RIASSUNTO
Abstract
Using an inorganic alkali in alkaline/surfactant/polymer (ASP) flooding resulted in scale precipitation when multivalent cations exist in mixing and/or formation brine in some extended field studies. This issue can be critical in the un-swept zones, which are the major target for EOR process, where formation brines can have high concentrations of Ca2+ and Mg2+. Any precipitation can result in plugging the pores in the target zone and loss of injectivity.
Several alkalis were evaluated to assess their scaling potential in high salinity formation brines and seawater as mixing brine at high temperatures. Four alkalis were evaluated; three inorganic alkalis sodium hydroxide, sodium carbonate, and sodium metaborate, and one organic alkali.
Eliminating the need to soften the mixing brine will result in expanding the ASP application to more challenging applications and will reduce the softening cost for the mixing water. Preventing precipitation of scale when chemical slug enters the unswept zones will result in improving the efficiency of the ASP flood, reduces chemical consumption and prevent scale precipitation that can cause formation damage and operational problems in the producers.
Introduction
The scaling problem caused by ASP injection in the Daqing pilot site was discussed by Wang et al. (2004). In the pilot test production wells in Daqing, scale was found in the wellbores and pumps, which resulted in a shutdown period of 114 days to replace and check the affected pumps, after that frequent pump malfunctions occurred from scale accumulations. Serious scale problems were found on the ground collecting and gathering system such as ground pipe walls, oil transferring stations, multipurpose stations, oil/water separating system, produced water disposal system, valves, pump heads and flow-meters. The authors studied the scaling mechanism and found that the injection of high-pH fluid will interact with formation brine and cause HCO3 - to produce CO3 2-. This will precipitate carbonate and hydroxide (Wang et al. 2004).
Strong alkalis have high reactivity with different reservoir rock minerals and hard ions in the formation and injected brines. Some researches suggested using different alkalis to overcome these complications. Recently, two alkalis were suggested, sodium metaborate and an organic alkali (Flateen et al. 2008, Berger and Lee 2006). Flateen et al. (2008) proposed sodium metaborate as a weaker alkali to avoid problems caused by strong alkalis in the ASP floods. The sodium metaborate showed a promising results and more tolerance to hard ions. Berger and Lee (2006) suggested using organic alkalis to replace inorganic alkalis in ASP process. They found that organic alkali can be mixed with similar chemicals used to formulate ASP with more tolerance for highsalinity and high-divalent cations concentrations. In our work we will investigate using these alkalis to reduce the negative impacts caused by interactions by strong alkalis.