The design of an offshore chemical EOR project involves not just reservoir engineering, logistics and chemical storage issues. Oil/water separation, produced water treatment, incompatibilities with other chemicals, precipitation on surface facilities and high salinity and hardness water source are among the items that must be evaluated.
For polymer flooding, as the main offshore injection water source is seawater or produced water, the salinity and hardness will decrease the viscosifying power of conventional Partially Hydrolyzed Polyacrylamides (PHPA). For small pilots, the chemical consumption of PHPA allows the polymer storage and preparation plant to be whole built on the FPSO deck. Full-field applications may consider independent units for chemical storage and preparation. Proposed full-field alternatives for reducing the polymer consumption are polymers with higher salinity tolerance or reducing the injection water salinity and hardness. Technical and economic analyses are required to compare the alternatives.
Another concern on offshore polymer flooding is the large well spacing, requiring polymers with extended thermal resistance in hard water. At elevated temperatures, even modified polyacrylamides cannot resist long enough. It's also recommended to avoid high shear rate regions during the injection, reducing the need of polymer overdosage to compensate the viscosity loss due to mechanical degradation.
The ASP/SP method has a considerable higher chemical consumption than polymer flooding. Moreover, regular alkalis cannot be used in hard water, requiring hardness insensitive alkalis or water softening. Water desalinization will be necessary to ensure that the salinity gradient required for the ASP/SP method can be attained. Like for polymer flooding, the application of the ASP/SP method may consider an independent unit for chemical storage and water desalinization/softening. Oil/water separation and produced water treatment must be designed in order to be able to treat the back produced fluids containing the EOR chemicals.
Chemical EOR methods are an important strategy to increase the recovery factor of offshore fields, especially where there is no gas available for EOR and thermal methods are not suitable. Important steps are being taken on logistics, chemical and process developments in order to overcome the challenges on making chemical EOR deepwater offshore technically and economically viable.