Produced Water Reuse Considerations For In-Situ Recovery: A Case Development

Abstract

Steam-assisted methods for in-situ recovery in Canada typically operate at steam to oil ratios of approximately 3 to 1 and generate in the order of 2 to 5 barrels of produced water per barrel of production. To raise the large quantities of steam required for reservoir stimulation, once-through type steam generators are most commonly used. They are typically designed to produce about 80 per cent quality steam from soft, oil-free feedwater. Suncor Inc operates a cyclic steam injection pilot project near Fort Kent, Alberta. In the early 19805, Suncor planned an expansion of the 180 m3/d (1,130 bbl/d) facility to 800 m3/d (1,130 bbl/d). The expansion necessitated the development of a reliable water supply. Preliminary investigations into the feasibility of reusing produced water as the sale source of supply for the project expansion revealed this to be a costly and technically high risk option, given the specific produced water characteristics. As a result, an innovative alternative was developed to use a blend of produced water and municipal effluent from a nearby town as the water supply. This paper presents the rationale for the selection of this unique water supply and the process design considerations/or the resulting water treatment system. Background Several operators of in-situ recovery projects are interested in reusing produced water for steam generation in their commercial or semi-commercial projects. This interest has been stimulated by the often scarce supplies of fresh water available in the principal oil-producing regions of western Canada, and environmental pressures to explore alternatives to produced water disposal by injection. Suncor's planned expansion of their pilot project near Fort Kent, Alberta was forecast to require a water supply of 3,700 m3/d (20,000 bbl/d) for steam generation. CH2M HILL was retained to investigate the feasibility of reusing produced water as the steam generator feedwater supply. Feedwater Quality Requirements The once-through steam generators used in the expansion operate at a pressure of 13.8 MPa and produce 80 per cent quality steam. Typical manufacturer-specified feedwater quality requirements for these units are listed in Table 1. The maximum concentrations are based primarily on past operating experience rather than solubility considerations. Higher concentrations than those listed reduce the performance and service life of the steam generators. High concentrations of oil and hardness lead to coking and scaling respectively. Localized hot spots in the fouled region contribute to tube burnout. Oxygen leads to tube corrosion and a reduced service life. Iron is believed to attack the protective magnetite layer on the tubes. Uncertainties exist as to the maximum TDS and silica concentrations that can be tolerated in once-through steam generators. Operation with higher silica and TDS concentrations than those listed has recently been reported and successful operation could result in an increase in the maximum limits by the manufacturers. However, at the time of design there was insufficient experience at high pressure (14 MPa) and 80 per cent steam quality to justify exceeding the manufacturer's recommended feedwater quality criteria as the design basis for a project of this size.