Study on the mechanical characteristics of the stress relaxation in the carbonate rock after high-temperature acidification

To elucidate the mechanism by which acidification influences wellbore stability in deep reservoir formations, this study investigates the rheological and mechanical behaviors of the carbonate rock subjected to high-temperature acid etching. A novel experimental system was developed to characterize the stress relaxation behavior of the acid-etched carbonate rock, and the characteristics of the stress relaxation curves under various acid etching conditions and strain levels were systematically analyzed. Combined with Burgers model and the Levenberg–Marquardt algorithm, the evolution of rheological parameters of the carbonate rock under different acid etching regimes was quantitatively evaluated. The results indicate that the acid-etched carbonate rock exhibit significant rheological mechanical properties due to the presence of developed microcracks and complex pore structures. Under the identical acid etching duration and temperature, the initial stress, residual stress, and time required for stress relaxation stabilization all increase with increasing the strain level. Overall, the stress relaxation magnitude prior to the core fracture ranges from 15 to 25 MPa, and the stabilization time for the core stress relaxation falls between 5 and 7 h. The stress relaxation behavior of the acid-etched carbonate core is well described by the Burgers model. At fixed strain levels and temperatures, the instantaneous shear modulus $$G_{1}$$ decreases linearly with extended acid etching time, whereas the instantaneous shear modulus $$G_{2}$$ and the viscosity coefficients $$\eta_{1}$$ and $$\eta_{2}$$ exhibit exponential degradation. The final variation ranges of the key rheological parameters are determined as follows: instantaneous shear modulus $$G_{1}$$ ranges from 5 × 103 to 2 × 104 MPa, instantaneous shear modulus $$G_{2}$$ ranges from 6 × 105 to 2 × 106 MPa, viscosity coefficient $$\eta_{1}$$ ranges from 2 × 107 to 8 × 107 MPa h, and viscosity coefficient $$\eta_{2}$$ ranges from 1 × 105 to 1.2 × 106 MPa h. Furthermore, the evolutionary equations correlating the global model fitting parameters with the porosity of acid-etched samples are established, using acid etching time as an intermediate variable. The results of this study provide a theoretical basis for the analysis of wellbore stability after acidification and the selection of acid fracturing completion methods of deep reservoirs.