In the previous part it is discussed that ion exchange at mineral surfaces promotes an alkaline environment needed for desorption of POC. This process leads to wettability alteration towards more water-wet conditions which results in increased capillary forces. (Austad et al., 2010; Piñerez Torrijos et al., 2016b). The wettability alteration is a result of CoBR-interactions at mineral pore surfaces. The process is time-dependent, and low flow rates could be needed to observe the LS EOR effect. Radial well geometries and reservoir heterogeneities result in low flow rates and low pressure drop in the main part of the reservoirs. The oil displacement could then be more dependent on capillary forces compared to the viscous forces.
In our experiments a low flow rate has been chosen, 4 PV/D, which will allow the chemical reactions to take place, so capillary forces could contribute to the recovery process. 4 PV/D corresponds approximately to the industry standard of 1ft/D (foot/Day).
The efficiency of LS brine injection has been tested by a large number of forced imbibition (viscous flooding) tests presented in the previous section. In this chapter, we will prove the idea of EOR by favorable
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wettability changes and an increase in the capillary forces using LS brine.
A series of spontaneous imbibition tests at Tres have been performed on core M3 using any of the individual brines, FWm, mSW and LSm, to study the potential of different brines on generating positive capillary forces. Both secondary and tertiary SI tests have been performed on restored core M3.
After the fourth restoration of core M3, M3-R4, the core was placed in the SI setup, and FWm was used as imbibing brine. The result is presented in figure 52. The ultimate oil recovery of 42 %OOIP was reached after 5 days. No chemical-induced wettability alteration is expected to take place because the core is already equilibrated with the FWM during core restoration. The imbibition by itself confirms the presence of positive capillary forces in the core.
Figure 52. Oil recovery test at Tres by spontaneous imbibition (SI) on core M3-R6 using mSW-LS brines, and in comparison, with spontaneous imbibition of LS in M3-R5 and FW-LS in core M3-R4.
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After eight days, the imbibing brine was changed to LSm, and 6 %OOIP extra oil is gradually recovered during the next five days, confirming wettability alteration and increased positive capillary forces during LSm
imbibition, figure 52.
After the fifth restoration, M3-R5, the core is exposed to the LSm in the secondary mode. As expected, the LS brine significantly increased the capillary forces compared to FW, due to wettability alteration, and a oil recovery plateau of 67 %OOIP was reached after six days. Comparing the recoveries in the same time frame confirms an increased rate of imbibition with LSm, which is a crucial parameter for optimized recovery processes.
Comparing the ultimate oil recoveries during SI and viscous flooding with LSM brine in secondary mode on core M3, SI with LSM gave the highest recovery of 67 %OOIP compared to 63%OOIP during viscous flooding. This confirms the key role of capillary forces during oil production from heterogeneous porous networks. Wettability alteration processes and capillary forces is normally ignored in mathematical reservoir modeling.
The final imbibition experiment, called M3-R6, was performed by SI with mSW followed by LSm brine. The result is presented in the figure 52. The ultimate oil recovery by mSW is 38 %OOIP, which is almost comparable with FWm, but the rate of imbibition is far slower. The result confirms the mSW is not smart water, and not able to induce increased capillary forces. But interestingly, when the imbibition brine is switched
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to LSM, a huge amount of extra oil was recovered reaching 68%OOIP after six days.
The results of all three spontaneous imbibition tests and two viscous flooding (Forced immbibtion, FI) tests performed on core M3 are summarized in table 12.
Table 12. Summary of the oil recovery tests by SI and VF performed on core M3.
Test emphasizes the importance of positive capillary forces generated by wettability alteration in the viscous flooding (FI) tests. Performing brine injection at low rates are essential for observing the capillary effects.
This is in line with the observations by Johannesen and Graue (2007) in their series of water flooding experiments in chalk, confirming that both SI and FI recovery curves reached almost the same plateau (similar residual oil saturations) when the flooding rate was at the lowest. This is in line with what hypothesized earlier that in the main part of the reservoir, where the pressure drop is the least, the spontaneous imbibition
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due to positive capillary forces are the main driving forces during smart water flooding process.
The recovery data presented in table 12, confirms that The LSM promoted the most water wet system, and also behaved the smartest brine for EOR purposes. The LSm brine gave the best sweep efficiency and showed the latest water breakthrough point during the FI test, figure 50. SI tests confirmed that the highest recovery is achieved in the most water wet system which is inconsistency with what Jadhunandan and Morrow (1995) stated that the highest oil recovery will be achieved in the neutral to slightly water-wet conditions.
Contrarily to the LSm brine, mSW could not contribute to increased capillary forces by wettability alteration compare to the base brine which is FWm.
The oil recovery process during FW injection into heterogeneous porous systems can be explained by viscous displacement of oil from larger high permeable pores, and some contribution of capillary forces, figure 53b.
When the flooding brine is switched to a Smart Water, the chemical wettability alteration will increase capillary forces and the oil recovery is increased by improving both the microscopic and macroscopic sweep efficiencies, figure 53c.
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(b)
(c)
Figure 53. Oil distribution and displacement efficiency in a heterogeneous porous network with large, medium and small pores during FW and Smart Water injection.
(a)Initial oil saturation in heterogeneous pore systems. (b) Residual oil saturation after FW injection at fractional slightly water-wet conditions where the oil displacement is controlled by viscous and capillary forces, and (c) Residual oil saturation after wettability alteration with Smart Water where the oil displacement is controlled by viscous and stronger capillary forces.
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