Estimation of Total Hydrocarbon in the Presence of Capillary Condensation for Unconventional Shale Reservoirs

Abstract

Abstract Accurate reserve estimates for unconventional shale gas reservoirs are critical to their economic assessment and field planning. Reserves are typically assessed with log and core measurements. Much of the log-derived information is validated with cuttings and core data. In particular, adsorption isotherms are measured in the laboratory and used to estimate ultimate recovery. Accurate shale gas reserve estimations require a storage model that incorporates the physics of small scale, i.e., nanopores. This model is critical for the interpretation of log data and the prediction of long-term production and ultimate recovery. Current models partition gas into two parts: gas adsorbed on the pore surface with density comparable to liquid and free gas in the shale rock pores and natural fractures. Such a storage model, however, fails to consider the extremely favorable conditions for capillary condensation in the kerogen-rich rocks: pores in the kerogen range from a few nanometers to a few hundred nanometers and the pore surfaces are strongly oil wet. These two conditions are highly favorable for capillary condensation to occur; consequently, a significant amount of hydrocarbon probably exists in kerogen pores in the liquid state. Capillary condensation in shale gas core plugs has been verified in a laboratory study and significantly more hydrocarbons were observed than one would have expected in a traditional model that does not include capillary condensation. We propose a new shale gas storage model for organic-rich shale rocks that accounts for capillary condensation. In this model part of the ‘gas’ exists in liquid phase inside kerogen pores. Capillary condensation is easy to model for single-component hydrocarbons. Under reservoir conditions, the hydrocarbons are usually a mixture of multiple components. This paper presents the method and theoretical result for estimating the total hydrocarbon volume for hydrocarbon mixtures in organic rich shales in the presence of capillary condensation.