Petrophysical Considerations in Evaluating and Producing Shale Gas Resources

Abstract

AbstractWe present a practical assessment of petrophysical properties of shales and their measurement in the lab and via logs. Gasbearing shale present unique measurement challenges due to their ultra-low permeability and complicated pore volume connectivity. The combination of low intrinsic permeability and gas sorption effects renders these reservoirs "unconventional". Advances in horizontal drilling and hydraulic stimulation have transformed gas-shale resources into economic reserves. Given their economic significance, there is a strong drive to understand gas shale petrophysical property measurements, both in the laboratory and in the subsurface. We note that various core analysis protocols are used in different laboratories leading to physical property measurements that are inconsistent, even when measured on identical sample sets. In addition, log analysis of kerogen-rich shale is ‘unconventional’ compared to classical techniques used in tight gas sands.As shale gas evaluation is becoming widely practiced among service companies and operators, we will focus on three reservoir assessment categories: storage capacity (gas-in-place), flow capacity (gas deliverability) and mechanical properties impacting hydraulic stimulation. Within each of these categories we have identified influential petrophysical properties such as rock composition, total organic carbon (TOC) content, porosity, saturation, permeability and mechanical properties. Specifically, we demonstrate the importance of estimating accurate mineral and kerogen content as these properties directly impact rock quality, hydraulic fracturing protocols, and gas-in-place estimations. In reviewing these practices, we also will show the need and possible direction of new technologies that will be required for making evaluations more accurate and quantitative in the future.