Over the last decade, multiple studies have outlined the challenges of adapting core analyses to unconventional rocks, from challenges in measuring and modeling very small, high surface area pores to round robin discrepancies among laboratories. In addition, multiple authors have demonstrated the challenges characterizing mobility in nanoporous rocks: including the scale and variability of the pore systems, the change in fluid composition as it reequilibrates to surface conditions, and uncertainties in wettability states. This study uses several different parallel and multiscale analyses to characterize the relationships between total porosity, pore size distribution, and fluid mobility in a broad variety of West Texas samples.
This study is designed to compare current core analyses for nanoporous rocks, to identify strengths and limitations of each analysis, and to design appropriate upscaling workflows to compare results from these analyses. We examined 50+ samples from West Texas with a broad range in mineralogy and organic matter content. The integration of SEM, N2 adsorption, NMR, GRI, HPMI, and thin section data on each of these samples enabled an understanding of pore size distributions, pore types and connectivity and inferred wettability. Results from the analytical comparisons - both where they agree and disagree - reveal insights into unconventional pore systems. We find that total porosity is well constrained by a tight agreement (± 1 p.u.) between crushed rock helium porosimetry and plug NMR, confirming the validity of both methodologies for measuring total pore volume. Low-field NMR relaxation measurements at several laboratory-controlled liquid saturation states assist in understanding potential liquid volumes at reservoir conditions and the nature of the liquid-pore wall interactions that reflect wettability behavior. High resolution (SEM) imaging is used to calibrate the distribution of pores and organic matter at multiple scales. The wettability state of organic-rich low-permeability unconventional reservoir samples is unclear since many samples spontaneously imbibe both water and light oil. While this may signify a neutral wetting state, another interpretation is that two pore systems, one water-wet and the other oil-wet, reside adjacent to each other in these rocks. This study focuses on whole rock samples rather than mineral/ organic isolates and employs parallel and multiscale analyses to pinpoint how each method informs aspects of reservoir quality. Take-aways include a stronger understanding of analytical capabilities and upscaling in organic-rich unconventional reservoirs.
The holy grail of reservoir characterization in unconventional, tight-rock (less than 20 microdarcy) plays is the ability to tie well production behavior to subsurface properties. Characterization challenges can exist from all angles - from decisions on the appropriateness of an analytic procedure, to employing proper upscaling, to ensuring apples-to-apples comparison of good quality production data. Regardless, a firm base understanding of the rock matrix and its properties is necessary prior to integration with well behavior. This study focuses on determining what portion of the pore system each core analysis method is measuring and with what precision and accuracy these analyses can be integrated together. When it comes to tight rocks, as an industry, we temporarily moved away from the core as strictly "ground truth" for well log calibration and rock characterization. At first, this allowance for wiggle room was with good reason: past round robin studies exposed large discrepancies in porosity, permeability, and saturations among vendors (Clarkson et al, 2012; Passey et al, 2010; Sondergeld et al, 2010; Luffel and Guidry, 1992; Luffel et al, 1993). However, Bohacs et al (2013) reported that vendor to vendor comparisons in crushed porosity improved to now be within ± 1 pu of each other. More recently, Blount et al (2017) confirmed this finding with a round robin study where crushed-rock porosity values from commercial laboratories fell within ± 0.05 pu for Dean Stark and ± 1 pu for retort.
