Technical & Economic Application Guidelines for Downhole Oil-Water Separation Technology

Abstract

Abstract Downhole Oil-Water Separation (DOWS) is a water management technology consisting of a hydrocyclone to separate oil from water together with some form of artificial lift to produce an oil enriched stream to surface and to inject the bulk of the produced water into a disposal zone. Field trials have proven DOWS's operational success at reducing the well's water production, but economic success has been sparser. Increased confidence in DOWS technology depends on the development of improved models and methodologies which:Realistically simulate the complete downhole separation process andEvaluate the operational and economical aspects of DOWS This paper summarises:The development of a hydrocyclone performance model,Its integration with a reservoir simulator andThe testing of this integrated model against a real field case leading to a reservoir development strategy for evaluating the optimum time for DOWS installation and DOWS economics. Our single or dual stage, hydrocyclone performance model allows the oil / water separation efficiency to be calculated for a range of operational conditions. A decision tree identifies the optimum DOWS installation time based on both the economical and the operational aspects of the technology. The impact of operational parameters on DOWS viability are assessed, ranked and compared. These include the importance of casing size, electrical power costs, properties of the water disposal zone, potential use of the reject water and the surface water treating costs. As expected, DOWS mechanical reliability is one of the key parameters controlling economic viability. Use of our DOWS technical and economic models described here, together with the reported application strategy, will allow confident installation of DOWS technology in the field. DOWS may then be counted as a reliable option in the water management armoury and a tool for improved oil recovery. Introduction Hydrocyclones separate substances of different densities by centrifugal forces and then the lighter phase flows to the overflow outlet and the heavier one to the underflow outlet (Fig. 1). Since the early 1980's, they have been employed in the oil industry to separate oil and water because of their compactness, low residence time, lack of moving parts and absence of fine clearence elements that could become blocked or need replacement. Early studies by Colman and Thew1 were used to design hydrocyclones to de-oil water prior to discharge to the sea, de-water oil to sales specifications, treating oil spills at sea and, more recently, in downhole oil-water separation. Hydrocyclone performance, as measured in terms of the separation efficiency, is controlled by the inlet fluid properties (differential fluid density, free gas volume and oil concentration) and the operational conditions (split ratio, flow rate and pressures). Multiple hydrocyclones can be connected in parallel, to accommodate flow rates greater than that achievable by a single unit, or in series, to enhance the separation efficiency. Several publications on the performance of de-oiling hydrocyclones are available1,2,3,4,5,6,7,8, but no performance model appears to have been developed to predict hydrocyclone parameters at different conditions. This paper describes the development of a hydrocyclone performance model and its link to a reservoir simulator for the full modelling of the DOWS technology. Besides, it is described a development strategy to evaluate the economic viability of the technology. The results of the economic/operational feasibility study of implementing the DOWS technology into a real field, using the DOWS model and the development strategies, are also shown.