Experimental investigation on the dynamic volume changes of varied-size pores during shale hydration

Abstract

In order to improve the seepage channels in shale, soaking operation is often conducted after fracturing to carry out full shale hydration by the fracturing fluid. Previous research has mainly focused on the changes in the overall pore volume of shale due to hydration, but there is a lack of understanding on the volume changes of varied-size pores during the hydration process. In this study, a nuclear magnetic resonance (NMR) measurement method was carried out to obtain the size distribution of the pores in the shale in its unaltered state using ethanol as the saturation medium. Subsequently, through experiments, the shale core was saturated with fracturing fluid, and the dynamic changes in the pore size distribution under hydration were monitored using the low-field NMR technique. In addition, the relationships between the type of clay, the salinity of the fracturing fluid and the volume changes of varied-size pore were analyzed. The results of this study indicate that when the shale and fracturing fluid underwent hydration, the increase in the volumes of varied-size pores was concentrated in a certain pore-size area. This pore -size area was related to the type of clay that the shale contained. There was a strong correlation between the mixed-layer illite-smectite content and the increase in the small meso-pore volume. And the same between the illite content and the increase in the micro-pore volume. The saline fracturing fluid had an inhibitory effect on the shale hydration, mainly in the micro-pore domain. The results of this study is significant to optimize the selection of shale gas reservoirs and fracturing fluids for use in soaking measures.