Transverse relaxation in sandstones due to the effect of internal field gradients and characterizing the pore structure of vuggy carbonates using NMR and Tracer analysis

[摘要] Nuclear magnetic resonance (NMR) has become an indispensable tool in petroleum industry for formation evaluation. This dissertation addresses two problems.• We aim at developing a theory to better understand the phenomena oftransverse relaxation in the presence of internal field gradients.• Chracterizing the pore structure of vuggy carbonates.We have developed a two dimensional model to study a system of claylined pore. We have identified three distinct relaxation regimes. The interplay of three time parameters characterize the transverse relaxation in three different regimes. In future work, useful geometric information can be extracted from from SEM imagesand the pore size distribution analysis of North Burbank sandstone to simulate transverse relaxation using our 2-D clay flake model and study diffusional coupling in the presence of internal field gradients.Carbonates reservoirs exhibit complex pore structure with micropores and macropores/vugs. Vuggy pore space can be divided into separate-vugs and touching-vugs, depending on vug interconnection. Separate vugs are connected only through interparticle pore networks and do not contribute to permeability.Touching vugs are independent of rock fabric and form an interconnected poresystem enhancing the permeability. Accurate characterization of pore structure of carbonate reservoirs is essential for design and implementation of enhancedoil recovery processes. However, characterizing pore structure in carbonates is a complex task due to the diverse variety of pore types seen in carbonates and extreme pore level heterogeneity. The carbonate samples which are focus of thisstudy are very heterogeneous in pore structures. Some of the sample rocks are breccia and other samples are fractured. In order to characterize the pore size invuggy carbonates, we use NMR along with tracer analysis. The distribution of porosity between micro and macro-porosity can be measured by NMR. However, NMR cannot predict if different sized vugs are connected or isolated. Tracer analysis is used to characterize the connectivity of the vug system and matrix.Modified version of differential capacitance model of Coats and Smith (1964) and a solution procedure developed by Baker (1975) is used to study dispersion andcapacitance effects in core-samples. The model has three dimensionless groups:1) flowing fraction (f), 2) dimensionless group for mass transfer (NM) characterizingthe mass transfer between flowing and stagnant phase and 3) dimensionless group for dispersion (NK) characterizing the extent of dispersion. In order to obtainunique set of model parameters from experimental data, we have developed an algorithm which uses effluent concentration data at two different flow rates toobtain the fitted parameter for both cases simultaneously. Tracer analysis givesvaluable insight on fraction of dead-end pores and dispersion and mass transfereffects at core scale. This can be used to model the flow of surfactant solutionthrough vuggy and fractured carbonates to evaluate the loss of surfactant due todynamic adsorption.

[发布机构] Rice University

[效力级别] Vuggy