Low Salinity Oil Recovery - Log-Inject-Log

Abstract

Abstract Many laboratory coreflood studies have shown increased oil recovery is achieved by waterflooding using low salinity water, compared with injection of seawater or high salinity produced water. The reasons for this improved oil recovery are thought to be due to effective wettability changes and / or controlled removal of clay constituents. This paper describes a log-inject-log field test, designed to identify whether this phenomenon could be observed within the near well region of a reservoir. The log-inject-log test was meticulously designed and executed, to ensure that flow rates were maintained at low rates, and that cross flow was minimised, to ensure that the results were representative of bulk reservoir waterflood displacement. A producing well was selected for the test to ensure that all saturation changes occurred under stringently controlled test conditions and that the results would not be affected by previous high rate water injection. 10 - 15 ‘pore volumes’ of high salinity brine, were injected into the ‘volume of interest’, to obtain a baseline residual oil saturation. This was followed by sequences of more dilute brine followed by high salinity brine for calibration purposes. Multiple log passes were conducted during injection of each brine. At least three further passes were run to ensure that a stable saturation value had been established after injection of each brine. Extensive water sampling was conducted to confirm brine salinities and increase confidence in the quantitative saturation results. The results were in line with previous laboratory tests from other fields, and showed 25 – 50% reduction in residual oil saturation when waterflooding with low salinity brine. Introduction There is increasing evidence from laboratory corefloods, supported by some field evidence, that waterflood performance is highly dependent on salinity of the injection water1,2,3. Laboratory corefloods suggest that as much as 50% additional oil could be produced if low salinity water (<4000 ppm) is injected into the reservoir, opposed to sea water or higher salinity production water. To ascertain whether these benefits seen in the laboratory are also observed in a reservoir environment, a modified log inject log test was performed to determine Sorw to both high and low salinity injection waters. This paper describes the design, execution, and analyses of this single well test. The Log Inject Log Test Techniques for determining Sorw by performing log inject log tests are well publicised in the literature. The technique is based on running multiple passes of pulsed neutron capture (PNC) logs after injection of two or more different brines, which have measurable differences in capture cross section. Assuming that formation porosity and the injectant brine cross section are known accurately, water saturation can be deduced from :Equation 1 Where:St1 is the PNC log response after injecting the first brineSt2 is the PNC log response after injecting the second brineSw1 is the capture cross section of the first brineSw2 is the capture cross section of the second brinef is the porosity of the formation. Therefore, by modifying this general approach, and injecting different brines of at least three different capture cross sections, it is possible to determine the Sorw after base saline injection and after injection of low salinity water.