Transient temperature-pressure field model of supercritical CO2 fracturing wellbore with tubing and annulus co-injection

The physical parameters of supercritical carbon dioxide (SC-CO2) fracturing fluid are sensitive to temperature and pressure. Accurate prediction of wellbore temperature and pressure during injection is critical for the fracturing efficiency. In this paper, a transient pressure-field model of SC-CO2 fracturing wellbore with tubing and annulus co-injection was established, which was adjusted to consider the Joule-Thomson effect, axial heat conduction, expansion/compression heat, and friction heat. The model predicted the variation patterns of wellbore temperature and pressure, comparing them with those under tubing injection conditions. Effects of tubing-annulus injection ratios and endothermic mode on bottom-hole temperature (BHT) and wellhead pressure (WHP) were analyzed in detail. Compared with pure tubing injection, co-injection provided higher BHT and lower WHP. At constant BHP, WHP dropped with time in the processes of tubing injection or co-injection, while the former provided a larger WHP drop. With an increase in tubing injection ratio, the difference between tubing and annulus BHT values slightly grew. When the tubing and annulus injection displacements were allocated according to their cross-sectional area ratio, their WHP was minimal. If CO2 was assumed to absorb all the friction heat, overestimation of BHT and WHP would be observed, which was more pronounced at larger injection displacements. This study's findings are considered instrumental in the design optimization and field application of SC-CO2 fracturing in unconventional reservoirs. (c) 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.