Experimental Analysis of Tight Rock Clay Cation Exchange Capacities and Modeling of Properties Critical to Reservoir Desiccation

Abstract

This study focuses on the role of clay cation exchange capacity (CEC) in natural reservoir desiccation and provides lab-measurement backed values by which CEC models in tight rocks may be independently calibrated against. Statistical models for organic maturation, CEC (XRF, XRD) data, and used for high resolution characterization shale intervals. High resolution CEC models were be calibrated against XRD/SEM measurement based estimations of clay CEC expected under modeled in-situ conditions. For estimation validation, samples of illite, smectite, chlorite, and kaolinite mixed in varying proportions, tested via XRD, XRF, imaged in SEM, and finally tested for CEC via wet chemistry. Resulting CEC values were placed in a composition vs CEC matrix for calibration of CEC models. Clay CEC and organic content are major factors in the response of a reservoir to fluids. A critical relationship, with potential solutions for the rapid decline curves commonly observed in unconventional wells. Consequently, understanding the mechanisms controlling natural fluid movement in unconventional tight reservoir is a critical economic issue. Our current hypothesis is that measured clay CEC will be significantly less than calculated values, due to the surface area dependent nature of cation exchange. CEC calculations often ignore grain size and morphology, resulting in models that do not respect real-world grain geometries, and pore-fluid interactions which may shift exchange values. Consequently, we also hypothesize that water saturation is proportional to CEC in tight rocks. Higher clay CEC values being associated with elevated water saturations. Unpublished preliminary statistical work has suggested that although the proposed relationship may not be 1:1, correlation is strong. Finally, we hypothesize that clay CEC, porosity, and organic maturity are the greatest influences on the effectiveness of natural reservoir desiccation.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019