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Low-salinity waterflooding in a low-permeability carbonate formation of a giant Middle East field
Alameri, Waleed
Alameri, Waleed
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2015
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The objective of this dissertation was three fold: (1) evaluate the potential of incremental oil recovery using low-salinity waterflooding in a specific carbonate reservoir, (2) develop the optimum water salinity to increase oil recovery from the reservoir, and (3) determine the underlying mechanisms of low salinity waterflooding. Five low-salinity waterflood experiments were performed on reservoir cores at reservoir conditions. A 1-D, two-phase, Buckley-Leverett model was developed to match experimental results and assess the wettability alteration due to the seawater and the low-salinity of injected brine. Ultimately, this model was used to history match the experimental results. Specifically, five seawater floods were conducted in several heterogeneous low-permeability carbonate cores, one of which included adding surfactant to the low-salinity waterfloods. The sequence of flooding was: seawater flood, two different concentrations of low-salinity waterflood, and in one experiment a final slug of surfactant diluted in low-salinity water. Core permeabilities were between 0.5 to 1.5 mD; porosity was in the range of 18 to 25%. Cores were aged in reservoir oil for eight weeks at reservoir pressure and temperature. Interfacial tension (IFT) measurements between oil-brine were conducted using pendant drop method. Captive oil-droplet contact angle measurements were made at different brine salinities. The carbonate core flood results show that removing NaCl from seawater and diluting the seawater twice and four times yielded about 8% incremental oil. An additional 5% oil recovery was obtained after the subsequent flood of surfactant diluted in low-salinity water. Oil-brine IFT increased with decreasing salinity both in presence and in absence of 1,000-ppm surfactant. From the captive oil-droplet contact angle measurements, it was observed that cleaned un-aged carbonate core discs began water-wet, and became more water-wet as salinity decreased (both in presence and in absence of 1000-ppm surfactant). The wettability of crude-aged carbonate core discs altered from oil-wet to intermediate-wet as salinity decreased. The wettability changed from intermediate-wet to water-wet with decreasing salinity in presence of 1,000-ppm surfactant. Moreover, addition of small amount of surfactant alters the wettability of crude-aged or cleaned un-aged carbonate core slabs towards water-wet. The degree of water-wetness achieved by surfactant solution depends on salinity level. To investigate the wettability alteration effect, a numerical 1-D, two-phase Buckley-Leverett model was used to mathematically simulate oil recovery during the experimental process of seawater and low-salinity waterflooding. The mathematical model matched the oil recovery with the adjustment of relative permeability functions to account for wettability alteration effects. Coreflooding, interfacial tension (IFT), and contact angle measurements were conducted to determine the possible mechanisms for improved oil recovery in carbonate reservoirs using low-salinity waterflooding after primary seawater injection. The following are the main conclusions: * Low-salinity waterflooding, after seawater injection, improves oil recovery in cores from facies 5 and 6, not with facies 3 because of burrows. * Diluting the seawater by a factor of two (LS1) is enough to achieve additional oil recovery. * The wettability of crude-aged carbonate core slabs alters from oil-wet to intermediate-wet with decreasing salinity.
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