Water sorption behaviour of gas shales: I. Role of clays

In this study, we investigate the role of clays on water sorption (adsorption and desorption) behaviour of gas shale samples. First, we conduct conventional sorption experiments on downhole shale samples by placing them in a controlled relative-humidity (RH) chamber. The full sorption cycle is obtained by step-wise increasing of RH from 0 to 1 followed by decreasing RH from 1 to 0 for obtaining adsorption and desorption isotherms, respectively. The results reveal that the samples with higher clay content have (i) higher amount of water adsorption, (ii) faster initial adsorption rate, and (iii) more water retention at the end of the desorption process (i.e. more hysteresis). We propose a modified sorption technique to account for water adsorption of clays by adding an early sorption cycle to the conventional method. In the modified process, the sample first goes through a cycle of adsorption at RH ≈ 0.215 followed by desorption at RH ≈ 0 (early sorption cycle). Then, we place the samples in a controlled-RH chamber for a full sorption experiment; which is adsorption from RH ≈ 0 to RH ≈ 1, followed by desorption to RH ≈ 0. The results of the early sorption cycle show similar trends to those we observed from the conventional sorption experiments. However, in the subsequent full sorption cycle (the modified adsorption and desorption isotherms), (i) the initial adsorption rate tends to be independent of clay content and (ii) the hysteresis is less significant for samples with higher clay content. These observations suggest that clays tend to remain saturated by water during the early sorption cycle in the modified technique. Moreover, we conduct spontaneous water imbibition experiments using deionized water to compare the total water uptake by imbibition and adsorption processes. The results indicate that for our samples, spontaneous water imbibition is always faster and yields higher water saturation compared to the water adsorption process. These observations suggest that water molecules probably cannot condense in all pores during water adsorption process. Additionally, the excess water saturation achieved by imbibition process increases by increasing the clay content. This observation suggests that water adsorption by clays (and the potential subsequent swelling and micro-fracture induction) is more prominent during imbibition process compared with that during adsorption process.