Seawater Depleted in NaCl as a ‘Smarter’ Enhanced Oil Recovery Fluid in Carbonates
Figure 1: Schematic Model of the Suggested Mechanism for the Wettability Alteration Induced by Seawater
AB Chromatographic Wettability Test
The chromatographic wettability test for carbonates developed by Strand et al.7
was used to determine the increase in water-wet area after spontaneous imbibition by the different fluids. The principle of the analysis is based on chromatographic separation between a
Ca2+ SO42- Mg2+ –
++ SO42-
+ –
CaCO3 (s) (A) Ca2 + and SO4 2 – are the active species. (B) Mg2 + and SO4 2– are the active species.2 Ca2+ Results and Discussions Figure 2: Spontaneous Imbibition at 100ºC
10 20 30 40 50 60 70 80
0 010 SSW 20
Time (days) SSW0NaCI
Figure 3: Spontaneous Imbibition at 110ºC
10 20 30 40 50 60 70 80
0 010 SSW 20 30 SSW0NaCI 40 Time (days) DiI SSW 20000 DiI SSW 10000
adsorption of polar components on the surface during ageing in the actual oil at 90ºC for four weeks.
Spontaneous Imbibition
The aged cores were immersed in the imbibing fluid in sealed steel cells. The spontaneous imbibition tests were performed at specified temperatures – 100ºC, 110ºC and 120ºC – with a back pressure of 10 bars to avoid boiling of the fluids. The produced oil during the test was collected in a burette, and oil recovery was determined versus time as percentage of original oil in place (%OOIP).
70 50 60 70 30 SSW4NaCI 40 50
The effect of salinity and composition of the displacing fluid was studied by a spontaneous imbibition process. Furthermore, the wettability changes were monitored after the imbibitions process in order to relate the difference in oil recovery to potential of the imbibing fluid to act as a wettability modifier.
Spontaneous Imbibition
The results from the spontaneous imbibition tests at 100°C, 110°C and 120°C are shown in Figures 2, 3 and 4, respectively. Neither Na+ nor Cl- are regarded as potential determining ions towards the chalk surface, which means that these ions are not part of the inner Stern layer.8
The ions are, however, active in the double layer and may have an influence on the access of the active ions, Ca2+, Mg2+ and SO42-, to the chalk surface. This was studied by
Improved oil recovery by low salinity flood is not possible for carbonates because the chemical mechanism for wettability modification is different for the two reservoir rocks.
comparing the imbibition of modified SSW, i.e. SSW depleted in NaCl (SSW0NaCl) and SSW4NaCl with original SSW as a base fluid. Thus, the concentration of the active ions was kept constant. At 100ºC (see Figure 2) the imbibition rate is low, and oil recovery after 30 days is in the range 30–40 %. The oil recovery increases as the concentration of NaCl in the seawater decreases. At 110ºC (see Figure 3) the difference in oil recovery between SSW and SSW0NaCl is more pronounced. After 30 days the difference is about 15 %. The plateau recovery for the fluid depleted in NaCl was 60 %, which is the recovery obtained for a completely water-wet core. No imbibition test was performed using SSW4NaCl at 110ºC. The same trend in recovery was observed at 120ºC, i.e. the recovery increased as the concentration of NaCl decreased (see Figure 4).
It is interesting to note that 70 % of OOIP was recovered by using seawater without NaCl present. This is a very high recovery by spontaneous imbibition for a low permeable Stevns Klint core. The imbibition rate was also very fast, and plateau recovery was reached within 15 days. On the other hand, when the amount of
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
tracer, SCN-, and SO42- at the water-wet sites of the rock. The area between the effluent curve of SCN- and SO42- is directly proportional to the water-wet area and, by using a completely water-wet
reference core, the water-wet fraction (WI) of the chalk core can be calculated.
Recovery factor (% OOIP)
Recovery factor (% OOIP)
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