Low-salinity waterflooding in a low-permeability carbonate formation of a giant middle east field.

摘要

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.