Experimental investigation of spontaneous imbibition process of nanofluid in ultralow permeable reservoir with nuclear magnetic resonance
Graphical abstract
In this work, we employed NMR technique to monitor the migration rule of nanofluid in sandstone cores during the spontaneous imbibition process. And EOR mechanism of the nanofluid was discussed.
Introduction
In recent years, nanofluids have attracted much interest as novel fluids because of their promising applications in different enhanced oil recovery (EOR) processes (Zargartalebi et al., 2015, Zhang et al., 2014, Hashemi et al., 2014, Franco et al., 2013, Wong and Leon, 2010, Wei et al., 2016). Compared with conventional fluids such as brine and surfactant solution, nanofluids composed of nanoparticles show special electrical, mechanical, magnetic, and optical properties, exhibiting great potential for EOR (Guo and Aryana, 2016, Li et al., 2017, Sharma et al., 2016, Suleimanov et al., 2011). In previous studies, the EOR potential of many nanoparticles such as silica (SiO2) (Hendraningrat et al., 2013, Maas et al., 2010, Perino et al., 2013, Monfared et al., 2016), zirconium oxide (ZrO2) (Karimi et al., 2012), aluminum oxide (Al2O3) (Esfandyari Bayat et al., 2014, Rahmatmand et al., 2016); and titanium oxide (TiO2) (Nakata and Fujishima, 2012, Ehtesabi et al., 2014) was demonstrated.
Nuclear magnetic resonance (NMR) is a rapid and nondestructive method to evaluate the pore structure and oil distribution of porous media. NMR has been used in gas and oil fields (Lai et al., 2016, Tinni et al., 2015, Meng et al., 2016, Liang et al., 2017). The characteristics of oil distribution in pores can be accurately investigated by combining NMR and spontaneous imbibition. NMR T2 distribution has also been used to study the factors influencing spontaneous imbibition, such as viscosity ratio, gravity, wettability, and boundary conditions (Zhang et al., 2000, Al-Mahrooqi et al., 2003, Wang et al., 2018).
Previous imbibition studies based on NMR mainly used synthetic brine or reservoir water as the imbibition agent. Yang et al. (2016) studied the water imbibition characteristics of tight cores with different mineralogy, pore connectivity, and pore-size distribution (PSD). They found that well-developed macropores (pore radius > 50 nm) have good pore connectivity and facilitated the majority of oil recovery during water imbibition. Wang et al. (2018) studied the spontaneous and forced imbibition characteristics in tight cores using NMR. In the presence of initial water saturation, the recovery contributions of mesopores (5.85 μm < pore radius < 58.5 μm) dominated in forced imbibition. Hun et al. (2016) investigated the water saturation distribution in shale cores during spontaneous imbibition using NMR. They found that the shale cores imbibed with water have a shorter advancing distance of water saturation front than the tight cores. However, spontaneous imbibition using a nanofluid in ultralow permeable sandstone cores using NMR has been rarely studied.
According to the findings of Hammond and Unsal, 2011, Hammond and Unsal, 2009, Hammond and Unsal, 2010) and Zhang et al., 2014, Wasan and Nikolov, 2003, Chengara et al., 2004, Kondiparty et al., 2012, Liu et al., 2012, Kondiparty et al., 2011) et al., mainly capillary force allows a water-based fluid to imbibe into pores and thus push the oil out of pores during spontaneous imbibition in ultralow permeable reservoirs. A nanofluid can displace the oil from a solid surface by low interfacial tension; wettability alteration, and structural disjoining pressure. Hence, it is essential to evaluate spontaneous imbibition using a nanofluid in ultralow permeable sandstone cores using NMR.
In this study, NMR technique was used to monitor the migration trend of a nanofluid in sandstone cores during spontaneous imbibition. A nanofluid based on functional silica nanoparticles with interfacial activity was prepared. NMR tests were carried out to study the characteristics of spontaneous imbibition including porosity, wettability, and oil distribution in cores. Based on the NMR results, PSD, wettability, and movable fluid distribution were evaluated. Spontaneous imbibition experiments indicate that oil recovery was obviously improved by using a nanofluid as the chemical agent for EOR. Finally, the EOR mechanism of nanofluid was elucidated.
Section snippets
Material and apparatus
The silica sol (10 nm, 30 wt%) used in this study was obtained from Ji Sheng YA. Co., Ltd., China. Adipic acid (AR) was purchased from Aladdin Reagent Co., Ltd., China. Chemicals including N, N-dimethylformamide (DMF, AR), n-heptane (AR), deuterium oxide (D2O), and sodium chloride (NaCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. The density of D2O is 1.105 g/cm3. Kerosene with a density of 0.8 g/cm3 was used as the oil phase. NaCl solution (3 wt%) with a density of 1.123 g/cm3
Mineralogy
The experiment was based on the Chinese Oil and Gas Industry Standard SY/T5163-2010 (Analysis Method for Clay minerals and Ordinary Nonclay Minerals in Sedimentary Rocks by the X-ray diffraction). The X-ray diffraction (XRD) analysis of sandstone core samples (Fig. 2) shows that the five cores mainly consist of quartz (88.9–91.0 wt%), moderate amount of clay (8.1–10.2 wt%), and slight calcite (1.2–2.0 wt%).
Characterization of functional silica nanoparticles
The structure and morphology of functional silica nanoparticles were observed using TEM (
Conclusion
In this paper, NMR technique was used to monitor the migration trend of nanofluid in sandstone cores during spontaneous imbibition. The conclusions are as follows:
(1) Combined spontaneous imbibition experiments and NMR measurements effectively reflect the characteristics of oil distribution in oil/nanofluid/rock system. The pore system of sandstone can be divided into micropores, mesopores, and macropores, and 70.82–71.98 wt% of oil was distributed in mesopores (50 nm < pore size ≤ 3000 nm);
(2)
Supporting information
T2 spectrum during the imbibition of core immersed in silica nanofluid with different concentrations and frequencies of oil distribution in pores before and after imbibition.
The interior wettability of kerosene-saturated cores (a) and after-imbibition cores (b) as the core is crushed.
Declaration of interest statement
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgment
This work was financially supported by the National Key Basic Research Program (No. 2015CB250904), National Science Fund (U1663206, 51425406), Chang Jiang Scholars Program (T2014152), and Climb Taishan Scholar Program in Shandong Province (tspd20161004).
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