Abstract

In the development of Elf Exploration Angola's Girassol field located approximately 200 km west of the coast of Angola in 1,500 m of water depth, 390,000 barrel per day of seawater will be needed for a waterflood operation. Formation water analysis indicate both barium and strontium levels of approximately 230 mg/l. By the injection of normal seawater containing 2,800 mg/l sulphate, scale tendency calculations indicate the possible formation of more than 320g barium sulphate per m3 of coproduced water. Conventional scale inhibitor squeeze treatments were not considered to be a reliable and cost effective scale control technique due to:

  1. Girassol's "daisy chain" concept with network of wells producing in the same gathering line and subsequent difficulties in monitoring residual inhibitor levels,

  2. difficulties in the placement of inhibitors in the reservoir which has high permeability and long perforation intervals, and

  3. the deep and remote subsea wells.

The high risks using scale inhibition for such a large scale waterflood project in a deepwater environment were considered unacceptable. After the extensive screening of all possible technical alternatives, Elf chose to remove the sulphate from the seawater using a selective nanofiltration membrane process previously used in the North Sea1,2. However, unlike the North Sea operations where scale inhibitors were originally used in combination with low sulphate seawater, Elf chose a specifically designed sulphate removal process with stringent high performance requirements that allows for the initial injection of seawater with approximately 20 mg/l sulphate for the leading edge of the waterflood. This will be followed by low sulphate seawater with approximately 40 mg/l sulphate. Based on

  1. the initial low sulphate level in the injection water,

  2. the projected scaling kinetics, and

  3. the partial precipitation of barium and sulphate in the reservoir before the seawater approaches the production bore, the calculated scale potential is essentially eliminated.

Introduction

Elf Exploration Angola is the operator of the Girassol FPSO in Block 17 off the coast of Angola. Girassol will have 23 subsea production wells, 14 subsea water injection wells, and 1 subsea gas injector well. Production from Girassol is estimated at 200,000 Barrel Oil per Day (BOPD) and 180,000 Barrel of Water per Day (BWPD) with some wells having production as high as 40,000 BOPD. Pressure maintenance by the injection of water is considered mandatory for the field development.

Various sources of water for pressure maintenance were evaluated. The use of water from an aquifer was rejected due to the cost of drilling and completing the subsea wells without any assurance that the aquifer would provide wells with sufficient rates and that the water found would be suitable for injection. The use of produced water was also evaluated. However, it was determined that there would be insufficient quantity, especially from the beginning of production. Consequently, seawater injection is the only realistic source of water for injection.

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