The thermal-fluid-mechanical (TFM) coupling method based on discrete element method (DEM) and the application of CO2 fracturing analysis

Due to the challenges of the complex temperature, fluid and mechanical coupling phenomenon during CO2 fracturing, a new thermal-fluid-mechanical (TFM) coupling method is proposed by using PFC2D fish code. This method utilizes the PFC2D fish code and combines the heat transfer difference algorithm with Cundall's fluidmechanical coupling algorithm to establish a TFM model for CO2 fracturing. Subsequently, the validation of the TFM coupling model is conducted using the analytical solution provided by the plane strain Khristianovic Geertsma-de Klerk (KGD) model. The reservoir temperature variation, pore pressure and crack propagation are studied considering many involved factors, such as injection fluid temperature, injection flow rate, in-situ stress and formation temperature. Crack number and length would increase with the increasing temperature difference between fracturing CO2 and reservoir temperature. The breakdown pressure reduces when there are large injection flow rate or temperature differences between fracturing CO2 and reservoir temperature, which can be optimized in the CO2 fracturing operation. Even CO2 fracturing could create complex fracture network, we must pay attention to the in-situ stress distribution and injection flow rate. Our simulation suggests that pore pressure and heat could prevent the fracture from initiating and propagating in the direction of the minimum principal stress, and the temperature influencing has contributed to the extra pressure increment in the pore domain, which could not be ignored in CO2 fracturing. From the above research, it is expected to offer a novel method to simulate the complex TFM coupling phenomenon and enhance our understanding of CO2 fracturing.

Automated summary

In this study, a new thermal-fluid-mechanical (TFM) coupling method using PFC2D fish code is proposed to simulate the complex temperature, fluid, and mechanical coupling phenomenon in CO2 fracturing. The TFM coupling model is validated using an analytical solution, and the effects of injection fluid temperature, injection flow rate, in-situ stress, and formation temperature on reservoir temperature variation, pore pressure, and crack propagation are studied. The results show that the crack number and length increase with the temperature difference between fracturing CO2 and reservoir temperature, and the breakdown pressure is reduced with larger injection flow rate or temperature differences. In addition, the pore pressure and heat play significant roles in preventing fracture initiation and propagation in certain directions. This study provides a novel method to simulate the complex TFM coupling phenomenon and enhances our understanding of CO2 fracturing.