In the last few years, iron sulfide scale has reduced overall production from the Barnóko-Mostno-Buszewo (BMB) field in northwest Poland. Despite challenging well environments, the Polish Oil & Gas Company (POGC) has been successful in mechanically removing the scale deposits and restoring production using underreaming technology conveyed on heavy-wall coiled tubing (CT). The successful campaign to clean three wells was completed using positive displacement motors (PDMs), mills and underreamers.

The BMB field is estimated to have geological reserves in the area of 400 million bbl (63.5 million tons) of oil and 1.02 tcf (28.9 Bcm) of gas. Recoverable estimates of 58.5 million bbl (9.3 million tons) of oil and 291 bcf (9.3 Bcm) of gas make it an important field for the Polish market in Eastern Europe. Total annual oil production from the BMB field is roughly 2.5 million bbl (400,000 tons) of oil. The oil reserves are located in the predominantly dolomite Zechstein formation.

Wells present challenges

The candidate wells selected for CT clean-outs presented operational challenges due to the type and hardness of the scale, high formation pressures and an H2S service environment. All three well candidates have static bottom hole pressures (SBHP) of up to 8,085 psi (550 bar) and wellhead pressures during operations were expected to exceed 4,410 psi (300 bar). With sour gas content in excess of 15% in some areas of the field, safety considerations (such as performing clean-out operations under hydrostatically balanced conditions) and proper equipment selection (such as employing a high-pressure CT work string) would be keys to the successful completion of the project.

BJ Services International B.V. was asked to provide heavy-wall 1.75-in. CT suitable to these extreme well conditions. With wellhead pressures greater than 4,410 psi (300 bar), collapse pressure on the CT string was of critical concern (engineers analyzed tubing force and predicted flow regimes with the company's CIRCA CT modeling software). If, during the operation, the fluid pumps were shut down and pressure was lost in the coil, the resulting pressure in the annulus could collapse the CT, so the CT wall thickness was designed to withstand such conditions.

A special 1.75-in. tapered CT string manufactured with 80 ksi grade material was ordered to meet well conditions. This 80 ksi-grade material reduces risk of hydrogen sulfide embrittlement compared with 90 ksi grade or higher material. The material selection factor was important based on the anticipated exposure time to sour gas. An H2S inhibitor would be used in the base oil circulating fluid to help protect the CT during operations, even though the chance of H2S attack is lower in a hydrocarbon environment. Since the high-pressure well conditions would result in higher fatigue in the CT string, the service life of the CT would be managed by engineering personnel onsite.

Seeking increased production

The primary objective of all three clean-out operations was to increase production by milling iron sulfide scale from the 27⁄8-in. production tubing. Once production tubing was cleaned, the second objective was to underream scale from the 7-in. perforated casing below the tubing shoe, since it was suspected that perforation and pore channels could be fully or partially plugged. To assist mechanical clean-out, US-40 mutual solvent (a hydrocarbon-based solvent solution) was used, but only after chemical compatibility tests determined the best materials for the CT stripper rubbers (which are responsible for holding wellhead pressure throughout the CT intervention).

Wells Buszewo-3, Buszewo-5 and Mostno-2 were successfully cleaned out. Operations required an average of 4.5 days per well. The clean-out operations comprised the following steps:

• Run in hole pumping oil at 1.26 bbl/min (200L/min) to mill-out scale from tubing;

• Run in hole with pilot mill through the 7-in. casing to total depth (TD);

• Run in hole with underreamer and 4.5-in. blades to TD; and

• Run in hole with underreamer and 6.0-in. blades to TD.

Several different runs were required to optimize the amount of material to be removed by each of the underreamer blades. A maximum motor size of 21⁄8-in. was used due to the restrictions in the wells. This limited the amount of torque available to drive the 6-in. underreamer, but did not compromise operations. The bottomhole assembly (BHA) for the milling run included (top-to-bottom): dimple-on connector, DFCV (double-flapper check valve), BOSS tool (ball-operated shear sub), dual circulating sub, PDM and bit.

The long tool string of almost 16.4 ft (5 m) was deployed in one piece with the use of a long riser section and CT support tower. The CT tower provided added safety and time efficiency (vs. a more conventional CT rig-up) by avoiding lengthy BHA changes and use of a "cherry-picker" lift during manual activities. Due to the hardness of the iron sulfide scale, a 21⁄4-in. pilot mill dressed with button-type tungsten carbide inserts was run. The underreamer blades also were dressed with button-type tungsten carbide inserts.

The onsite CT monitoring and modeling software aided the clean-out operations. In one instance, for example, CT weight during tripping did not match predicted weights. Onsite modeling helped identify corkscrewed production tubing as the probable cause. The clean-out procedure was adjusted to remain within CT tension limits and maintain proper and controlled weight-on-bit.

The following table details the conditions for the three candidate wells:

Pressure data

Milling began once the top of the hold-up was found and continued until TD was reached. Pumping was shut down during underreaming operations while passing nipples and restrictions in the production tubing to prevent damage to the tubing or blades. When the CT passed below the production shoe, pumping continued. After design depth was reached, a few passes were made with the fingers open to ensure production casing was free of obstructions.

Following the milling and underreaming clean-outs with CT, wireline was able to tag TD to confirm the clean-outs were successful. It was difficult to quantify the volume or weight of scale removed from the wells since no sample collection was available. Indication that scale was successfully removed was obtained by monitoring the downhole motor pressure. CT pressure indicated how hard the downhole motor was working over different sections of the well.

Production information was not available following each of the treatments, but initial pressures and production indicated improvements had been made on all accounts as each well came back to its original levels and draw-down rate. POGC has plans to continue this type of treatment on other wells in the field. These plans will allow new well technologies, i.e. safely controlled CT operations in extreme high-pressure, H2S environments, to expand and grow in the Eastern European market. 7
Germany's biggest offshore producing oil field is set for a major makeover to turn it into to a more reliable production center for North Sea crude from a highly sensitive offshore location.

John Bradbury, Offshore Editor

Equal partners Wintershall and RWE-DEA are planning to install a new export pipeline capable of carrying 1 million tonnes of oil a year from the Mittelplate field - Germany's biggest offshore field.

Mittelplate is exploited from a shallow water artificial island located nearly 5 miles (8 km) from shore in tidal land in Germany's North Sea sector and via extended reach onshore wells.

Mittelplate has been producing for nearly 16 years since 1987 and until now US $580.72 million has been invested in the project.

But this summer should see a major change from dependence on barges for oil export from the offshore part of the field with the installation of a pipeline export system. It will carry both crude and produced water from the offshore site, which is in a specially protected bird sanctuary in the Wadden Sea, to an onshore process facility.

The Mittelplate consortium, a joint venture created between German operator RWE-DEA and partner Wintershall, is installing two 4.6-mile (7.5-km) pipelines from the offshore platform to an onshore site this summer. One will carry liquids from the Mittelplate island to an onshore site near Freiderichskoog, northwest Germany, where oil, water and gas will be separated. Another line will return produced water back to the island for re-injection. Originally the plan was to install flexible pipe but because of the tidal lands, the pipeline will instead be drilled into position, in 0.8-mile (1.4 km) sections.

But first the landfall section and the island connection have to be completed.

RWE DEA spokesman Harald Graeser said, "The pipeline will be laid mid July. But we have to work around and complete the landfalls for the island and the land station and that needs some time."

Once the pipeline is laid over the mud flats to the offshore island, a further 1.75-mile (2.8 km) onshore section will be laid, using corrosion resistant steel pipe, to a land station at Dieksand.

By providing the new pipeline, the consortium will not only improve reliability of the export route from the offshore facility, but also cut barge movements in the North Sea and their attendant oil spill risk. "About 1,000 oil transports through the tidal flats in special-design tug-and-barge units would be eliminated," consortium members have said in their argument for the fixed export pipeline.

Until now, weather and tidal conditions have restricted output by the double-hulled barges and tugs used to carry Mittelplate offshore production to the port of Brunsbuttel, to 5.03 million bbl per year. Once the new export line is in place, and with six new onshore production wells to tap the eastern end of the field with a new drilling rig, the consortium aims to raise Mittelplate production to 7.55 million bbl a year. The total cost of the new pipeline is $129 million - and another $64.5 million is being invested in the new rig.

Bernard Schmidt, head of exploration and production for RWE-DEA, said Mittelplate is Germany's largest oil deposit. "Sixty-five percent of Germany's crude is found there," he said, speaking recently at an energy forum in Mannheim, Germany, convened by parent company BASF. Schmidt said remaining reserves in Mittelplate are estimated at 449 million bbl.

Wintershall completed its internal approvals for the pipeline in 2003 but there is a further period of public objection to the plan, which closed November 2004. "There was a technical challenge and this means that we have 3 months to put this 5-mile (8-km) pipeline into the ground," Schmidt said, speaking through a translator. At best, it seems that the pipeline will not be installed much before the summer 2005.

But assuming the new export line can be laid soon, it could provide the outlet for further nearby reserves. Schmidt indicated Wintershall is already looking at new deposits around the Mittelplate area.

"There is major exploration potential both north and south and we will tackle these. Once we have fixed the pipeline, we can increase production," Schmidt said.

In February 2003 Mittelplate reached 74.9 million bbl of oil production. Of that total, 7.3 million tonnes was from the offshore location where there are 17 injection and production wells, and 2.7 million tonnes were produced onshore at Friedrichskoog on the German coast, where there are a further seven wells. These onshore wells have stepouts between 5 miles and 5.6 miles (8 km and 9 km), almost as long as those at BP's Wytch Farm field in southern England.

Germany's Mittelplate field is in tidal lands in the Wadden Sea, which is also a bird sanctuary and home to other wildlife. Because of the fragility of the natural environment there, the offshore platform features a totally closed operating system that is continuously monitored and controlled to prevent discharges to the surrounding environment. Also, the artificial island is protected so the tidal lands cannot penetrate the production facility.

"There are multiple safeguards against possible risks," the consortium said. Even the cargo barges used to export crude are double hulled with separate compartments to minimize any crude spillage.

Once laying of the new pipeline is complete, the enhanced output expected from the field will allow the offshore island to be vacated earlier than originally planned, lessening the impact on the wetland location where it is sited. RWE-DEA said, "The environmental balance will be improved in spite of higher production volumes, with the added benefit of a shorter total duration of production and an earlier end to activities in the sensitive tideland ecosystem," consortium members have said previously.