Advances in Low-Field Nuclear Magnetic Resonance Analysis Applied to Geophysics and Petroleum Exploration

Low-field Nuclear Magnetic Resonance (NMR) analysis is an excellent technique for rapid measurement of rock physical properties. This technology leverages the resonance of hydrogen nuclei in oil and water under a magnetic field, excited by electromagnetic waves. By measuring the signals generated during relaxation, it can detect the presence and distribution of oil and water, and calculate rock physical parameters. In petroleum core analysis, NMR offers non-destructive testing, multi-parameter measurement, rapid results, and pollution-free operation, making it highly suitable for field applications. It has emerged as a new method for rock property analysis, gaining widespread attention and expanding its application areas in the petroleum industry.

1. International Development

Nuclear Magnetic Resonance (NMR) as a physical phenomenon was independently discovered in 1946 by Purcell at Harvard University and Bloch at Stanford University. Extensive experiments and theoretical studies established that the NMR relaxation time of fluids is closely related to pore size. In 1966, Seevers found a correlation between NMR relaxation time and rock permeability. Subsequent research introduced the concept of the Free Fluid Index and methods to measure sandstone porosity, permeability, and Free Fluid Index using NMR. By 1979, the theory of NMR relaxation in rock porous media was formalised. From the 1990s, R.L. Kleinberg studied hydrogen index (HI), relaxation times (T1, T2), and diffusion coefficients (D). Later, technical advances enabled determination of formation pore size distribution, capillary pressure curves, and methods to calculate movable fluid from NMR data, greatly improving the technology. In 1990, the MRIL-B NMR logging tool by NUMARGS was deployed, followed by Schlumberger’s CMR, TCMR, and CMR200 series, and in 1998 the CMR-Plus. Atlas released MRIL-A/B, MRIL-C, and MRIL-C/TP tools, while Halliburton launched the MRIL-Prime in 1998. These innovations enhanced measurement speed and precision, expanded reservoir property acquisition, and made NMR logging increasingly suitable for field applications.

2. Domestic Development

Chinese companies and research institutions have also conducted extensive research on NMR technology, following two main lines of development and application.

The first line, led by the Exploration and Development Research Institute of the former China National Petroleum Corporation and a joint laboratory with the Chinese Academy of Sciences—NMR Key Laboratory—introduced the country’s first advanced superconducting NMR imaging instrument in 1991. This facilitated extensive research in petroleum core analysis, flow mechanics, and instrument development. By 1997, China developed an advanced low-field NMR core analysis system, with pulse sequences and multi-relaxation inversion techniques adapted for complex, heterogeneous reservoirs. This enabled rapid, non-destructive measurement of porosity, permeability, and other rock properties. China became the first country to achieve full-diameter core NMR testing, creating a novel method to evaluate recoverable reserves in low-permeability reservoirs. In 1998, the first PC-based NMR logging interpretation software was developed. Subsequent research included determination of movable fluid cut-offs, permeability interpretation models, and calibration work. NMR technology was applied to nearly ten major domestic oil and gas fields, such as Daqing, Shengli, Xinjiang, and Qinghai, resolving production challenges and delivering significant technical, economic, and social benefits. These studies have guided the development of NMR expertise and cultivated professional talent in China.

The second line, based on over fifty years of accumulation at Ministry of Education key laboratories, saw Shanghai Niumag Electronics Technology Co., Ltd. continuously track advanced technologies. In recent years, they developed NMR core analyzers, NMR core imaging systems, and online NMR oil-water analysis instruments. These products have been adopted by organisations such as Shanghai ShenKai Petroleum Engineering Co., China National Offshore Oil Corporation, and China University of Petroleum (Beijing), receiving strong feedback.

3. Main Application Areas of Low-field NMR Analysis

1. Cuttings Logging Analysis: NMR technology allows analysis of drill cuttings to determine reservoir permeability, porosity, pore size distribution, and movable fluids, reducing coring volume and providing real-time geological information to guide exploration and production.

2. Borehole Core Analysis: Rapid, non-destructive testing of cores extracted from well walls provides movable water content, evaluates oil and water production, and supports oilfield development decisions.

3. NMR Logging Calibration: Enhances the accuracy of NMR logging interpretation.

4. Oil Saturation Analysis: Enables fast, low-contamination testing.

5. Development Experiment and Flow Mechanism Analysis: Provides dynamic, timely measurements of oil and water saturation and changes in oil-filled porosity during experimental procedures.

6. Calculating Recoverable Reserves: Utilises movable fluid and movable oil measurements to calculate recoverable reserves, supporting field development planning.

7. Pre-Assessment for Reservoir Stimulation: Based on measured parameters such as movable fluid fraction, pore size distribution, and permeability, guidance can be provided for selecting target layers and establishing a pre-assessment method for reservoir stimulation.

Low-field NMR has made significant progress in rapidly acquiring rock physical parameters and is undergoing extensive application research in other areas of petroleum exploration and development. For example, in Qinghai’s Shizigou Oilfield, self-developed NMR interpretation software confirmed oil-bearing capabilities in certain reservoir matrices and identified eleven favourable layers, providing critical support for reserve evaluation and development decisions. In Xinjiang’s Xiaoguai Oilfield, with reservoirs up to 70 meters thick, conventional core analysis could not explain low production after fracturing and acidising. NMR movable fluid testing demonstrated that the reservoir matrix lacked industrial oil flow capacity, informing development strategy adjustments. In Daqing’s Toutai Oilfield, where high injection-to-production ratios and poor development efficiency were observed, NMR was used to study low-permeability reservoir properties, movable resources, and development methods, achieving positive results.

Using low-field NMR technology, quantitative analysis of rock parameters—including porosity, effective porosity, permeability, movable fluid saturation, oil saturation, movable water saturation, T2 relaxation times, and cut-off values—along with semi-quantitative pore size analysis, plays a crucial role in cuttings logging, recoverable reserve calculations, pre-assessment for reservoir stimulation, experimental flow mechanism studies, and logging calibration. NMR enables rapid measurement of movable fluids and reservoir properties in low-permeability reservoirs, providing essential guidance for evaluating recoverable resources, assessing development potential, and designing development strategies.