The development and production of Germany’s most significant oil deposits on the Mittelplate drilling and production island have been under way for more than 25 years. The Mittelplate facility, operated by RWE Dea AG with Wintershall as partner, is an artificial island built on top of a sandbank in the Wadden Sea tidal flats.
The Mittelplate development (Figure 1) is located within one of Europe’s most environmentally sensitive coastal areas and has been a major success in part due to the creative way in which the large but shallow field is being developed using extended-reach multilateral well technology. As with most drilling and production facilities after 25 years of service, the island has a limited number of remaining available slots; therefore, every remaining slot must contact the maximum amount of reservoir.
The first multilateral well constructed from the Mittelplate island was MPA 23, a dual-lateral (mother bore plus one lateral) design contacting three individual Dogger Beta reservoir targets. The mother bore was drilled through the X24 and X25 targets, while the lateral was drilled to the X19 target.
A high-pressure TAML L 5 junction was required for this project since substantial drawdowns are required to produce this reservoir using electric submersible pumps. In addition, there is a high-pressure water drive system in place. Therefore, the multilateral junction had to be fully isolated to prevent damaging the junction rock and also protected from the ingress of injection water should it break through to the junction area.
Most failings during the construction phase of a multilateral well can be attributed to debris-related issues, so debris must be considered at every stage. From the very outset, the MPA 23 team took the risks and resultant effects of debris very seriously and set out to design the well with a plan to manage and mitigate the effects of debris.
As the well has two branches, there was a need to plug each bore in turn after perforation to protect the reservoir from debris and hydrostatic pressure fluctuations and to allow packers to be set and locator seal stems to be pressure-tested. When the team started to compare and evaluate barrier options, a list was generated of must-haves, needs, and wants. More than 30 different tools were reviewed, ranked, and compared. The decision was made that each one of them carried more risk than the team was willing to accept. The top six highest ranked requirements were:
- Debris tolerance;
- Ability to hold pressure from both above and below;
- Ability for tools to pass through easily and safely;
- Independent activation;
- Ability to be used in the mainbore as well as the lateral or laterals of a multilateral; and
- Ability to be removed, retrieved, or opened on demand.
For every tool considered, a secondary and tertiary method of opening or removing the barrier also was considered. Since the barriers were to be installed in the horizontal sections of the well, it was necessary to have coiled tubing (CT) equipment ready and available as a contingency.
However, with the significant cost and time to prepare and have the CT equipment on standby, it was decided that the CT should be used as the primary method of opening and retrieving the selected barrier.
It was obvious to the team that the most reliable barrier would be the one with the least amount of moving components and potential leak paths. As shown in Figure 2, the barriers were set within the well, each hung off from the bottom of a permanent packer. As the L 5 junction has two 2 -in. tubing strings to seal the junction, the internal diameter access to the barriers is only 2.44 in.
The team decided to design a completely new barrier. It had to be debris-tolerant, have no moving parts, and be able to withstand a 7,500-psi differential pressure rating. Removal would be with a drill-out bottomhole assembly (BHA) run on CT. However, the search was now on for a barrier material that could be milled or drilled out with the least amount of resultant debris and time.
The first material used to form a drillable plug was a carbon fiber resin mix. The drillability of this material had been qualified and tested previously, and the debris generated was known to be small dust-like cuttings. However, what was not known was the pressure rating of this material as a barrier. During extensive trials and testing, it became obvious that the carbon fiber alone could not hold the required 7,500 psi.
It was then decided to have a barrier manufactured with a steel bulkhead and a backup carbon fiber-shaped plug. The plug, made from a combination of both steel and carbon fiber, was called a combo plug.
The plug body with integral steel plate was designed with the aid of finite element analysis (FEA) to fine-tune the shape and minimize the steel mass required to be drilled out. The steel plate takes all the pressure, while the carbon fiber portion supports the steel plate during milling and prevents punch-through. The shape of the steel plate and backup carbon fiber plug were designed such that the shaped drill-out mill would drill the plate and the carbon fiber plug from the center outward.
Punch-through is an effect seen when a material is being drilled out. As it becomes increasingly thin, the weight on the milling BHA overcomes the thinning material and punches through, resulting in very rough edges and an increased possibility of tools becoming stuck as they pass through.
A rigorous test program was carried out to qualify the FEA-designed combo plug. From this test program the optimum dill bit and motor BHA were selected. These resulted in the debris pieces being less than 0.5 mm in size, with a final drill out time per plug of less than 20 minutes.
Two fully qualified plugs were installed within the MP 23 well as shown in Figure 2. As can be seen from the simple design in Figure 3, debris can be washed, circulated, and drilled out from on top of the barrier without any resultant harm or malfunction to the combo plug.
The combo plugs allowed the TAML L 5 completion to be installed with packers set and tested before being selectively drilled out with CT. Both plugs were drilled out within 20 minutes, and the plugs will be run on all future wells of this design.
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