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Description
In acid fracturing of carbonate geothermal reservoirs, high temperatures result in rapid acid-rock reaction rates and short effective acid migration distances. Pre-injection of cold water can lower the temperature at the fracture surface and delay the reaction. However, the numerical solution of the multi-field coupling of thermal-flow-chemical-mechanical interactions between the fracture and the matrix during the pre-cooling and acid injection process is highly challenging. This study establishes a laboratory-scale single-fracture THMC numerical model that couples the physical fields of heat transfer, convective mass transfer, chemical reactions, and the evolution of porosity-permeability relationships in porous media. The acid-rock reaction accounts for the influence of the temperature field, employing first-order Arrhenius kinetics and a local thermal non-equilibrium hypothesis to describe the temperatures of the porous matrix and fluid. The evolution of porosity and permeability is modeled using empirical constitutive equations. During the pre-cooling stage, only the thermal-flow field is solved, while chemical reactions are activated during the acid injection stage. The model was validated by comparing it with experimental results from carbonate rock acid etching, with a relative error in etching depth of less than 5%. Based on this, single-factor sensitivity analyses were conducted on pre-cooling time, reservoir temperature, acid injection rate, and acid concentration, revealing the mechanisms by which these parameters influence the acid etching effectiveness of fractures. The results indicate that during conventional acid fracturing, most of the acid is consumed by reactions near the wellbore; higher temperatures lead to more intense acid etching at the inlet end and make acid migration along the fracture more difficult. In the laboratory-scale simulations of this model, the introduction of pre-cooling reduces the near-wellbore temperature by 40–80 °C, extending the effective acid etching distance from 7.27 cm to 8.69 cm—a relative increase of approximately 19.5%. There is an optimal pre-cooling duration; excessive duration leads to diminishing returns due to thermal penetration saturation. This study provides a theoretical basis for understanding the mechanisms and optimizing parameters of pre-cooling-assisted deep acid fracturing.
| Country | China |
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