Numerous comprehensive well logging suites have been deployed in recent years to assess unconventional oil and gas reservoirs. These technologies produce extensive and rich data sets, that while powerful, can introduce significant technical challenges during integration into formation evaluation workflows. A key challenge arises when integrating advanced dielectric and geochemical spectroscopy logs, each of which yields independent outputs for water saturation and salinity through distinct measurement physics. Present workflows designed to reconcile these and other quantities do not rigorously and consistently incorporate known measurement sensitivities and uncertainties, and can vary between log analysts. In unconventional reservoirs, they also rely on user inputs of kerogen thermal maturity and hydrocarbon properties, which can be difficult to ascertain a priori. This underscores the need for a consistent, quantitative interpretation methodology that can precisely integrate advanced logs into unconventional formation evaluation workflows.
A novel multiphysics inversion approach has been recently introduced to optimize the various formation property sensitivities of two primary advanced logs: multifrequency dielectric dispersion and geochemical spectroscopy. One of the primary limitations of standalone dielectric interpretation is its insensitivity to salinity estimation at high formation water salinity ranges, which is complemented by chlorine and formation sigma measurements in the multiphysics inversion. Additionally, nuclear magnetic resonance (NMR) porosity, when incorporated into the inversion with the spectral total organic carbon (TOC) measurement, provides unique sensitivities for constraining kerogen and hydrocarbon properties. The multiphysics inversion method quantitatively accounts for these differences in physical sensitivities, as well as their measurement uncertainties, in the solutions for water volume, saturation, formation water salinity, and kerogen and hydrocarbon densities. Combined with advanced mineralogy and porosity evaluations from the logging suites, this methodology produces unified, optimized outputs for advanced well log interpretation while maximizing the extracted value of the acquired advanced measurement suite.
We present case studies as a first demonstration of the new multiphysics inversion technique, integrated into complete petrophysical workflows, for utilizing multifrequency dielectric, geochemical spectroscopy, and NMR logging tools in unconventional reservoir evaluation. We assess reservoirs in the Permian Basin of the United States, including logged intervals over the organic shale pore-fluid systems of the Wolfcamp formation and in the Vaca Muerta Formation, Neuquén Basin of Argentina. Through these studies, we demonstrate how the standardized multiphysics inversion approach supersedes individualized advanced log reconciliation and minimizes uncertainties in their integration. We provide comparisons to stand-alone interpretations of each advanced log to illustrate the benefits of a simultaneous inversion with discussions of the limitations presented by each approach. Validation of formation water salinity and kerogen properties is performed against available published local data. Finally, we discuss the conditions under which these workflows can be logically extended to conventional reservoir evaluation and Carbon Capture and Storage (CCS) applications, broadening their applicability beyond unconventional hydrocarbon reservoirs.
