Hydration State of Montmorillonites and its Effect on Swelling Behavior

Abstract Swelling of montmorillonites (MMTs) causes many complications over a well's life cycle. Previous X-ray diffraction (XRD) studies with controlled air humidity revealed the stepwise crystalline swelling of MMTs. However, the effect of the initial hydration state of MMTs on the swelling kinetics remains unclear. The objective of this work is to address these gaps in understanding and provide a basis for designing prospective shale chemical inhibitors for drilling, completion, or stimulation fluids. Natural montmorillonite samples—MMT-I and MMT-II—were used for this study. Using these materials instead of shales minimizes the impact of mineral impurities, structural features, compaction history, and rock mechanics. The samples were characterized for mineral composition using XRD, elemental composition using X-ray fluorescence (XRF), and cation exchange capacity (CEC) using the methylene blue test. Sodium (Na) is the dominant exchangeable cation in both MMTs. However, up to 0.1 atoms of calcium (Ca) per structural unit and trace amounts of magnesium (Mg) and iron (Fe) are also present. MMT-I and MMT-II have a CEC of 82 and 104 meq/100g, respectively. To prepare for swelling tests, the initial water content in samples was adjusted by equilibration at air humidities of 49–98% RH for up to 33 days. Water state and content in each sample were evaluated by XRD and thermogravimetry. Swelling kinetics in 8 wt% potassium chloride (KCl) solution was measured using a linear swell meter (LSM) at 150°F and 100 psi. Equilibrated at the same air humidity, MMT-II contains up to 55.7% more water and exhibits a higher degree of crystalline swelling than MMT-I. MMT-II with an initial water content of 62.7 mg H2O/g clay at Aw = 0.56 swells 64.3%. However, the same MMT prehydrated with 221.4 mg H2O/g clay at Aw = 0.94 swells only 26.0%. Thus, experiments using the ambient-equilibrated (dried) laboratory samples overestimate the degree of swelling. To aid in the interpretation of the swelling mechanisms, this work presents the correlations of initial water content and d(001)-spacing between the 2:1 phyllosilicate layers of MMTs. The dramatic impact of the initial hydration state on swelling behavior indicates that the performance of different chemicals should be compared at similar sample hydration states. Understanding the swelling behavior of montmorillonites across a wide range of water contents provides a basis for determining the efficiency of shale inhibitors.