Laboratory Measurement of Total Porosity and Fluid Saturations for Unconventional Tight Rocks: Methodologies, Challenges, and Comparison

Core-to-log calibration is critical in building petrophysical models for formation evaluation. Despite much research and progress over the past decades, laboratory characterization of unconventional tight rocks is still challenging. Total porosity and fluid saturations obtained by different core analysis methods (crushed rock analysis (CRA) or Gas Research Institute (GRI)/retort/nuclear magnetic resonance (NMR)) may vary even after the best practices have been applied to these measurements. This is due to the nature of the tests and assumptions/uncertainties associated with each technique. Operators can better evaluate the core data and select the more efficient testing methods while being cost effective when the differences in applications and limitations of these techniques are understood. This paper will discuss the methodologies and uncertainties of several common laboratory techniques (CRA/retort/NMR) for measuring total porosity and fluid saturations for unconventional tight rocks. This will be accomplished through the result review of an integrated study conducted on core samples from the Bakken petroleum system, including Upper and Lower Bakken Shale, Middle Bakken, and Three Forks Formations. Results will be divided into two groups: shale and non-shale. Implications for data variances in fluid types, volumetrics, and mobility among these techniques will be reviewed. Comparisons focused on differences in as-received vs. extracted state analyses are also addressed. Other geochemical and mineralogical analyses such as total organic carbon (TOC), X-ray diffraction (XRD), and rock characterization using mercury injection capillary pressure (MICP) are adopted to aid in data interpretation. In addition to low porosity and low permeability, the ultrahigh salinity of Bakken Formation water also poses challenges to measurement and data processing. Results highlight the in-situ fluid saturation variations that may or may not be reproducible in the laboratory environment. Applying integrated workflows can help assure data quality and improve data accuracy.