Experimental investigation of acid imbibition on matrix and fractured carbonate rich shales

Abstract Matrix and fracture permeability of carbonate rich tight cores from Niobrara, Bakken, and Three Forks formations were measured before and after exposing the core samples to spontaneous imbibition using 10 wt % KCl and 1 wt % HCl acid. A hydraulic fracture was modeled by saw-cutting a given core sample. Sand proppant of 0.02 lbs/ft 2 concentration and 30/80 mesh size with monolayer packing was used for the propped fracture cores. Permeability was measured at varying net stress of 1000 psia up to 6000 psia. Before and after dilute HCl acid imbibition matrix and fracture permeability vs. stress and rock hardness measurements were investigated. Brine and dilute acid imbibition effect on proppant embedment, rock softening/weakening, and fracture roughness were assessed. Brine imbibition before and after dilute acid imbibition was also studied to investigate possible wettability alteration. After acid soaking/imbibition, up to twenty fold increase in matrix permeability was observed for carbonate rich Niobrara cores. A matrix permeability decrease was observed in clay rich Three Forks cores. On the other hand, permeability decrease after dilute HCl acid imbibition was observed on both propped and un-propped fractured Niobrara cores. Matrix permeability increase after acid imbibition is attributed to the enhancement of pore throat connectivity and nano- and micro-fracture etching. Permeability decrease for the un-propped fractured Niobrara cores is attributed to fracture surface softening leading to decrease in fracture roughness and fracture aperture at in-situ stress conditions. And, permeability decrease for the propped fracture cores is attributed to decrease in fracture width or aperture caused by proppant embedment as brine and acid weakens the fracture face fabric. It is concluded that dilute acid injection of pre and post hydraulic fracturing stimulation in ultra-tight carbonate rich mudrocks could improve the overall deliverability of stimulated reservoir volume. This is because, despite decrease in fracture permeability after acid imbibition, matrix permeability increase is more significant for the overall deliverability of ultra-tight carbonate rich mudrocks. Moreover, favorable wettability alteration can be achieved by dilute acid imbibition.