A High Field Solid-State Nuclear Magnetic Resonance Experimental Study for Clay and Shale Swelling
Authors: Alzahrani Mohammed S. , Tutuncu Azra N. , Yang Yuan 
Published: March 2015

Reservoir characterization provides one of the key factors of understanding complex shale formations. In this study, an attempt has been made to investigate the NMR response for pure clays in order to eventually use NMR response in developing permeability and porosity correlations for shale formations. The methodology in the study involves using the application of high field solid-state NMR (400 MHz) and incorporating the chemistry of fluids knowledge in saturated clay systems. The high NMR application has the capability to investigate the interlayer and inter-particle spaces in pure clays and detect the change in the structure of clays in different systems through the analysis of the silicon spectra.

The expansion mechanisms of clays have been studied extensively and several theories have been developed over the years. The results of this study have been tied to these mechanisms in different clay/fluid systems to comprehend how the associated interlayer or inter-particle porosity affect the total volume change in the formation. The results obtained in our research suggest that the water is adsorbed on the external and internal layers of the clay at the same time, yet at different rates. Under the interaction with highly saline solution, there is a noticably slower diffusion into interlayer space compared to the low salinity or de-ionized water confirmed by the behavior of surface to volume ratios at different porosities. This might be explained as a result of the strong double layer effect (on external surfaces) associated with the high ion concentration. Moreover, the silicon spectra for the saturated montmorillonite showed a minor production of OH- group when exposed to the 8% KCl solution. Yet, no spectra could be detected with the 17.9% KCl suggesting a change in the structure of the clay and hence the adsorption properties of the clays surfaces.

Introduction

By reviewing the earlier research reported in the literature for clays and the NMR techniques implemented in the petroleum industry, it is evident that the NMR technique has not been widely used to describe the clay swelling behavior. Moreover, most of the reviewed NMR applications in the petroleum industry depend on the low field (1MHz or 2MHz) NMR measurements that typically could not have the capability to describe the main structural groups in the clay. The high field NMR can define the main groups in the structure, e.g. 1H, 13C, and 29Si groups. It provides information on the chemical bond breakage or formation taking place under different conditions. Thus, the utilization of the high field NMR appears to have potential benefits in this matter. Moreover, studying the swelling mechanisms and using the relaxation time measurements and distribution curves to confirm them are of high importance to build better understanding of reservoir characterization. In addition, the high field NMR has the capability to investigate the interlayer (in nano-meter) and inter-particle pore spaces (in nano-meter) in the pure clays. The measurement methods used in the industry to estimate the porosity and pore throat sizes for various formations of interest are summarized in Figure 1 (Nelson, 2009). Typically, the only method used in determining the porosity in unconventional formations with pore dimensions less that 1.0 nm is the computational chemistry method. The high field NMR application, with its high resolution and sensitivity, provides information to study the porous systems in this range.