Vincent Noël

Stanford Synchrotron Radiation Lightsource

ABSTRACT

Imbibition – the process of a wetting fluid displacing a nonwetting fluid in porous media – 
profoundly  affects  geochemical  reaction  networks  in  stimulated  geological  formations.  Recent 
experimental studies of the spontaneous imbibition of hydraulic fracture fluid in dry shales shows 
that  acidic  fluid  penetration  and  calcite  dissolution  occur  at  similar  rates  at  the  pore  scale, 
indicating that reaction fronts evolve rapidly than expected. It was further shown that the spatial 
extent and rate of imbibition was moderated by alteration of pore structure. These investigations 
raised important questions about the role of readily solubilized mineral phases in controlling the 
overall imbibition process. Here, we report the results of set of studies designed to isolate the 
impact of calcite solubilization as a function of carbonate mineral content on matrix geochemical 
reactivity and pore space modification concomitant with imbibition.     

We tracked acidic fluid transport in shales by monitoring the rate and spatial extent of 
bromide tracer transport using synchrotron X-ray fluorescence, while simultaneously imaging the 
spatial  profiles  of  Ca.  Our  approach  thus  yields  a  direct  record  of  time-resolved  selective  ion 
transport resulting from the penetration of acidic fluids and attendant mineral transformations. We 
show that the variability in calcite content of shales can directly affect the rate and spatial extent 
of imbibition. Although the reaction of the acid spear head with carbonates in shales enhances 
calcite  dissolution  and  increases  porosity,  the  spatial  extent  of  calcite  dissolution  in  the  shale 
matrix is limited due to a rapid neutralization of pH. Consequently, increased calcite content in the 
shale matrix inhibits the spatial extent of the pore-volume increase and, by extension, the spatial 
extent of imbibition. 

Our study demonstrates that modification of pore space due to reaction during imbibition in dry 
shales  profoundly  effects  the  spatial  extent  of  fluid  penetration.  Of  the  various  controls  on 
reactivity,  the  abundance  and  distribution  of  calcite  importantly  and  rapidly  (couples  hours) 
contributes to the evolution of fluid composition and spatial extent of chemically reacted regions. 
This conclusion has important implications for implementation of subsurface stimulation.

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Email: noel@slac.stanford.edu