Experimental study and isotherm models of water vapor adsorption in shale rocks

The understanding of water vapor adsorption and equilibrium in the low permeability matrix of gas shale is crucial for predicting and optimizing gas productivity in shale gas reservoirs. In this study, water vapor adsorption isotherms for gas shale samples from the Lower Silurian Longmaxi Formation in Southern China were measured gravimetrically at two temperatures (30 °C and 50 °C) under the relative humidity ranging from 11.1% to 97.0%, and four different isotherm models were used to fit the experimental data and to analyze water vapor adsorption on shale rocks. The experimental results showed that water vapor adsorption for shale rocks followed a typeⅡsigmoid shape over the humidity range. At the lower humidity range, the monolayer-multilayer adsorption was the dominant process while capillary condensation and temperature effects became significant with relative humidity increasing. As the amount of total organic carbon increases, water adsorption weakens while calcite has an inhibitory effect. Through quantifying the average relative error (ARE), coefficient of determination (R2) and chi-square (χ2) of the isotherm models relative to data, the GAB isotherm model was identified to be the best-fitting isotherm to describe the water adsorption process in shale rocks. Moreover, the FHH plot was used to analyze and distinguish the states of water retention by adsorption and capillary condensation.