Subsea Water Injection-Water Quality Management

Abstract Subsea processing systems for removal and re-injection of produced water have been in operation for some time and are being studied for use in several fields. For some types of reservoirs there is concern that poor injection water quality may reduce the water injectivity of the well. In some cases a well intervention might be required. This injectivity damage can be caused by dispersed oil drops and solids particles plugging the well injection face. There is thus a need for reliable, automatic sensors of water quality to protect injection wells. To be of use in well protection, both dispersed particle size distribution and number of each size of particle must be determined. This current work focuses on instrument determination of particle size as a primary indicator of potential value of a technology. There are currently no sensors for discrimination of oil from solids with the required range and accuracy. However some possibilities are now emerging. This paper summarizes work done by the Subsea Processing Consortium (SPC) over the last 3 years, and describes two promising lab-tested techniques. Introduction This paper covers water quality sensing for water injection purposes. Water quality sensing remains a developing technology. This paper presents the work done thus far and the conclusions reached by the SPC Consortium. Background Water quality has been a topic of interest ever since work began on subsea separation. A multitude of sensor principles have been investigated along the way. A collaboration was formed in 2001 with 2 vendors and 2 oil companies (Vetco, AkerKvaerner, BP and ChevronTexaco) with the aim of investigating and qualifying equipment for subsea processing. One of the areas investigated was water quality sensing. At this point, there is no qualified sensor available with suitable performance for this application. This paper will present the main areas covered thus far. Effect on reservoir injectivity of off-specification water For our application, we define injection water quality by 2 parameters: oil content, and solids content. Both of these parameters affect the injectivity of a water injection well. We therefore use the term "water quality" instead of "oil in water" to include both parameters. Water quality specification refers to the allowable limit of contaminants of both solids and oil drops. Some wells are very sensitive to injected water quality. For example, formations in West Africa were reviewed in which an expensive intervention could be required after just 15 minutes of off-specification water injection. Significantly, only the dispersed oil has an effect on injectivity. Dissolved oil and gas will proceed unhindered into the reservoir. For solids, only grains larger than a certain size (typically 1/3 of pore size) are a problem. Smaller particles seem to have little impact on injectivity. Solids are much more damaging to a reservoir than oil. Some test demonstrate up to 70 times more on a ppm-for-ppm basis. The solids may be oil-wetted or water-wetted. A combination of oil and oil-wetted solids is likely more detrimental than oil or sand alone.