Evaluation approach of rock brittleness index for fracturing acidizing based on energy evolution theory and damage constitutive relation

Acidizing, as an essential approach for well stimulation of sandstone or carbonate reservoirs, greatly affects the brittleness of the rock mass. Therefore, it is of great significance to develop a scientific brittleness evaluation methodology for the acid-corroded rock. In this paper, firstly, a damage constitutive model considering the compression hardening process of the acid-corroded sandstone under uniaxial loading is established and verified. Then, the evolution formulae of the relevant mechanical and fitting parameters are derived, and the stress-strain curve of the sandstone subjected to acid corrosion with soaking time is predicted. Finally, a theoretical model for evaluating the brittleness index (BI) of the acid-corroded sandstone based on energy evolution theory and damage constitutive relation is proposed. Based on this model, the BI of the sandstone subjected to acid corrosion is calculated and analyzed, and the BI of the acid-corroded sandstone with the soaking time is predicted. Results show that the BI of the sandstone is negatively correlated with the soaking time, and the rate of descent of the BI decreases with the increment of the soaking time. In addition, the decline degree of the BI has a negative correlation with the pH value. On the other hand, the temperature (25°C, 50°C, and 75°C) has greater weakening effects on the BI compared with the impact of the pressure (5 MPa, 10 MPa, and 15 MPa). Besides, days 50 and 120 are two turning points where the decreasing rates of the BI change from rapid to slow and slow to almost constant, respectively. Furthermore, a coefficient (θ) is proposed to quantify the effect of the acid corrosion, and some suggestions are provided for the application of the acidizing treatment.