The Reservoir Producibility Index: a Metric to Assess Reservoir Quality in Tight-Oil Plays from Logs

摘要

Methods for formation evaluation in unconventional reservoirs are evolving quickly as improved logging methodologies are developed. In gas-producing shale reservoirs, reservoir quality (RQ) is strongly correlated to the total organic carbon (TOC) content, because the kerogen, composed largely of organic carbon, hosts the pore system necessary for storing and transporting gas. In tight-oil formations, however, TOC alone is not as strongly correlated to RQ because storage and transport through inorganic pores can also be significant. Instead, the volume of oil the only producible organic component in tight oil plays is considered a dominant factor impacting RQ, whereas the immobile organic components (kerogen and bitumen) are typically neglected. It has also been argued that immobile components are not inconsequential, but are actually negative RQ indicators in tight-oil plays because they can adsorb oil, swell, and clog pore throats. Indeed, methods to assess RQ in tight-oil formations based on cuttings and core analysis, where the oil content is considered a positive RQ indicator and the immobile organic content is considered a negative RQ indicator, have been proven valuable in many basins, such as the Williston, where the organic-lean middle member of the Bakken formation and Three Forks Formation are generally completed instead of the organic-rich upper and lower members of the Bakken Formation. However, surface measurements alone are inherently limited because the oil content of cuttings and cores from tight-oil formations may be unrepresentative of reservoir conditions due to core alteration, evaporative losses, etc. Here we present a new metric for evaluating RQ in tight-oil formations, the Reservoir Productivity Index (RPI). The RPI accounts for the positive RQ properties of oil and the negative RQ properties of immobile organic carbon, and it avoids challenges regarding representativeness of surface analysis because the oil content and other measurements are based on in-situ logs, principally nuclear magnetic resonance (NMR) and nuclear spectroscopy. A term representing richness is included in the RPI, making a single metric that captures many of the factors describing tight-oil RQ without requiring extensive log interpretation. Several examples of the use of the RPI for evaluating RQ in tight oil plays are shown.