Effect of clay–water interactions on clay swelling by X-ray diffraction

Clay swelling is a very common phenomenon observed in sedimentary rocks and soils. It is one of the major causes for permeability reduction in hydrocarbon reservoirs. The aim of this work is to characterize the swelling behavior of bentonite and of two clays from Brazilian petrolific basins. X-ray diffraction (XRD) was used to quantify the swelling characteristics of these clays under contact with salt solutions, as in the reservoir. The measurement of interplanar spacing of the swollen clay was measured 24 h after the preparation of the paste. Their diffractograms displayed well-defined peaks at low angles, even for (0 0 1) d-spacing values larger than 50 إ. Two types of mechanisms for clay swelling were identified: crystalline swelling (limited d-spacing increase) and osmotic swelling (large d-spacing values). The critical salt concentration (CSC) value, i.e., the salt concentration at which clay changes from crystalline to osmotic swelling, was obtained. The method used in this work allows one to determine the compatibility between reservoir clays and foreign fluids and to avoid the osmotic swelling conditions that can cause permeability reduction in natural hydrocarbon reservoirs.