New Solutions for Subsea Produced Water Separation and Treatment in Deepwater

Abstract

Subsea processing is becoming necessary for subsea production enhancement in deepwater. For brown fields, when water production is increasing, subsea oil/water separation with water re-injection into the reservoir is a relevant solution. When operated subsea, the produced water treatment will increase oil recovery from mature fields and generate spare topside capacity in order to tie back new wells. Indeed, this solution will provide process capacity for oil (reduction of total liquids) and capacity for injection water (reduction of seawater needs). Current oil/water separation units (Troll and Tordis operated both by Statoil in North Sea) are installed in relatively shallow waters and re-inject produced water into disposal wells. For deepwater applications, the large diameter vessels used on these two projects are not practical, due to the very thick wall required to sustain high pressure (either hydrostatic pressure or wellhead shut in pressure). Saipem is currently developing two bulk liquid separation systems to fit deepwater requirements, one based on gravity separation using several pipes working in parallel, and the second one based on cyclonic separation with compact equipment. After this first separation stage, if produced water is re-injected into injection wells, additional water treatment is then required to meet the stringent requirements of oil-in-water and solids-in-water contents. On this basis, Saipem and Veolia Water have partnered together to develop a series of solutions for deep water produced water treatment, targeting a water quality compliant with operators' requirements to inject produced water safely in existing or new injection wells. One of the developed solutions is based on the use of ceramic membranes which bring advantages compared to more conventional cyclonic systems, giving a much better water quality. This paper will present the global solutions developed for deepwater applications, from the first separation stage to the produced water treatment stage, giving the expected performances and the current maturity of the whole system. Introduction The production of a large number of deepwater fields requires water injection in order to sweep the oil to the production wells (water flooding) and/or to maintain the reservoir pressure by replacing the volume of the produced fluids. The injected water has conventionally been treated seawater while the water coming from the production stream is separated on topsides and disposed to the sea. As produced water cuts increase, produced water reinjection has become common, the oily water being treated on the topsides prior to injection. This paper addresses Subsea Produced Water Re-Injection (SPWRI) applied to mature deepwater fields producing a large amount of water. It consists of separating most of the produced water on the seabed and reinjecting it into the reservoir through water injection wells.

Gas Flotation of Petroleum Produced Water: A Review on Status, Fundamental Aspects, and Perspectives

“…These conditions also imply some changes in the process units compared to topside (offshore) design. Subsea separators and hydrocyclones for PW treatment are currently in operation or test use, − while new separation concepts have been proposed. , When it comes to CFUs, however, their performance at high outer pressure is still a key gap for possible subsea use. Moreover, the marinization, gas and production chemical logistics, and reject handling are other important reasons why subsea CFUs are not yet qualified.…”

“…Moreover, the marinization, gas and production chemical logistics, and reject handling are other important reasons why subsea CFUs are not yet qualified. One main challenge is that they rely on direct injection of pressurized gas or the use of recirculation pumps to drive eductors . Lately, a new type of eductor was introduced, which lets the flotation unit operate without the need of energy input or pressurized gas feed.…”