Knowledge of water-rock interactions on the surface of fractures is important to develop an understanding of the geological structures and changes within the formation, to determine the water pollutants in produced water during hydraulic fracturing, and to manage effectively the produced waters. To obtain this knowledge, the water-shale interaction is investigated on carbonate-rich (Eagle Ford), organic-rich (Green River), clay-rich (Barnett), and other minerals rich (Marcellus) shale samples.
Crushed shale samples were exposed to water for three weeks at reservoir conditions. The water and rock samples before and after each static experiment subjected to several analyses. The change in the rock mineralogy was defined with XRD, the elemental composition of rock was determined with XPS and SEM-EDS. The water was analyzed for its anions and cations, total dissolved solids (TDS), conductivity, pH, total organic carbon (TOC), and average particle sizes of colloids. The stability of the colloids are characterized with Zeta potential.
It has been observed that Barnett rock is high in illite content, the greatest calcite concentration is determined for Eagle Ford rock. The sulfate source of water correlates with the atomic percent of the sulfur and oxygen elements determined through XPS analyses. The magnesium content of water correlates mainly with illite amount in the rock and calcium concentration associates with the calcite and gypsum content of the rock samples. The greatest dissolution rate belongs to the minerals which yields sulfate in the water, then, gypsum and calcite which yield calcium cation in the water come second, and the lowest dissolution rates obtained from the magnesium containing minerals mainly dolomite. TDS of the water samples shows that Green River has the least tendency to interact with water and Barnett has the greatest tendency. Zeta potential values indicate that the particles in the water interacted with Eagle Ford have the highest tendency for precipitation, since it has the lowest zeta potential value.
This study provides correlations on water-rock interaction for four different shale samples. These correlations can be used to understand the water-rock interactions for different rocks by conducting simple XRD and XPS measurements only on rock samples.