Theoretical and Conceptual Investigation of the Effect of Pore Water Chemistry on Formation Breakdown Pressure Estimation using Conventional Models in Hydraulic Fracturing Operation

The three main factors affecting formation breakdown pressure (PB) include rock mechanical and petrophysical properties, fluid properties and in-situ stress fields. Boreholes in different reservoir sections within the same rock type and reservoir and even under the same stress fields may exhibit different PB behaviors due to differences in pore water chemistry (different ionic composition and ionic strength) and geochemical interactions between pore water and rock. The PB can be estimated based on different theoretical and empirical models. However, in these models, the geochemical effect is not directly captured. Besides, the accuracy of PB estimation using these relations is still unknown. In this study, the accuracy of PB estimation using the conventional models and the effects of pore water chemistry on PB were investigated using published data from literature. Also, a new model was developed based on the concept of effective stress, disjoining pressure and the superposition principle for stress distribution around well-bore. The results indicate that most of the conventional models are unsuccessful in PB prediction for hydraulic fracturing operations. As an example, the popular Hubert-Willis model has an average error of 34.7%. Our study indicates that pore water chemistry has a substantial effect on rock wettability and PB due to the change of inter-granular forces, based on the DLVO theory, affecting the rock strength. The results of the new conceptual model show a decrease in the PB of chalkstone with an increase in the concentration of the sulfate ions in the pore water.