and ESP was seen as the best solution.
RWE Dea decided to run the IWC on a separate, combined 3 ½-in. (8.9-cm), 4 ½-in. (11.43-cm) and 7-in. (17.78-cm) tubing string, with IWC components such as gauges, valves, and packers installed in the 3 ½-in. and 4 ½-in. sections. The ESP was installed on a separate 3 ½-in. string in the IWC’s 7-in. section, thereby effectively preventing mechanical interference to the IWC technology during an ESP change-out.
Production performance
After successful drilling of A20 and the trouble-free installation of the completion, the well has been in regular production since December 2006. The IWC has provided considerably improved reservoir monitoring, and allows response to changing production behavior without resorting to time-consuming and costly well interventions.
The experience gained will assist in subsequent projects on Mittelplate, such as the upcoming multi-lateral well development of the Beta reservoir, and could also be applied to other RWE Dea projects elsewhere in the world.
