Abstract
Based on cores from tight oil reservoirs in Ordos Basin, water flooding experiments with both low and high displacement pressures were carried out. Combined with NMR, quantitative analysis approaches for produced oil under different microscopic effects were established for quantitative research of the microscopic mechanism of water flooding in tight reservoirs. The research indicated that under low displacement pressure, oil recovery mechanisms of hydrophilic cores mainly include displacement, imbibition, and denudation, and those of wetting cores mainly include displacement and imbibition. After increasing the displacement pressure, both hydrophilic and neutral wetting cores have a certain increase in oil recovery. The common point is that both oil controlled by small throats and remaining oil droplets controlled by traps in large pores have been activated, while the discrepancy is that the oil film of the neutral wetting core boundary layer becomes thinner, which improves oil recovery ratio, and these mechanisms have less effect on hydrophilic cores. The amount of oil produced by each type of oil recovery mode was quantitatively analyzed. Percentages of produced oil in hydrophilic cores by flooding and imbibition and denudation are 15% and 12%, respectively, which are the main oil recovery mechanisms; percentages of produced oil in neutral wetting cores by displacement and imbibition are 25% and 2%, respectively, with displacement as the main oil recovery mechanism. After increasing the displacement pressure, oil produced by hydrophilic and neutral wetting cores increased by 6% and 9%, respectively, indicating that with increasing the displacement pressure, a part of the boundary layer of oil could be produced in neutral wetting cores.
Highlights
Tight oil, whose strategic significance has become increasingly prominent due to its effective development in domestic China, is widely distributed (Jia et al, 2012; Zou et al, 2012; Wu et al, 2016; Zhang et al, 2016; 2018, Yang et al, 2013; Guo et al, 2016; Wu et al, 2015; and Dejam et al, 2018)
There are few research studies on the microscopic recovery mechanism of water flooding in tight reservoirs with no report on the quantitative characterization of oil recovery in the microscopic mechanism, and it is necessary to explore the quantitative characterization of different recovery mechanisms in water flooding
Since the displacement pressure is only 0.32 MPa, which is relatively low, the absorption at the surface of the pores in the tight reservoir is very strong, suggesting that such displacement pressure cannot drive out the oil on the surface of the large pores of the cores; on the other hand, the tight reservoir is featured by mixed wettability, with some of the pore throats showing hydrophilic characteristic and some of the pore throats showing lipophilic characteristic
Summary
2009; Law and Curtis, 2002; Mullen, 2010; Zhong et al, 2012; Yao et al, 2013; and Quan et al, 2011). Gu et al (2017) revealed the microscopic influence mechanism of the permeability of tight reservoirs on the efficiency of oil imbibition recovery. Qualitative visual observation and theoretical analysis of the microscopic mechanism of water flooding in low permeability reservoirs (Huang, 1999; Guo et al, 1990; and Wang and Sun, 2010) with different wettability values have been done. Targeting at tight cores with different wettability values, this study combined nuclear magnetic resonance technology with water flooding experiments under different pressures to establish a quantitative analysis approach for oil production from different microscopic actions and quantitatively analyze the microscopic mechanism of water flooding. The water flooding experiment under the pressure of 0.32 MPa and 5.12 MPa is carried out for each core, and the nuclear magnetic resonance T2 spectrum is measured separately
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