Real-time oil-saturation monitoring in rock cores with low-field NMR

Flow and structural analysis of sedimentary rocks by core flooding and nuclear magnetic resonance: A review

Fluid flow analyses and investigations of associated structural variations in rock formations are important due to the complex nature of rocks and the high heterogeneity that exists within fluid-rock systems. Variations in fluid-rock parameters need to be ascertained over time with continuous or cyclic fluid injection into subsurface rocks for enhanced oil recovery and other subsurface applications. This Review introduces the use of the core flooding-nuclear magnetic resonance (NMR) technique for analysis of combined fluid flow and structural features in subsurface fluid-rock systems. It presents a summary of the results realized by various researchers in this area of study. The influence of several conditions, such as geochemical interactions, wettability, inherent heterogeneities in fluid flow and rock properties, and variations in these parameters, is analyzed. We investigate NMR measurements for both single fluid phase saturation and multiphase saturation. Additionally, the processes for identifying and distinguishing different fluid phases are emphasized in this study. Furthermore, capillary pressure and its influence on fluid-rock parameters are also discussed. Although this study emphasizes subsurface rocks and enhanced oil recovery, the experimental combination is also extended to core flooding using several other injection fluids and porous media. Finally, research gaps pertaining to core flooding-NMR systems regarding fluid flow, structural changes, fluid-rock systems, and instrumentation are pointed out. Transient flow analysis involving structural variations is suggested for future work in this regard.

The Effect of Clay Content on the Spin-Spin NMR Relaxation Time Measured in Porous Media

Clays, hydrous aluminous phyllosilicates, have a significant impact on the interpretation of physical measurements and properties of porous media. In particular, the presence of paramagnetic and/or ferromagnetic ions like iron, nickel, and magnesium in clays can complicate the analysis of nuclear magnetic resonance (NMR) data for porous media characterization. This is due to the internal magnetic field gradient induced by the clay minerals. In this study, we aim to investigate the impact of clay content on spin-spin relaxation time (T ₂), which is strongly influenced by the pore surface chemistry. Seven rock core plugs, characterized with variable clay content, were used for this purpose. The clay mineralogy and volume were determined by means of quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN). The T ₂ relaxation time was measured using a Carr-Purcell-Meiboom-Gill (CPMG) sequence with variable echo spacing (T _E). The maximum percentage difference in dominant T ₂ values (MRDT ₂) between shortest and longest echo spacing was subsequently correlated with clay content obtained from QEMSCAN. Our results show that the reduction in T ₂ distribution with increasing echo time T _E is more significant in samples characterized by higher clay contents. The MRDT ₂ was found to be strongly correlated with clay content. An analytical equation is presented expressing MRDT ₂ as a function of clay content providing a quick and non-destructive approach for clay content estimation. Moreover, the MRDT ₂-clay content relationship showed a nonlinear behavior: MRDT ₂ increases drastically as the clay content increases up to 15%, beyond which the rate of MRDT ₂ change with clay content diminishes. This behavior could be attributed to the clay distribution. At higher clay contents (above 15%), it is more likely for clay to form clusters (structural clays), which will not significantly increase the clay surface in contact with the pore fluid. Further, experimental data suggests that ignoring the impact of clay on internal magnetic gradients and T ₂ signal may result in considerable underestimation of the actual pore size distribution.

Low-field NMR for quality control on oils

Oil is a prominent, but multifaceted material class with a wide variety of applications. Technical oils, crude oils as well as edibles are main subclasses. In this review, the question is addressed how low-field NMR can contribute in oil characterization as an analytical tool, mainly with respect to quality control. Prerequisite in the development of a quality control application, however, is a detailed understanding of the oils and of the measurement. Low-field NMR is known as a rich methodical toolbox that was and is explored and further developed to address questions about oils, their quality, and usability as raw materials, during production and formulation as well as in use.

Monitoring Electrochemical Reactions in Situ with Low Field NMR: A Mini-Review

The number of applications of time domain NMR using low-field spectrometers in research and development has been steadily increasing in recent years with applications ranging from quality control of industrial products to the study of physical and chemical properties of a wide array of solid and liquid samples to, most recently, electrochemical studies. In this mini-review we summarize the progress that has been achieved in the coupling between time domain NMR (using low-field spectrometers) and electrochemistry and how the challenges that this coupling poses have been overcome over the years. We also highlight the effect that the static magnetic field of the NMR spectrometer has on the electrochemical systems.

Fast spatially-resolved T2 measurements with constant-gradient CPMG

Speed of acquisition is paramount for the application of magnetic resonance to flow experiments through porous rocks. One popular method for imaging core floods is the spatially resolved T₂ experiment which can separate fluids either by their viscosity contrast or by doping one fluid with a relaxation agent. Existing techniques for spatial-T₂ may suffer from long acquisition times and eddy currents due to the pulsing of magnetic field gradients. Here, we propose a constant gradient method for 1d spatially-resolved T₂ which embraces the speed of frequency encoding techniques and avoids eddy currents by the absence of any gradient ramps during the radio frequency (r.f.) pulse train. We provide the operating envelope for this kind of experiment, which is restricted due to the slice selectivity of the r.f. pulses in the presence of the magnetic field gradient. Additionally, we show that the effects of self-diffusion and the mixing of T₁ and T₂ contributions are manageable. As an illustration, we have applied this technique to an enhanced oil recovery experiment. The two fluid phases were tracked without any doping and with a time resolution of 40 s. In this case, the increased time resolution allowed us to observe dynamic flow phenomena such as fluid fingering and the calculation of the velocity of the fluid displacement fronts.

Microfluidics: an enabling screening technology for enhanced oil recovery (EOR)

Oil production is a critical industrial process that affects the entire world population and any improvements in its efficiency while reducing its environmental impact are of utmost societal importance. The paper reviews recent applications of microfluidics and microtechnology to study processes of oil extraction and recovery. It shows that microfluidic devices can be useful tools in investigation and visualization of such processes used in the oil & gas industry as fluid propagation, flooding, fracturing, emulsification and many others. Critical macro-scale processes that define oil extraction and recovery are controlled by the micro-scale processes based on wetting, adhesion, surface tension, colloids and other concepts of microfluidics. A growing number of research efforts demonstrates that microfluidics is becoming, albeit slowly, an accepted methodology in this area. We propose several areas of development where implementation of microfluidics may bring about deeper understanding and hence better control over the processes of oil recovery based on fluid propagation, droplet generation, wettability control. Studies of processes such as hydraulic fracturing, sand particle propagation in porous networks, high throughput screening of chemicals (for example, emulsifiers and surfactants) in microfluidic devices that simulate oil reservoirs are proposed to improve our understanding of these complicated physico-chemical systems. We also discuss why methods of additive manufacturing (3D printing) should be evaluated for quick prototyping and modification of the three-dimensional structures replicating natural oil-bearing rock formations for studies accessible to a wider audience of researchers.

Viscoelastic polymer flows and elastic turbulence in three-dimensional porous structures

Viscoelastic polymer solutions flowing through reservoir rocks have been found to improve oil displacement efficiency when the aqueous-phase shear-rate exceeds a critical value. A possible mechanism for this enhanced recovery is elastic turbulence that causes breakup and mobilization of trapped oil ganglia. Here, we apply nuclear magnetic resonance (NMR) pulsed field gradient (PFG) diffusion measurements in a novel way to detect increased motion of disconnected oil ganglia. The data are acquired directly from a three-dimensional (3D) opaque porous structure (sandstone) when viscoelastic fluctuations are expected to be present in the continuous phase. The measured increase in motion of trapped ganglia provides unequivocal evidence of fluctuations in the flowing phase in a fully complex 3D system. This work provides direct evidence of elastic turbulence in a realistic reservoir rock - a measurement that cannot be readily achieved by conventional laboratory methods. We support the NMR data with optical microscopy studies of fluctuating ganglia in simple two-dimensional (2D) microfluidic networks, with consistent apparent rheological behaviour of the aqueous phase, to provide conclusive evidence of elastic turbulence in the 3D structure and hence validate the proposed flow-fluctuation mechanism for enhanced oil recovery.