Abstract
Research on the Gangxi III area in the Dagang Oilfield shows that there was still a significant amount of oil remaining in oil reservoirs after many years of polymer flooding. This is a potential target for enhanced oil recovery (EOR). Surfactant–polymer (SP) flooding is an effective chemical EOR method for mobilizing residual oil and improving displacement efficiency macroscopically, but the microscopic oil displacement efficiency in pores of different sizes is unclear. Nuclear magnetic resonance (NMR) is an efficient method for quantifying oil saturation in the rock matrix and analyzing pore structures. In this paper, the threshold values of different pore sizes were established from the relationship between mercury injection curves and NMR T 2 spectrums. The distribution and migration of residual oil in different flooding processes was evaluated by quantitatively analyzing the change of the relaxation time. The oil displaced from pores of different sizes after the water flood, polymer flood, and the SP flood was calculated, respectively. Experimental results indicate that (1) the residual oil in medium pores contributed the most to the incremental oil recovery for the SP flood, ranging from 40 % to 49 %, and small pores usually contributed <30 %; (2) the residual oil after the SP flood was mainly distributed in small and medium pores; the residual oil in medium pores accounted for 47.3 %–54.7 %, while that trapped in small pores was 25.7 %–42.5 %. The residual oil in small and medium pores was the main target for EOR after the SP flood in oilfields.
Highlights
After over 30 years of water flooding and polymer flooding, the Dagang Oilfield is a mature oil field with its water cut reaching the economic limit
Experimental results indicate that (1) the residual oil in medium pores contributed the most to the incremental oil recovery for the SP flood, ranging from 40 % to 49 %, and small pores usually contributed \30 %; (2) the residual oil after the SP flood was mainly distributed in small and medium pores; the residual oil in medium pores accounted for 47.3 %–54.7 %, while that trapped in small pores was 25.7 %–42.5 %
The residual oil in small and medium pores was the main target for enhanced oil recovery (EOR) after the SP flood in oilfields
Summary
After over 30 years of water flooding and polymer flooding, the Dagang Oilfield is a mature oil field with its water cut reaching the economic limit. A widely used traditional method is to measure residual oil through analysis of cast thin sections of a reservoir core (Zao et al 2009). Fractured cores, unconsolidated sands, and mud cannot be cut into slices using this method Another experimental technique is to use a microvisualized model instead of a reservoir core to simulate a displacement process as well as the distribution of residual oil (Wang et al 2010). This method does not take into account the influence of interstitial matter on the distribution of residual oil. The NMR technique was used to evaluate the residual oil distribution after water flooding, polymer flooding, and SP flooding to study the ‘‘kickoff’’ mechanism of the residual oil
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