The Mare's Tail – The answer to a cost effective produced water management in deepwater environment?
Authors: Oyeneyin M. B. , McLellan Glen , Vijayakumar Bhavani , Hussain Mamdud , Bichan Roy , Weir Nigel 
Published: December 2009

The deepwater environments form the cornerstone of future oilfield developments many of the hydrocarbon reservoirs of which are characterised by High Pressure (HP) and High Temperature (HT). A key feature of HP-HT reservoirs is the rapid depressurisation in the early production life of the reservoir. One of the most critical issues associated with the high drawdown is early water ingress and sand production. Continuous water production is also a key phenomenon with mature/depleted reservoirs. Keeping production costs to a minimum whilst keeping production targets high, requires putting in place an effective management of the produced water either by re-injection or by discharge. The challenge is in meeting the stringent operational requirements and environmental disposal regulations that define the level of oil in water and solids content before re-injection or discharge. In the UK, for example, legislation is becoming increasingly strict with the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR) commission – which came into force in 2007 – reducing the total discharge tonnage of oil allowed by 15% compared to the levels permitted in 2000. The 1st generation of the Mare's Tail coalescer was initially developed by Opus Plus through a joint industry project in 1998 to meet this challenge. Through utilising the technology, clients have enjoyed a number of key benefits, including a greatly improved quality of produced water being discharged to the sea – without the use of deoiling chemicals.

The Mare's Tail works by coalescing small oil droplets found in produced water, into significantly larger sizes so that the droplets can then be separated more efficiently. A spool cartridge contains a fibrous coalescer element, which is fixed at the inlet end to facilitate inspection and removal. Fluids enter the inlet nozzle and flow along the spool piece in the same direction as the coalescer media and then, as the fluids travel along the oleophilic fibres, small oil droplets are attracted to the surface and coalesce with other droplets as they migrate towards the outlet. The direction of flow along the fibres, rather than across it as with more conventional technologies, means that any solids are passed through the Mare's Tail rather than building up within the media. The technology has proved to be particularly suited to FPSO applications but never tried out for any subsea processing.

Environmental regulations relating to the discharge of oil into the sea continue to tighten significantly across the globe. To meet this new challenge and improve the efficiency of the Mare's tail, the development of a second generation of the system has now been initiated in a collaborative project between RGU & Opus Plus funded through the UK Knowledge Transfer Project Scheme.

In this paper the 2nd generation Mare's Tail development programme is presented. The paper presents the unique coalescence operating mechanism of the 2nd generation Mare's Tail, planned improvements over the 1st generation system and its importance/relevance to improved produced water management in deepwater environment. Potential for application in subsea processing compared to FPSO installation is also highlighted.

Supporting this optimisation process is a newly developed support design and optimisation algorithm the details of which are also presented. Preliminary validation of algorithm predictions have been carried out using selected field data the results of which have been found to be in agreement. Stochastic analysis has been carried out the results of which are presented to demonstrate how the algorithm can be utilised in the optimisation of the design parameters such that every developed Mare's Tail is fit for purpose. The algorithm can also be utilised to optimise operational parameters in real time as well as any onsite problem diagnosis thus minimising any flat time or loss of production.

The paper is concluded with a presentation of the Opus test facility which will be used to test the Mare's Tail.