Effects of pH on the quality of the clay cap in geothermal reservoirs

Areas with geothermal potential have been widely explored in more than 20 countries and with possibility of expansion as it is considered clean energy. Successful explorations in Indonesia, New Zealand and the Philippines have led to exploitation, where reservoir water is pumped up for the conversion into energy. Hereby reservoir water is stripped of dissolved salts, before the conversion and pumped back into the reservoir with different pH and dissolved ion content. For longer periods of time, the clay cap in the geothermal reservoir has been in equilibrium with the geothermal fluids. Is the change in chemical composition of the geothermal fluids, due to extensive exploitation, detrimental for the quality of the clay cap? In order to answer this question, montmorillonite and kaolinite, common clay cap minerals were investigated under different conditions. For this research an autoclave was employed with the clay minerals in solution at 230 bar, 250ºC, typical reservoir conditions. To speed up the process of clay conversion, extreme conditions were employed: the pH of geothermal fluid was varied between 0 and 14 and the chemical composition was varied between ultrapure water and 1M NaCl. The clay minerals and the consequent alterations were investigated with XRD and ASD. The XRD diffractograms give information about the mineralogical conversion within the clay mineral under investigation. The ASD spectra give extra information about crystallinity of the clay minerals and the amorphous fraction created due to the extreme reservoir conditions.