Challenges for Water Quality Measurement for Produced Water Handling Subsea
Authors: Yang Ming 
Published: April 2012

Subsea processing and separation of produced water from oil and gas is aninnovative and effective technology that contributes to maximizing recovery ofoffshore oil and gas resources. Separating water at the seabed and then eitherdisposing of it via subsea re-injection or discharge offers many advantages interms of flow assurance, easy separation, reduced use of production chemicals, and saving on energy (by not bringing the water to the surface). For a widetake-up of subsea processing systems, water quality measurement is criticallyimportant.

Despite the success of Troll pilot and also the first full scale system atTordis by Statoil, it is clear that there is still a technology gap regardingsubsea water quality measurement instrumentation. Without such devices, monitoring of water quality can only be achieved by taking subsea samples usingROVs (Remotely Operated Vehicles) and bringing them up to the surface foranalysis. This is not only extremely expensive and time consuming, but moreimportantly it does little to help oil production that requires continuous andtimely data for control and operations.

There are many challenges in developing reliable and accurate subsea waterquality measurement devices. These challenges lie in the fact that they willneed to work reliably at a water depth as much as 3000 m with the temperatureand pressure of process fluids far higher than that experienced on the surface. Testing and qualification will play a vital role in the technology developmentand deployment. Standards have now being developed for testing and qualifyingsubsea equipment. A number of Joint Industry Projects (JIPs), which aim ataccelerating the development and deployment process of these technologies, areunderway. These JIPs are thought to offer the best routes to close thetechnology gap.

This paper provides an overview of the current state of subsea separation andproduced water re-injection (PWRI) (and/or discharge) practices and thechallenges faced by the industry in developing subsea water quality measurementdevices. The paper presents information and findings from a recently completedJIP relating to potential subsea water quality measurement technologies, functional specifications, and qualification test requirements.

Background

Subsea processing is an innovative and effective technology that contributes tomaximising recovery of oil and gas resources. Subsea separation and PWRI formsan integral part of subsea processing strategy. One advantage of subseaseparation and PWRI, as shown schematically in Figure 1, is that only thehydrocarbon products are brought to the surface, while the separated water isre-injected directly into the wells either for disposal or for reservoirpressure maintenance.

Subsea separation has direct benefits by:Reducing back pressure on production wells, leading to increased oilproduction rates (typically 10% - 25%) and higher total oil recovery (typically5% - 10%).Enabling marginal field developments both deepwater and or longtie-backs.Enabling tie-in of satellite developments into existing infrastructure.Saving in flowline/riser capacity and energy that would otherwise be requiredto bring the water to the surface.