Water Quality Aspects of North Sea Injection Water
Authors: Mitchell Robert W. , Finch Edward M. 
Published: December 2007

This paper discusses current water quality assessment methods and some of the parameters that can affect the suitability of North Sea water as an injection fluid. Also included are general comments on the North Sea and its biomass. Examples are drawn from tests carried out during the commissioning of the Forties field seawater injection system in 1976-78.

Introduction

The use of seawater for subsurface injection is well known in many oil production areas. An area of particular interest at the present time is the North Sea, where the number of offshore seawater injection facilities is increasing rapidly. The high expense of offshore operations plus the high capital cost of the platform very often impose severe constraints on the design engineers, particularly in terms of the size and weight of treatment facilities. An important factor affecting the success of a waterflood project is the quality of the water being injected. Generally, the design basis is such that the water quality should enable injection for the life of the project at a minimum injection pressure and minimum cost. However, very often the quality standards required for injecting a particular water into a particular formation are not known in the early stages of the project.

Design Considerations

The water quality requirements for successful injection of seawater into North Sea reservoirs is a highly controversial topic at the present time. Water quality is affected by several types of contaminants including suspended solids, scale, oil, bacteria, corrosion products, and marine organisms. Generally, the major water quality problems associated with a seawater flood are due to colloidal iron formation and the presence of organisms. In the past, such problems have been overcome by using other sand or diatomaceous earth filters or by hydraulic fracturing of the formation. However, some operators have experienced problems with the latter approach due to sand influx into the injectors and fracture breakthrough into the producing wells. In an offshore situation, sand or diatomaceous earth filters can be employed only at considerable cost due to their size and weight. In this respect it is essential, therefore, that water quality specifications and designs are adequate but not ultraconservative.North Sea water generally is good quality, containing between 0.2 to 0.8 mg/L suspended solids as determined by membrane filtration techniques. The Forties field seawater injection system has been described previously, and a schematic diagram of the system (capable of handling 150,000 B/D of water) is shown in Fig. 1. Because of the general good quality of water present in the Forties area and weight limitations on the platform, two back-washable cartridge filters are employed. A four-pod filter containing an 80-micron stainless steel element located downstream of the seawater winning pumps provides the first stage of filtration (coarse). A second stage of filtration (fine) is achieved using another filter skid, containing a fiber element, located immediately upstream of the main injection pumps.Initially, there were several problems associated with the fine filters, mainly concerning the backwash and filtration efficiency. To overcome those problems, a considerable development effort was carried out by BP Petroleum Development during 1976–1978. Although that study was aimed primarily at obtaining an efficient backwashable filter element, the data also were employed to investigate present water quality prediction methods. This is of considerable importance as there is very often the need to assess water quality in an unknown environment.

JPT

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