No abstract

A modified ASP flooding technology, in which polymer and alkali-surfactant solution are injected alternatively for several turns, rather than being injected simultaneously as the traditional way, is studied by both lab experiments and a field test. Experimental results show that the viscosity of polymer solution will be reduced by 30% if alkalis and surfactants are added, while the dynamical interfacial tension is much higher in ASP-oil-water system than in AS-oil-water system. To give full play of these three chemicals on oil recovery, we alternatively inject polymer and alkali-surfactant slugs, and this modified ASP flooding is named A/S-P alternating flooding. A pilot test is then carried out in a water flooded reservoir since November 2008, which is employed in a block of 28 injection wells and 40 production wells. In the pilot test, alkali-surfactant slugs and polymer slugs are alternatively injected for five turns after a leading polymer slug. Injecting pressure increases in each polymer slug and decreases in the following A/S slug. In the following polymer slug, the injecting pressure of test area is 1.5MPa lower than contrast area and the injection rate per well is 9m3/d higher, while the production is 18% more. Profile tests show that the proportion of injectable thickness to total effective thickness keeps 80% or higher during the whole flooding process of the test area, while the proportion of contrast area is only 70% in the late stage. The minimum water cut during A/S-P alternating flooding is 82.0%, which is 6.1% lower than ASP flooding. However, the water cut increases significantly when A/S slugs are injected, which is an important weakness of A/S-P flooding. When chemical flooding is over, 17.8% of the original oil in place (OOIP) has been exploited by A/S-P alternating flooding, which is more than 7% higher than the contrast area flooded by ASP, while the total chemical cost is 11.3% lower. Therefore, A/S-P alternating flooding can be a cost effective enhanced oil recovery technology. A/S-P alternating flooding has more injection and production, displaces more zones, gets a higher oil recovery and uses fewer chemicals, which can be a cost effective technology.