The prospects and potential opportunities of low salinity water flooding for offshore applications in sandstones

Abstract

Low salinity water flooding (LSWF) is an emerging enhanced oil recovery (EOR) technology with enormous potential for offshore applications in sandstones. The objective of this study is to provide a 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 (SRUs) for seawater injection in offshore fields. Furthermore, the potential of targeting offshore oil fields with de-sulfated seawater (DSSW) 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 reservoir condition corefloods. The LSWF field tests implemented in offshore fields mainly comprise of single well chemical tracer tests (SWCTT) and interwell trials. However, the reported interwell field trials are restricted to unconfined pilots, which makes the injection and production allocation difficult. Therefore, confined pilots are recommended for future consideration of LSWF field trials to provide better estimations on swept volumes and accurately determine the improvements in the displacement efficiency.Globally there are more than 80 sulfate removal units in operation for de-sulfated seawater (DSSW) injection in offshore waterflooding. All these offshore fields with DSSW injection either currently ongoing or planned can become potential candidates for screening and switch over to LSWF EOR for the following two reasons: (1) 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 can become primary targets to be replaced by un-complexed cations in low salinity water for EOR. (2) The de-sulfated seawater injection processes can easily be switched to LSWF by retrofitting the existing sulfate removal units with reverse osmosis (RO) desalination systems and upgrading the facilities to handle larger water volumes. Such retrofitting is considered sufficient to generate low salinity water, which can bring significant gains to increase oil recovery with minimal additional investment. The availability of RO desalination systems on offshore platforms can also provide potential opportunities for the cost-effective implementation of tertiary chemical EOR technologies especially polymer flooding.The novelty of this study is that it provides useful guidelines for reservoir screening and offshore field implementation of LSWF processes. Also, the new potential of evaluating the offshore oil fields with existing DSSW injection for switching over to LSWF has been identified. These findings will have a potential impact on enhancing the LSWF prospects and opportunities for EOR in different offshore fields.