Abstract

The Mittelplate oil field is located in the Tidelands National Park in the German North Sea. Its location in an ecologically sensitive area was a challenge during well completion planning. The horizontal well A 7a penetrates the Dogger-delta sandstone in the Jurassic trough on the flank of a salt dome. The well's productivity is a very important factor in the economics of field development. The specific reservoir conditions were decisive for well design and completion fluid selection. To retain all options for future production phases, a concept using a cemented liner and subsequent perforation was selected. Formation damage control was supported by short workover time due to new completion techniques. To avoid formation damage during the initial stages of perforation and completion, a non-damaging completion fluid was evaluated in laboratory investigations. The objective was to verify the rheological properties, carrying capacity, and time stability. Minimizing fluid loss during completion guarantees optimum well productivity. A xanthan-based carrier fluid with oil-soluble resins serving as bridging agents was selected. Intensive quality control of the chemicals ensured the use of correct batches in the field, contributing to the positive results of the project.

Introduction

The Mittelplate oil field is located in the German North Sea tidelands (Fig.1). It was placed on production in 1987. Within an early pilot project, a long-term production test was conducted to study the reservoir's oil recovery potential. An artificial drilling and production island, where environmental protection has high priority, was constructed on a tideland elevation around the Mittelplate 1 discovery well (Fig. 2). The concrete structure measures 70by 95 m and is protected by sheet pile walls up to 11 m on the most exposed side. The rig cellar is made of reinforced concrete and has 44 slots. The drilling rig is mounted on hydraulic skids. The crude is loaded in the island harbor basin. It has a specially designed lock gate which seals off the loading area. In the event of an accident, the crude would be safely contained in the harbor.

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