Dow Water Solutions

Sulfate Removal from Injected Water in Oilfield Operations

The FILMTEC™ SR90 nanofiltration membrane (and the resultant sulfate removal technology) has been developed in a cooperative effort between The Dow Chemical Company and the Marathon Oil Company to selectively remove sulfate from seawater that is used for waterflood injection operations.

There are two major advantages in the removal of sulfate from injected seawater.

• Prevention of barium and strontium sulfate scale precipitation. When normal high sulfate seawater is injected into reservoirs which have formation water containing barium and strontium, mixing occurs forming a supersaturated barium and / or strontium sulfate solution. Upon pressure decreases in and around the production wells, the supersaturated barium and / or strontium sulfate solution is no longer stable and precipitation occurs. The result is scale formation in the production tubing and / or plugging of reservoir rock around the production well. Petroleum reserves are often lost. By removing sulfate from injected seawater, the potential for scaling is prevented. This is contrasted with traditional scale remediation treatments and procedures addressing scale problems after they occur. In deep water and other complex oil developments, sulfate removal, and the subsequent prevention of scale, provides significant cost advantages.
   
• Souring Control / Mitigation in reservoirs containing barium and strontium. By removing the sulfate in the injected seawater, there simply is a reduced source of sulfur that can be converted to hydrogen sulfide by thermophilic sulfate reducing bacteria. Consequently, well souring does not occur. Examples of sweet oilfields that became sour upon waterflood breakthrough following the injection of high sulfate seawater, include Prudhoe Bay, Kuparuk, and Gulfaks in the North Sea.  By removing the sulfate from the injection water providing subsequent souring control, there is a reduced need for sour safe metallurgy, costly hydrogen sulfide removal equipment, and health and safety concerns. Along with sulfate removal in normal scale prone reservoirs, the Reservoir Souring Mitigation (RSM) Technology is being developed by Dow and Marathon to ensure no hydrogen sulfide generation for reservoirs - regardless of barium or strontium content.

The nanofiltration membrane also removes all particles greater than one one-thousands of a micron resulting in high quality injection water free of silica and bacterial materials thereby insuring continued injection rates reflective of initial reservoir conditions. 

Sulfate removal provides maximum benefits in oil developments exhibiting the following characteristics.

a)  Deep water operations with subsea vs. dry well heads. Chemical squeeze inhibitor treatments generally cannot be applied when access to a dry well head is not available.
b)  Horizontal or multilateral reservoir developments where there is difficulty / risk in the placement of chemical squeeze inhibitors.
c)  Costly production wells where the loss of one well due to scaling could approximate the cost of a sulfate removal unit.
d) Mild / Moderate scaling potential in combination with costly and / or complex reservoir developments.

It has been observed that sulfate removal is being accepted as the "default" scale control technique in most West African and Brazilian operations. Seven additional sulfate removal facilities are either in operation or under construction in the North Sea. Total sulfate removal capacity is approximately 4MM BWPD for existing systems and systems under construction.

Products

FILMTEC SR90-400i and SR90-440i  nanofiltration membranes are used exclusively in offshore oilfield applications. In addition to the membrane elements, the product includes end use know-how, proprietary operation procedures, risk assessments, detailed system designs, and a sublicense of the Marathon Oil Company patent, which allows an end user to practice the technology. Extensive field experience, product technical manuals (under confidentiality and sublicensing agreements) and research has resulted in a one-third reduction in weight, space, and cost requirements since the inception of the technology. Systems are designed specifically to ensure the elimination of scale and for particular platform requirements and reservoir characteristics.

Features

Eliminating Scale

Removal of sulfate eliminates the sulfate component from injection water that causes scale formation.  Reducing or eliminating scale:

1. Eliminates the existing scale inhibitor squeeze treatments.
   a. Cost of such treatments.
   b. Deferred oil production during squeeze treatments.
   c. Necessary monitoring of residual scale inhibitors.
   d. Potential reservoir damage from these treatments.
   e. Discharge of scale inhibitors and resultant environmental concerns.
   
2. Eliminates potential coprecipitation of radium 226 which results in radioactive barium sulfate scale and resultant handling and disposal costs.
   
3. Potential loss of a well (reservoir reserves) due to scaling and resultant unsuccessful well workovers.
   
4. The cost of a well workover. In some areas, two or three well workovers often equal the cost of a sulfate removal facility.

Eliminating Hydrogen Sulfide

An auxiliary benefit is that by reducing sulfate injected into barium and / or strontium rich reservoirs, one eliminates the source of sulfur that is converted to hydrogen sulfide by sulfate reducing bacteria. This applies only to wells such as those in the North Sea that were sweet until waterflood breakthrough when they turned sour. Eliminating hydrogen sulfide generation:

1. Reduces safety, health, and environmental concerns by eliminating the production of deadly hydrogen sulfide.
   
2. Reduces the cost related to sour gas and oil treatment or dedicated "sour safe" pipelines.
   
3. Allows use of less costly metallurgy for the operation due to reduced stress cracking and corrosion.
   
4. Reduces sulfide scale problems (ferric sulfide is exceptionally troublesome). 
   
5. Reduces the potential for necessary addition equipment on a platform or FPSO with limited space and weight capacities.
   
6. Improves the economics of a discovery to "tip the scales" in pursuing v. abandoning a new prospect via risk analysis.

Competitive Offerings

The Dow / Marathon Sulfate Removal Technology is a patented process offered by four worldwide non-exclusive licensees. The licensees / system suppliers include: Aker Kværner Process Systems of Lysaker, Norway; NATCO Group of Houston, TX, USA and Farnborough, UK; Siemens - US Filter of Houston, TX, USA and Oslo, Norway; and VWS Westgarth Ltd. of East Kilbride, Scotland and Gloucester, UK as well as Houston TX, USA.

Case Histories

Low Sulfate Seawater Mitigates Barite Scale - Jim Hardy and Ian Sims, Marathon Oil UK Ltd. Oil and Gas Journal, 02 Decemeber 1996.

Barite Scale Prevention for Elf Angola's Girassol Field Using Sulphate Removal Technology, Van Khoi Vu, Elf Exploration Angola, Paris France, Deep Offshore Technology Conference, October 1999. 

Eliminating the Need for Scale Inhibition Treatments for Elf Exploration Angola's Girassol Field, Van Khoi Vu, Elf Exploration Angola, Paris France,  SPE 60229, January 2000.

Low Sulfate Water Injection Knocks Off Production Hit-List, Offshore Magazine, Pennwell, April 1998

Seawater Membrane Filtration Used to Control Scale in Syd Arne Reservoir, Offshore Magazine, Pennwell, August 2001, English

New System at Heidrun removes sulfate from seawater, Offshore Magazine, Pennwell, May 2004. 

Deepwater project economics demand sulfate removal to ensure scale-fee operation, Offshore Magazine, Pennwell, May 2003.

The Advancement of Sulfate Removal from Seawater in Offshore Waterflood Operations (972KB PDF) presented at CORROSION/2002
NACE International Web Site
©NACE International. All rights reserved. Paper Number 02314 reproduced with permission from CORROSION/2002 Annual Conference and Exhibition, Denver, CO.

Presentations / Reference List

For in-depth discussions and review of how Sulfate Removal may provide field operation cost reductions for each particular reservoir development, please contact either:

The Dow Chemical Company (Scott Beardsleyat (952) 897-4290 ssbeardsley@dow.com) or Marathon Oil (George Southwell at (303) 692-0139 george@prattllc.net)