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ABSTRACT The Gryphon field in UK North Sea Block 9/18b requires full voidage replacement by water injection. A typical Eocene oilfield, it will produce at high watercuts from an early stage. Due to technical considerations (and environmental benefit), the produced water is to be reinjected. Produced water is compatible with the reservoir (from which it originated). Voidage replacement, however, requires an additional source of water to make up the necessary volume. Seawater has a high sulphate content and the Gryphon formation water contains Barium and Strontium. Breakthrough of injected seawater at the producers would cause immediate and severe sulphate scale deposition. Preventive measures were considered (use of a sulphate removal plant; squeeze treatments), but found unsatisfactory. The massive Middle Eocene aquifer, which overlies the Gryphon reservoir, provides an alternative, compatible source of water. Direct downward injection in a single wellbore was investigated, bur n scheme involving a separate water source well was preferred. The aquifer water and produced water are commingled before injection. INTRODUCTION For reservoir management reasons, optimal drainage of the Gryphon reservoir requires full voidage replacement by water injection. The production profile (Figure 1) features a peak oil production of approximately 50,000 bbls/d and a plateau liquids production of 80,000 bbls/d. Characteristic of an Eocene field is the short peak oil production followed by a rapid increase in water cut due to the relative thinness of the oil leg (190 ft in Gryphon) and the unfavorable water-oil mobility ratio (about 6: 1), despite the use of horizontal wells to minimize water coning. Large volumes of injection water will therefore be required. At breakthrough of injected water at the producers, any incompatibility between injected water and Gryphon reservoir water could have severe effects because of the large volumes of water involved. Seawater and Gryphon formation water are incompatible, and solutions to the potential problem of sulphate scaling were sought. SEAWATER Table 1 shows the composition of seawater at the Gryphon location in the North Sea. Table 2 lists the composition of Gryphon reservoir water. A scale prediction program (Ref. 1) indicates that the high sulphate content of seawater will cause the formation of Barium and/or Strontium deposits if mixed with reservoir water in almost any proportion. The barium sulphate deposition, which reaches a maximum at only 5% seawater breakthrough, is severe and immediate. In principle, a scale inhibitor squeeze treatment can be designed to prevent the deposition of scale. Treatment would be necessary at the first trace of seawater breakthrough; use of a radioactive tracer in the seawater would assist in its early detection. However, a more serious problem to solve is the placement of the squeeze chemicals along the length of a horizontal producer. Uncertainties regarding adequate diversion of the chemicals and coverage of the entire production interval, compounded by the cost and downtime attributable to a coiled tubing operation on a subsea producer, led to a search for alternative solutions. |
