Shale Gas Characterization and Property Determination by Digital Rock Physics

Abstract

Abstract Unconventional shale reservoirs differ largely from conventional sandstone and carbonate reservoirs in their origin, geologic evolution and current occurrence. Shale is a wide variety of rocks that are composed of extremely fine-grained particles with very small porosity and permeability values in the order of few porosity units and nano-darcy range, respectively. Shale formations are very complex at the core scale, and exhibit large vertical variations in lithology and Total Organic Carbon (TOC) at a small scale that renders core characterization and sweet spot detection very challenging. Shale formations are also very complex at the nano-scale pore level where the pores have different porosity types that are detected within the kerogen volume. These complexities led to further research and development of advanced application of high resolution X-ray CT scanning on full diameter core sections to characterize shale mineralogy, porosity and rock facies so that accurate evaluation of the sweet spot locations could be made for further detailed petrophysical and petrographic studies. In this work, argillaceous shale gas cores were imaged using high resolution dual energy X-ray CT scanning. This imaging technique produces continuous whole core scans at 0.5mm spacing and derives accurate bulk density and effective atomic number (Zeff) logs along the core intervals which were crucial in determining lithology, porosity, and rock facies. Additionally, integrated XRD data and energy dispersive spectrum (EDS) analysis were acquired to confirm the mineral framework composition of the core. Smaller core plugs and subsamples representing the main variations in the core were extracted for much higher resolution X-ray CT scanning and Scanning Electron Microscopy (SEM) analysis. Porosity was mainly found in organic matter and was determined from 2D and 3D SEM images by image segmentation process. Horizontal fluid flow was only possible through the organic matter and the simulations of 3D FIB-SEM volumes by solving Stokes equation using Lattice Boltzmann Method (LBM). A clear trend was observed between porosity and permeability while correlating with identified facies in the core. Silica-rich facies gave higher Phie-K characteristics compared to the low clay-rich facies. This is mainly caused by pressure compaction effect in the soft clay-rich samples. High percentages of organic matter were not found to be good indication for high porosity or permeability in the clay-rich shale samples. The depositional facies was found to have great effect on the pore types, rock fabric and reservoir properties. The results and interpretations entailed in this study provide further insights and enhance our understanding of heterogeneity of the organic rich shale reservoir rock.