ZTE Characterization of Fracture Connectivity in Porous Media

Abstract:

Characterization of porous space is necessary to understand transport properties in tight hydrocarbon reservoirs. Techniques such as micro-tomography (µCT) and nuclear magnetic resonance (NMR) methods have been used to this end for decades. Magnetic Resonance Imaging (MRI) offers direct visualization of pore architecture through a vast number of pulse sequences, which is unavailable through traditional methods. In this work, the MRI pulse sequence Zero Echo Time (ZTE) was tested in the study of sandstone and carbonate samples, including induced fractures. ZTE has been used previously to image highly porous geological material with relative success due to the small flip angle (5°) and early application of the gradient system. In this research, ZTE was used to study porous systems tighter than those previously reported. We show that ZTE can resolve a combination of fractures and pores simultaneously. Pore Cluster Analysis (PCA), combined with ZTE, can be used to analyze connectivity of relatively large rock sample volumes. Results show that sandstone pore bodies should be resolvable using MRI, but not pore throats. In the Indiana Limestone case, most of the large pores should also be discernible. We additionally show that inducing fractures increases pore connectivity. Lastly, we show that the Madison Limestone sample bulk porosity lies in the large pores, which should be resolved with MRI. This limestone pore-space connectivity is increased by fracture induction. By combining Time-Domain NMR pore-size analysis and PCA, we show that careful selection of resolution is necessary to understand transport in porous media. Therefore, dynamic and transport properties can be studied in relatively tight porous media.