Integration of Thermal and Solvent Extraction Methods for Improved Characterization of Hydrocarbon in Place and Producibility

Dean Stark and Retort are solvent and thermal fluid extraction methods used for evaluating the water and oil content in unconventional reservoir rocks and for calculating their porosity and pore fluid saturations. Unfortunately, they provide different answers and these differences lead to different interpretations of reservoir quality, hydrocarbon in place, and producibility. In this paper we show that the two methods are accurate and correct, but on their own they provide incomplete information. We show that when combined, they provide a more complete explanation of the distribution of water and oil in the rock, including the separation of loosely bound and strongly bound water in the clay system, the water incorporated into the clay structure, and the various hydrocarbon constituents present (light, intermediate, and high molecular weight). This separation of hydrocarbon constituents provides a first pass estimation of their mobility. We show that the above measurements are consistent with RockEval measurements, Thermogravimetric Analysis, and with other solvent extraction methods. We also show that NMR measurements provide an independent confirmation of the liquid-filled pore saturations (oil and water). NMR also provides independent information on the solid/liquid-surface interactions that reduce the mobility of hydrocarbons compared with intrinsic mobility based on molecular weight. In this paper we describe the workflow for integrating the various thermal and solvent extraction methods and provide examples from the Wolfcamp formation. Results show that once the various equipment are calibrated properly, thermal extraction and solvent extraction methods provide identical measurements of water content. Results also show that once the process of hydrocarbon evaporation and distillation as a function of time and temperature is understood, the evaluation of the hydrocarbons obtained from solvent extraction and thermal extraction are consistent, and can be combined to provide a better quantification of the hydrocarbon constituents (light, intermediate, and high molecular weight). These are also consistent with RockEval results. Hydrocarbon fractionation was verified with GC analysis of the thermally extracted constituents at various temperatures. Results of the analysis also provide a measure of the solid organic carbon, kerogen, and coke in the rock. Understanding the distributions of mobile and less mobile hydrocarbons as a function of depth should be considered in the analysis of reservoir quality and is an additional variable for the selection of the optimal lateral landing depths.