Pore-scale investigation of residual oil displacement in surfactant–polymer flooding using nuclear magnetic resonance experiments
Pet. Sci.
Pore-scale investigation of residual oil displacement in surfactant- polymer flooding using nuclear magnetic resonance experiments
Zhe-Yu Liu 0 1
Yi-Qiang Li 0 1
Ming-Hui Cui 0 1
Fu-Yong Wang 0 1
A. G. Prasiddhianti 0 1
0 Edited by Yan-Hua Sun
1 EOR Research Institute, China University of Petroleum , Beijing 102249 , China
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 T2 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.
Nuclear magnetic resonance (NMR); Surfactant-polymer (SP) flood; Residual oil distribution; Displacement mechanism; Core displacement test
1 Introduction
After over 30 years of water flooding and polymer
flooding, the Dagang Oilfield is now a mature oil field with its
water cut reaching the economic limit. However, as much
as 70 % of the original oil in place (OOIP) may remain in
the reservoir after secondary recovery processes
(Sorbie
1991)
. A large portion of the residual oil is capillary
trapped
(Lake 1989)
. To tackle the residual oil saturation
and revitalize this reservoir, a tertiary recovery is required.
Surfactant–polymer (SP) flooding has been proved to be an
efficient tertiary method for most major oilfields in China.
To apply SP flooding in the Dagang Oilfield and optimize
the process in heterogeneous reservoirs, it is necessary to
predict the residual oil after the SP flood and identify the
displacement efficiency in pores of different sizes.
Several experimental/numerical techniques have been
proposed to measure or predict the residual oil distribution
after displacement processes. For example, a widely used
traditional method is to measure residual oil through
analysis of cast thin sections of a reservoir core
(Zao et al.
2009)
. This method damages reservoir cores while
obtaining slices. Furthermore, 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). However, this method does not take
into account the influence of interstitial matter on the
distribution of residual oil. X-ray computed tomography is
often used to detect the rock matrix, but it is not sensitive to
fluid changes
(Vinegar 1986; de Argandona et al. 1999; Liu
2013)
. Numerical simulations require some assumptions in
order to achieve mathematical completeness. In addition,
reservoir parameters are uncertain and hard to determine.
Therefore, there is usually a discrepancy between
simulation results and actual conditions
(Li et al. 2006)
. Nuclear
magnetic resonance (NMR) is a quick, accurate,
non-destructive, and widely used technology for core testing
(Kleinberg and Vinegar 1996; Xie and Xiao 2007; Zhao
et al. 2011)
. In NMR measurements, the received signals
originate only from fluids in pores. To differentiate
hydrocarbons from brine, brine is doped with paramagnetic
ions to shield the signals from water, so that the signals
only come from the oil. NMR T2 relaxation time represents
the fluid content in the pores of different sizes. The longer
T2 relaxation time corresponds to the larger pores, and vice
versa
(Williams et al. 1991; Gleeson et al. 1993; Cowan
1997)
. The residual oil distributions in pores of different
sizes are quantified through the T2 distribution analysis,
and the accurate oil saturation can be calculated to
investigate the oil movement in pores of different sizes. In our
study, the NMR (...truncated)