The impact of paramagnetic-rich clay distribution on NMR measurements

Fundamental porous media properties such as effective porosity, permeability and saturation can be greatly affected by the clay content. This is particularly important in the case paramagnetic-rich clays such as chlorite which is very common in subsurface reservoirs. In addition to clay content and mineralogy, determining clay distribution patterns is essential for formation evaluation. There are two main patterns of clay distribution relevant to this research: structural and dispersed. In the dispersed case, clay is distributed between the grains filling the inter-granular pore space which cause a significant reduction in pore sizes and permeability. On the other hand, structural clay is defined as an integral part of the grain framework and its effect on reservoir quality is negligible since it does not fill the inter-granular pores. The most commonly used wireline logging tool for downhole estimation of pore size and permeability is Nuclear Magnetic Resonance (NMR). Earlier studies showed that the content of paramagnetic-rich clay (such as chlorite) can significantly impact the estimation of pore size and permeability derived from NMR tool. Nevertheless, the impact of paramagnetic-rich clay distribution patterns on NMR measurements and the derived petrophysical properties is not well understood yet. Current NMR-derived permeability models do not take into account variations in clay distribution which may cause significant errors in the estimation of permeability. Therefore, this proposed project aims to investigate the impact of paramagnetic-rich clay distribution patterns on the NMR signal. In doing so, this work will particularly address the following research questions: 1) how the paramagnetic-rich clay distribution (dispersed versus structural) affect the T2 relaxation times and internal magnetic field gradient?; 2) how NMR measurements might be utilized for the determination of the clay distribution patterns?, and 3) how clay distribution impacts the pore size and permeability estimates derived from NMR T2 relaxation data? The method proposed in this research involves the use of well controlled experimental measurements, and numerical simulation. In the experimental part, we will prepare various well controlled clay-glass beads samples where we can control all parameters (such as clay content and minerology) and vary only the clay distribution. The laboratory NMR measurements on such samples can provide an important insight about how clay distribution impact T2 measurements and complicate the derived permeability values. Additionally, experiment results can provide important input parameters and benchmarking for the numerical simulation through which various scenarios can be tested. The outcomes of this proposed work can improve the interpretation of NMR data obtained from laboratory or downhole tools. This has direct implications for improving the NMR-derived estimation of pore sizes and permeability for subsurface water or hydrocarbon reservoirs. Moreover, developing a technique to determine the pattern of clay distribution at any given clay volume can inform the correction and interpretation of other petrophyscial properties such as fluid saturation. The expected results and significance described here can lead to more accurate evaluation of subsurface reservoir quality, volume and production which are relevant to various fields and applications including oil and gas industry, CO2 sequestration and water resources evaluation.