While acid fracturing has been applied extensively to carbonate formations, very little attention has been given to its application in shales. Many calcareous mudstones (commonly referred to as shales) such as the Eagle Ford and Bakken have a high calcium carbonate content, and are, therefore, viable candidates for stimulation by acid fracturing. However, the significant differences in well completions, mineralogy, pore structure and petrophysical properties between carbonates and shales, lead us to believe that acid fracturing in shales will present some unique challenges in such rocks.

An experimental study was conducted to investigate how acid treatment affects the petrophysical properties and pore structure of shales. Different concentrations of hydrochloric acid were forced into a shale matrix. The permeability of the core sample was compared before and after acidizing. The acidized zone was first identified by micro-CT scanning to identify the wormholes created by the acid. The acidized matrix was used to study changes in 3D pore structure by CT scanning and by nitrogen adsorption and mercury injection. Based on the 3D pore structure, the change in permeability of the acidized zone was calculated with the lattice Boltzmann method. Changes to the shale microstructure and mineralogy at specific locations before and after acidizing were also compared by SEM-EDS.

Preferential channels for acid flow developed in the shale during acidizing, and the acidized zone was found to be confined to a region surrounding these channels. The permeability of whole core sample increased significantly (from nano-Darcy to micro-Darcy level). The permeability of the acidized zone was even higher and reached around 100 mD. Acidizing created preferred flow channels with roughness on their surfaces. The almost complete dissolution of the carbonate minerals, made the pores in the clay more accessible. Porosity was found to increase (based on pore size distribution, and 3D pore structures/ 2D microscopic images).

This study demonstrates some of the changes that can occur when shales are subjected to acid treatment. These findings can be used to design acid fracture treatments in shales i.e. make decisions about acid concentration, acid type and injection rates.