A fracture initiation model for carbon dioxide fracturing considering the bottom hole pressure and temperature condition

The properties of CO2 under different conditions significantly affect the breakdown pressure during CO2 fracturing. In this study, the fracture initiation model is combined with real bottom-hole temperature-pressure condition by considering CO2 borehole pressurization rate. The results show that the breakdown pressure can be reduced by 26.2%-35.5%. With a constant pump rate of 0.5 m(3)/min, bottom-hole pressure exceeds the breakdown pressure at approximately 408s, meanwhile, bottom-hole temperature decreases by 4.38% and borehole pressurization rate expands by 2.4 times. As the pump rate increases, the pressurization rate reaches its peak at a pump rate of 6.0 m(3)/min, and the downtrend is delayed with inlet pressure increasing. Case study shows that the breakdown pressure predicted by the new model agrees well with in-situ data, and the range of optimal inlet pump rate is 1.9-2.5 m(3)/min under the in-situ inlet condition (T-0 = 258 K and p(0) = 6 MPa). Furthermore, the range of optimal inlet pump rate expands with inlet pressure increasing and shrinks with inlet temperature increasing. This new initiation pressure model can provide more insights for CO2 fracturing mechanisms and design.