The Forties Field Sea-Water Injection System
Authors: Mitchell R.W. 
Published: December 2007

This paper summarizes the design, commissioning, and initial operation of an offshore seawater injection plan in the Forties field of the North Sea. Water sampling and core injectivity, studies are discussed in relation to the subsequent design of surface facilities. Typical results obtained with the system are described in terms of both surface and reservoir requirements.

Introduction
Pressure maintenance in the Forties field reservoir is Pressure maintenance in the Forties field reservoir is considered essential for satisfactory oil recovery. Initial water sampling and core injectivity studies indicated that sea water would be a satisfactory injection fluid. Based on these studies, surface facilities were designed to prevent the buildup of crustacea, to remove oxygen, to filter prevent the buildup of crustacea, to remove oxygen, to filter solids, and for high pressure injection at rates of about 150,000 B/D per platform.
Before beginning injection, the chemical treatment requirements for the sea-water injection system were determined. The principal requirement was an effective microbiological treatment. While corrosion and scale control is not a significant problem now, the use of oxygen scavengers, scale and corrosion inhibitors, and iron chelating agents also are discussed. The quality of water required for successful injection into the Forties reservoir was investigated at length. Typical results obtained with the injection system are presented in relation to core injectivity studies, injection data, and the type of well completion used.

Water Sampling and Feasibility Studies
Water sampling studies were conducted on North Sea water during 1972 by BP Petroleum Development, Ltd., research personnel. Tests were performed using both discrete and continuous sampling methods to determine the average water quality of North Sea water. Fig. 1 shows typical results of the suspended solids (retained on a 0.8 mu Millipore filter) as a function of sampling depth. Generally, the suspended content is at a minimum near 150 to 200 ft. Although not shown in Fig. 1, the suspended solids content of sea water was greatest during the summer. This might result from the higher levels of organic matter in sea water during the summer. Chemical and bacterial analyses of the samples were as expected for sea water.
Feasibility studies to establish the efficiency of the Plenty and Sons backwashing filter for removing Plenty and Sons backwashing filter for removing suspended matter from North Sea water were performed on BP's exploration rig Sea Quest at the end of 1972. During these tests, various filter elements and polyelectrolyte filter aids also were investigated. Particle-size distribution data from these short-term tests indicated that a reduction of about 60 to 70 percent of particles above 3 mu could be obtained with the Plenty filter. Adding polyelectrolyte appeared to have little effect on filter polyelectrolyte appeared to have little effect on filter performance, except at extremely high concentrations. performance, except at extremely high concentrations. The chlorine demand for sea water also was investigated during these trials and was found to be quite low (about 0.3 ppm).
Laboratory core studies using North Sea water and Forties reservoir rock have shown that hydration of clays is not a problem. In addition, capillary pressure data indicated that the effective hydraulic pore-throat diameter of the Forties aquifer is about 15 mu. No significant permeability reduction resulting from perforation permeability reduction resulting from perforation plugging should occur if the injection water is filtered to 5 mu. plugging should occur if the injection water is filtered to 5 mu.
JPT
P. 877