Shale hydration and swelling are major hindrances for using water-based drilling fluids to develop shale gas. Quantitative investigation of the hydration behavior of clay minerals is helpful to further develop high-performance inhibitors to use with water-based drilling fluids. In this work, the hydration behavior of sodium montmorillonite (Na-MMT) was investigated by isothermal adsorption, X-ray diffraction, and thermogravimetric analysis (TGA) with microporous crucible technology and low-field nuclear magnetic resonance (LF-NMR). The adsorbed water content and the basal spacing of Na-MMT were found to be a function of relative humidity. The adsorbed water on Na-MMT of the first step by TGA was the cationic interlayer water. The adsorbed water of the second step was the interlayer surface water. The adsorbed water of the third step was the sum of the part of interlayer surface water and free water. The experimental results of the cationic interlayer water and interlayer surface water measured using LF-NMR were consistent with the TGA results. According to the above results, it was inferred that if the cationic interlayer water and interlayer surface water can be removed, the surface hydration of Na-MMT could be completely inhibited.

Shale gas as green and clean energy is attracting more interest. However, shale hydration and swelling have become the major challenges using water‐based drilling fluids, replacing oil‐based drilling fluids, for developing shale gas. In this work, a low molecular weight branched polyamine (NETS) as clay swelling inhibitor was synthesized. The NETS performed excellent inhibition property by linear expansion experiments and cuttings hot‐rolling recovery. The inhibition mechanism of NETS was investigated using X‐ray diffraction, fourier transform infrared spectroscopy, cation exchange capacity, X‐ray photoelectron spectroscopy, scanning electron microscopy, and zeta potential. The results show that the NETS can adsorb on Na‐Mt, decrease the basal spacing of wet Na‐Mt from 1.91 to 1.43 nm, and replace majority sodium ions from interlayer of Na‐Mt. However, replaceability of sodium ions and inhibition of water molecules of ethylenediamine are worse than NETS. These indicate that the primary amines of NETS had strong interaction with Na‐Mt, tensed clay layers and decrease the basal spacing. The combination of long hydrophobic chain and primary amines makes NETS to adsorb on surface of Na‐Mt, intercalate into interlayer of Na‐Mt with tilted monolayer arrangement, and replace sodium ions. The more the number of primary amine groups, the better the inhibition performance. Therefore, NETS has a great potential to be an outstanding shale inhibitor of water‐based drilling fluids in the future.

Shale hydration and swelling are the major problems in using water-based drilling fluids in developing shale gas. In this work, the intercalation inhibiting mechanism of alkyl chain length on the swelling of sodium montmorillonite (Na-MMT) has been innovatively investigated using isothermal adsorption, adsorption kinetics, scanning electron microscopy, X-ray diffraction, cation exchange capacity, elemental analysis, and X-ray photoelectron spectroscopy techniques. The results indicate that an increase in the alkyl chain length of the inhibitor reduces the saturated adsorption capacity of the inhibitor on Na-MMT accordingly, thus reducing the dosage of the inhibitor. The adsorption rate constant increased rapidly when the length of the alkyl chain was increased, which also reduced the degree of hydration of the clay minerals and improved the inhibitory potential. When a flat-lying monolayer of alkyl chain containing four to seven carbon atoms was inserted, the d-spacing of Na-MMT was minimized compared to the hydrated Na-MMT, which in turn replaced the sodium ions in the interlayer space of Na-MMT. The inhibitor was strongly attached in the Na-MMT interlayer between the silicon–oxygen tetrahedron and the primary amine groups. Finally, the water molecules were expelled from the interlayer of Na-MMT. Therefore, the alkyl chain length inhibitors should be considered to design a better class of inhibitors when using the water-based drilling fluid system.