A Critical Review of Low Salinity Water Flooding for Offshore Applications and Potential Opportunities

Abstract

Low salinity water flooding (LSWF) is an emerging enhanced oil recovery (EOR) technology with enormous potential for offshore applications. Numerous laboratory experiments and field trials of LSWF have been conducted to evaluate the EOR benefits and understand the underlying recovery mechanisms. The objective of this study is to provide a critical review on LSWF offshore field applications and summarize the key lessons learned. A review was also conducted on the capabilities of existing sulfate removal units for seawater injection in offshore fields. Furthermore, the potential of targeting offshore oil fields with de-sulfated seawater injection, either ongoing or planned, as primary candidates to switch over to LSWF EOR has been investigated. For LSWF field trials, the chance of success can be significantly improved when it is based on key laboratory screening tests such as corefloods at reservoir conditions. The methodologies implemented for LSWF offshore field trials mainly involved Single Well Chemical Tracer Test (SWCTT) and inter-well field trials. However, the inter-well field trials implemented so far are restricted to unconfined pilots, which makes the production and injection allocation more difficult. Therefore, confined pilots are recommended for future consideration of LSWF field trials to provide better estimations on swept volume and improvements in the oil displacement efficiency. Globally, there are more than 80 sulfate removal units currently in operation for offshore seawater flooding with approximately 10 million BWPD of cumulative de-sulfated seawater injection (DSSW) capacity for offshore water floods in the North Sea, the Gulf of Mexico, West Africa, and Brazil. All these fields with DSSW injection either ongoing or planned can become potential candidates to switch to LSWF EOR for the following two reasons: (1) The primary purpose for sulfate removal from sea water is to prevent scaling due to often high concentration of divalent cations in formation water and high sulfate concentration in seawater. The divalent cations can act as bridges between negatively charged rock surfaces and negatively charged polar oil components to increase the oil-wet tendency. These bridges become primary targets to be replaced by un-complexed cations in low salinity water for EOR. (2) The de-sulfated sea water injection process can easily be switched to LSWF by replacing the existing nanofiltration membranes in the sulfate removal facilities with reverse osmosis membranes and upgrading the facilities to increase the water treatment capacity and generate the desired low salinity water if these reservoirs fit the screening criteria and have a positive outcome of LSWF evaluation. Such retrofitting to the seawater treatment facilities on offshore platforms can bring significant gains to increase oil recovery with minimal additional investment. The novelty of this study is that it provides some useful practical guidelines for reservoir screening and offshore field implementation of LSWF. Also, the new potential of evaluating the offshore oil fields with existing de-sulfated seawater injection for switching over to LSWF has been identified. These findings will have a potential impact on increasing the prospects and opportunities of LSWF for EOR in different offshore fields.