Piolot Study On Seawater Nanofiltration For Offshore Oil Fields Water Injection Operations

Water injection is seen as one of the most important method of oil production. Due to the large quantitiy of seawater, it should be the main or even unique source of water injection for offshore oil fields. However, Ca²⁺, Mg²⁺ and SO₄^2- ions can be deposited by the underwater exsisting ions, leading to problems for oil production. Nanofiltration (NF) can be used to effectively remove the ions mentioned above, and there are some successful examples in foreign oil fields. However, litter applications in this area have been reported domestically and reverent researches are lacking. Consequently it has valuable significance to initiate the investigation in this area.In this thesis, ultrafiltration (UF) process was used to pretreat raw water of NF process, and comprehensive studies were carried on about the operating parameters and membrane fouling. Meanwhile, the stability of pretreatment and NF process was investigated respectively, and the compatibility of softened seawater and formation water was analyzed, together with the sensivity of soften seawater in the reservoir. Several parameters were examined including the turbidity of UF process and SDI under long-term operation with the variance of seawater. Results showed that during the running of 1700 hours, even experienced the high fluctuation of seawater quality in typhoon, the UF produced water still had relatively stable quality.Based on the results of UF process, the research of NF process on softening seawater was carrried out, and the results showed as to four different types of NF membrane (NFI,NFII,NFIII,NFIV), the permeating flux increased as the rise of operating pressure, however the ratio of rise reduced when the pressure was over certain value. The rejection of Ca²⁺,Mg²⁺,SO₄^2- for each type was stable , among which SO₄^2- is over 95% for all types while Ca²⁺ and Mg²⁺ vary remarkably: Ca²⁺ 50%-61%,Mg²⁺ 85%-91% for NFII, Ca²⁺ 50%-61%,Mg²⁺ 71%-81% for NFIII and Ca²⁺ 47%-56%,Mg²⁺ 64%-70% for NFIV. The study on membrane fouling of NFII was carried out. SEM and EDS were used to identify the surface of fouled membrane, and the result showed the existence of Ca²⁺,Mg²⁺ were in the form of chlorides and sulfates.Based on the previous results of the pressure effect on NF process, optimization on the NF pressure was done, and softened seawater was prepared with 70% recovery rates under the optimized pressure. Static compatibility investigation and computer simulation are used to investigate the compatibility of softened seawater and formation core. Results showed that the compatibility of NFI, NFII and NFIII with formation water are ideally, and NFIV soften water had the risk of forming BaSO4 when used as injecting water, but still superior to seawater itself. NFI softened water is harmless to reservoir comparing with the mediate harm for NFV softened water.To conclude, compared with seawater, the NF softened seawater is more compatible with formation water, and no precipitation was formed in the process of mixture between formation water with NFI, NFII and NFIII softened water respectively. The softened water was harmless to formation, or only mediately. It is proved that the NF-UF integrated system can provide high quality injection water for offshore oil fields, and found ground foundation for its wide application in this area.