Reactive Transport Modeling of Shale–Fluid Interactions after Imbibition of Fracturing Fluids

Injection of hydraulic
fracturing fluid (HFF) into shale formations
for unconventional oil/gas production results in chemical reactions
in the shale matrix. Our recent experimental study determined the
depths to which different types of reactions between the shale matrix
and the HFF extended. In the present study, we built continuum-scale
reactive transport models to understand the coupling of chemical reactions
and the transport of aqueous species in these shale–HFF systems.
Calibration of the model with our previous experimental results reveals
that it takes hours to months for the shale matrix to completely neutralize
the imbibed acids, depending primarily upon the carbonate content
of the shale. Both the HFF pH and pore pH affect the location of barite
precipitation, resulting in unique barite precipitation profiles extending
millimeters into calcite-rich Eagle Ford shale but only tens of micrometers
into low-carbonate Marcellus shale. In addition, dissolved oxygen
and extracted bitumen are key to reproducing the experimental observation
of Fe­(III) (oxyhydr)­oxide formation in the shale matrix as a result
of pyrite dissolution in the shales. A comparison between the modeling
results of porosity in the present study to experimentally measured
permeabilities in our previous study suggests that chemical reactions
occurring at a greater depth than the observable reaction zone might
have impacted permeability. Our model serves as a benchmark for efficiently
modeling water–rock interactions in similar systems where bulk
rock samples react with a solution in batch reactors. Important reactive
transport processes were ascertained via modeling, which allows for
quantitative prediction of shale–HFF interactions in shale
matrices given the shale and HFF compositions.