Combined interpretation of NMR and TGA measurements to quantify the impact of relative humidity on hydration of clay minerals

Abstract

Reliability of core measurement and analysis can be highly dependent on the humidity of measurement environment. However, impact of humidity on core measurements is often assumed to be negligible. This paper quantified the influence of relative humidity on the hydration of clay minerals (i.e., montmorillonite, illite, chlorite, and kaolinite) using nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). The influence of clay hydration on the assessment of fluid volume in organic-rich mudrocks was also investigated. The NMR T2 (spin-spin relaxation) magnetic decay of the clay samples showed existence of water even after the samples were dried in an oven at 95°C. The TGA measurements in clay minerals confirmed the weight loss between 95°C and dehydroxylation temperature, which is approximately the same volume as that recorded by NMR measurement in dried samples. This recorded water volume corresponded to desorption of water molecules bound to interlayer cations (i.e., interlayer water). In the case of the hydration water, the NMR-based water vapor adsorption experiments showed that the volume of water accumulated on the surface of clay minerals increased significantly as the relative humidity increased. The cation exchange capacity (CEC) measurements of clay minerals demonstrated that the volume of hydration water was strongly related to the CEC. The volume of interlayer water was, however, not related to the CEC. In the case of organic-rich mudrocks, water vapor adsorption experiments showed >250% change in water volume when relative humidity varies in the range of 0% to 95%, which further verified the significant impact of humidity on core measurements and the necessity of applying corrections for such an impact.