The contact angle of water on feldspar in a reservoir rock

Abstract

The wettability of a reservoir rock plays an essential role in affecting the distribution and transportation of fluids (water, oil, gas, etc.) in reservoir pores that are constructed with micro-sized mineral particles. To study the wettability of a reservoir rock, several quantitative and qualitative methods have been developed, among which the Amott method, the USBM method and the contact angle method are most frequently employed. As limited by the micro-size of minerals in reservoir rock, the methods mentioned above are all difficult to be applied to evaluating the wettability of a specific mineral in an actual reservoir rock. Recently, atomic force microscopy (AFM) has been used to measure the contact angles of water droplets on quartz in a sandstone reservoir rock. However, the wettability of feldspar, one of the major minerals of reservoir rock, has not been studied yet at the pore scale. In this study, the contact angles of micro-sized water droplets on both K-feldspar and Na-feldspar in a reservoir rock, which was taken from a sandstone core in Ordos Basin, China, were measured with AFM under tapping mode. Before measurement, the slice was simply mechanically polished, cleaned with deionized water, and then dried in nitrogen of high purity. In order to create micro-sized water droplets on the feldspar surfaces, a large droplet of around 2 mm in diameter was first gently generated, and then the slice was immediately moved into the chamber of the AFM. Inside the chamber a certain amount of water was sprayed around the test rock slice to get the chamber to achieve the equilibrium state of evaporation and condensation of water. Generally it took about 12 h to reach the equilibrium state, when no significant changes in droplet size and contact angle were observed in ~10 min. As the result of evaporation, some micro- sized water droplets emerged on feldspar surfaces. The AFM results obtained show that the micro-sized droplets are asymmetrical so that the contact angles, which are in the range of 24.3°−38.6° and 29.5°−39.1° for water on K-feldspar and Na-feldspar, are different at different points along the triple-phase contact lines, possibly caused by the roughness and heterogeneity of the surface and the difference in crystal face, etc. As compared with previous studies the current results indicate that the average contact angles of water on both K-feldspar and Na-feldspar are smaller than those for water on quartz (~40°), implying that quartz cannot be simply applied to represent all minerals of a reservoir rock for wettability evaluation. The current results of contact angles of water on K-feldspar and Na-feldspar are about 10° smaller than those obtained on crushed pure feldspars by indirect thin-layer wicking method in reference studies. Because the current investigations were conducted on an actual reservoir rock, these results might better describe the wettability of feldspars in reservoir.