Matrix acidizing is typically used to remove drilling and completion damage to reservoir conductivity around the wellbore and dissolve calcite in natural fractures. Despite being a common procedure, few studies have investigated the effect of matrix acidizing on the physical properties and oil recovery factors in shales. This paper describes the effect of HCl treatment on porosity, spontaneous imbibition, and crack distribution in samples from the Eagle Ford, Mancos, Barnett and Marcellus shale formations. Samples measuring 2.54-3.81 cm in diameter and 2.54-5.08 cm in length were exposed to acid with concentrations between 0.8-3% active HCl (4 and 15% of 20% diluted HCl). Active acid concentrations of 0.8% were optimal for balancing enhanced oil recovery while maintaining samples structural stability and integrity. The Pre and post HCl treated samples were saturated with Soltrol 130™. Oil recovery was determined by measuring the quantities of oil displaced by uptake of saline water. Recovery factors for Mancos and Eagle Ford shale samples improved from 4 and 16% for non-treated samples to 36 and 71% for the treated samples. In contrast, no change in Barnett and Marcellus shale recovery factors were observed. Porosities were calculated using X-ray computed tomography (CT-scanning). Average porosities prior to acid treatment were 1.6-3.7% versus 1.6-10.7% after acid treatment. The CT-scanning studies of treated samples also shows that in the Mancos shale randomly oriented cracks developed, while in the Barnett and Marcellus shales cracks developed along bedding planes; very little crack development was observed in Eagle Ford shales. These results are encouraging for the application of matrix acidizing in shale formations to improve microfractures conductivity at distances along strike from the wellbore.