In the field of oil and gas field development, reservoir transformation is a key step in improving oil recovery efficiency. In recent years, nanofluids have shown great potential as a new type of working fluid due to their unique properties. Among them, the dispersibility of nanofluid particles improves the effectiveness of reservoir modification, and the hydrophilicity of nanofluid particles improves the effectiveness of reservoir modification, which has become a hot research topic in the industry. Excellent dispersibility ensures that nanoparticles can penetrate deep into the micropores of the reservoir, while strong hydrophilicity can effectively change the wettability of the rock surface, reduce crude oil adhesion, and jointly improve oil recovery efficiency. However, how to scientifically and quantitatively characterize these two characteristics has always been a bottleneck in technological optimization. In this context, low field nuclear magnetic resonance technology, with its advantages of non-destructive, quantitative, and dynamic monitoring, provides a revolutionary analytical tool for revealing the mechanism of nanofluid interactions.

Low field nuclear magnetic resonance technology: principles and application foundations

Low field nuclear magnetic resonance (LF-NMR) technology is mainly based on measuring the relaxation behavior of hydrogen nuclei (protons) in a magnetic field. In nanofluid systems, the relaxation time of water molecules' hydrogen nuclei (mainly T ₂ relaxation time) in the fluid is extremely sensitive to its microenvironment. When nanoparticles are uniformly dispersed in a liquid, a huge solid-liquid interface is formed; The hydrophilicity of the particle surface directly affects the binding state of its surface water molecules. These microscopic changes will significantly alter the relaxation rate of water molecule protons, which can be accurately captured by LF-NMR. By analyzing the T ₂ relaxation time distribution spectrum, researchers can directly correlate macroscopic fluid properties with microscopic particle states, achieving a leap from "seeing" to "understanding".

Three core applications in the study of nanofluid properties

Quantitative evaluation of particle dispersibility

The aggregation of nanoparticles significantly reduces their effective surface area and migration ability. LF-NMR directly reflects dispersion by measuring the T ₂ relaxation time spectrum of the system: the shorter the T ₂ time, the larger the particle specific surface area and better dispersion, indicating that more nanoparticles have effectively formed interfaces; On the contrary, prolonged T ₂ time indicates particle aggregation and decreased dispersibility. This provides clear quantitative indicators for optimizing the preparation process of nanofluids, such as surface modification and dispersant selection, to ensure that particles can enter the reservoir in the optimal dispersion state.

Accurate analysis of hydrophilicity/wettability

The hydrophilicity of the particle surface determines the strength of its interaction with formation water. The linear relationship between relaxation rate and particle surface area in LF-NMR can be used to determine the degree of coverage and binding of water molecules on the particle surface. Nanoparticles with strong hydrophilicity will adsorb and strongly bind more water molecules, resulting in a significant reduction in the overall T ₂ relaxation time of the system. By comparing the T ₂ changes of nanofluids before and after different treatments, the improvement effect of surface modification technology on hydrophilicity can be accurately evaluated, thereby guiding the development of nanofluids with stronger wettability reversal ability.

Real time dynamic monitoring of dispersion stability

The long-term stability of nanofluids under reservoir conditions is crucial. LF-NMR technology can perform continuous and non-destructive measurements on the same sample. By tracking the changes in T ₂ spectra over time, real-time monitoring of the sedimentation and agglomeration process of nanoparticles in liquid can be achieved. This dynamic tracking capability enables researchers to evaluate the long-term stability of dispersed systems under simulated formation temperature and pressure, providing key data for screening nanofluid formulations suitable for long-term oil recovery.

Significant advantages compared to traditional methods

Traditional methods for evaluating the dispersibility and stability of nanofluids, such as particle size analysis, turbidity testing, sedimentation observation, etc., often suffer from limitations such as sample destruction, one-sided results, and difficulty in reflecting micro interface changes in real time. Low field nuclear magnetic resonance technology exhibits unique advantages:

Non destructive quantification: Testing without damaging the sample can obtain true relaxation information and achieve absolute quantitative analysis.

Microscopic sensitivity: extremely sensitive to changes in molecular states at the interface between nanoparticles and fluids, directly related to microscopic mechanisms and macroscopic properties.

Comprehensive: One test can simultaneously obtain multidimensional information about dispersion state, hydrophilicity, and uniformity.

Dynamic tracking: capable of conducting long-term stability studies on the same sample, revealing patterns of evolution over time.

In summary, low field nuclear magnetic resonance technology, as a powerful analytical tool, is deeply empowering the research and application of nanofluids in the field of enhanced oil recovery. It provides a solid data foundation for the formulation design, performance optimization, and effect prediction of nanofluids by quantitatively analyzing the intrinsic relationship between the improvement of reservoir modification effect through the dispersion of nanofluid particles and the improvement of reservoir modification effect through the hydrophilicity of nanofluid particles. With the further popularization and deepening of this technology, it will continue to promote the development of more efficient and intelligent nanofluid reservoir transformation technology, opening up new technological paths for increasing production and efficiency in oil and gas fields.