Abstract
Abstract Acidizing as a stimulation technique does not always yield the desired sustainable conductivity especially compared to hydraulic fracturing. In addition to that, hydraulic fracturing provides a great contact with shale formations due to its deeper developed conductivity that is sustained by the existence of proppant. However, hydraulic fracture conductivity declines because of a variety of reasons: increasing effective stress, proppant fatigue due to stress cycling, and digenesis. Here, we studied the potential of combining the benefits of propped hydraulic fracturing and acidizing by introducing low concentrations of acid slugs in the hydraulic fracture treatment of shale formations. The acid slugs intended to improve the micro-fracture conductivity deep in the formation and near the wellbore. Experiments using low concentrations of HCl (up to 3 wt% active HCl) showed a great enhancement in oil recovery factors and shale porosity. The treatment was carried out at 200°F. Furthermore, the effect of the HCl treatment on samples from Mancos, Eagle Ford, Barnett, and Marcellus shale formations was examined. The samples used were 2.54-3.81 cm in diameter and 0.76-5.08 cm in length. The porosity measurements on the cores before the treatment range from 0.23-7.7% versus 0.2-35.8% after the treatment. Spontaneous imbibitions showed recovery factors of 2.5-13% from non-treated samples against 4-53% from the treated samples. The treated samples were saturated with more oil compared to the non-treated samples due to the improvement in porosity and permeability by the acid treatment. CT scan images also have been taken for the samples pre and post the acid treatments to show the effect of the treatment on the porosity and cracks distribution along the cores. Our study clearly demonstrates that injecting slugs of low concentrations of HCl near and away from the wellbore adds permanent improved conductivity to hydraulic fracturing that is resulted from the partial dissolution of calcite.
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