Mechanism Study on Foam Flooding for Daqing Reservoirs After Polymer Flooding

Abstract Polymer flooding has been carried out widely in Daqing oilfield. The average oil recovery was 53% and the water cut was more than 90% after polymer flooding. The remaining oil was highly dispersed in distributary channels. New EOR techniques with better profile control ability than polymer flooding need to be applied. The mechanism on foam flooding after polymer flooding was studied based on core flood experiments and X-ray computed technology (CT) in this paper. The EOR effect for foam flooding after polymer flooding was evaluated employing both 2D and 3D core flood experiments. Artificial three-layer positive rhythm sand-packed cores were employed. The length, width and height for 2D cores were 28.0 cm, 4.5 cm and 4.5 cm, respectively. And the length, width and height for 3D cores were 50.0 cm, 50.0 cm and 20.0 cm, respectively. 1/4 five-spot pattern was employed for 3D core flood experiments. Particularly, the oil saturation profile was monitored with CT during whole 2D flooding process. The fluid distribution and sweep efficiency during the flood were described based on CT images. EOR effect evaluation with 2 D and 3D models indicated that foam flooding could contribute 11.7% and 16.1% OOIP recovery even if the recovery after polymer flooding was more than 50%, respectively. The obvious increasing in injection pressure during the foam flooding indicated that foam fluid had stronger profile control ability than polymer. The distribution of residual oil in three-layer cores during the flooding was monitored. Firstly, the injected water mainly fingered through high-permeability zones during water flooding. The residual oil after water flooding was mainly distributed in the middle and low permeability layers. Next, polymer flooding could subsequently increase sweep efficiency by the deceasing in mobility ratio. The residual oil after polymer flooding was highly dispersed. In the end, foam could be employed to further recover considerable oil after polymer flooding. Pore blocking effect might play a great role in EOR mechanism for foam flooding. The evolution of fluid distribution was demonstrated clearly during foam propagation. The difference among the CT value of three phases (eliminating the contribution of the rock) was contributed to the addition of sodium iodide in aqueous phase. Core flooding experiments monitored with CT proved that foam flooding could become a further EOR method for Daqing reservoirs after polymer flooding. Foam demonstrated better pore blocking ability than polymer within middle and high permeability layers. Foam could effectively block thief zone for highly heterogeneous reservoirs and increase sweep efficiency significantly after polymer flooding. It might become a promising EOR method for Daqing reservoirs after polymer flooding due to its profile control ability.