Effect of fluid uptake on the micro and meso-pores of shale rocks at different pH

Abstract:

Stimulation fluids for hydrocarbon formations have evolved over time. Their properties and composition are dictated by reservoir characteristics and operational requirements. The addition of certain additives such as crosslinkers, polymers, and surfactants, among others, have been of interest due to their apparent benefits. However, other characteristics, more related to fluid chemistry, i.e. salinity, ionic strength and pH have proven to impact the results of the stimulation process. pH is a particularly important variable, because fluids used in fracking processes have gone from acidic to alkaline solutions. pH reflects the ability of a stimulation fluid to prevent or induce mineral dissolution and precipitation, as well as to alter the organic matter present in the rock, mainly through dissolution. The resulting alterations of the petrophysical properties of the rock have been observed and analyzed for fractures and macropores. However, for meso and micropores, illustrations of acidic alteration remain elusive. Experimental insight at that scale is necessary to understand how fluid-rock interactions affect the reservoir in the long term. To gain insight on the effect of pH on meso and micropores, three different unconventional rock samples with strikingly different composition and organic matter content were reacted with a KCl brine at different pH values (3, 6, 7, 10 and 12), at atmospheric conditions. Changes in petrophysical properties were monitored through the application of time-domain NMR over time. Water chemistry gas adsorption data were obtained at the end of the reaction period (30 days). In addition, rock structures were imaged by Focused Ion Beam Scanning Electron Microscope (FIB-SEM) on selected samples post reaction. Preliminary FIB-SEM and NMR data suggest that changes occurring in the meso and micropore regions relate to the presence of organic matter content. The effect of mineral dissolution and precipitation is more appreciable on the rock surface.