Experimental investigation of proppant transport in hydraulically fractured wells using supercritical CO2

Supercritical CO2 fracturing is a promising waterless stimulation technology, and pumping proppant during fracturing is an essential part of the frac job design to obtain a high-conductivity flow channel after the artificial fracture is created. To date, only a few laboratory experiments have been conducted using supercritical CO2 directly to carry the proppant. In this study, an experimental system was designed to allow investigation of supercritical CO2 transport of proppants within a fracture. To determine the supercritical CO2 transport proppant mechanism and the impact of key operational variables, a series of laboratory experiments were performed considering various pumping schemes. The results demonstrate that the mechanism of supercritical CO2 transport proppant includes various flow patterns such as scouring, throwing and fluidization, among which fluidization is the key to the further migration of proppant. Continued injection of clear-fluid after the addition of proppant is complete, will help the dunes extend deeper into the fracture. Fluid velocity has a significant effect on supercritical CO2 transport proppant. While under the premise of constant mass flow rate, increasing the injection temperature, decreasing the injection pressure or reducing the proppant concentration will make the dune equilibrium height lower and the dune laying distance longer. Meanwhile, small-sized proppant particles can still be easily lifted and suspended in supercritical CO2, and reducing proppant size significantly increases the transportability. Furthermore, dune equilibrium height correlates significantly with the variation of the Reynolds number of the fluid flow. Therefore, from a practical field operation point of view, using Reynolds number as a controlling parameter seems to be a viable option for selection of optimum pumping operational variables and prediction of proppant migration within the fracture. The results of this study are expected to provide guidance for field operators to better design operational variables (e.g. CO2 injection rates, proppant load and pumping schedule, etc.) used for supercritical CO2 fracturing. •An innovative experimental system was designed to study the supercritical CO2 transport proppant.•The mechanism of supercritical CO2 transport proppant includes scouring, throwing and fluidization.•Reducing proppant size increases the transport efficiency.•Dune equilibrium height correlates significantly with the Reynolds number of t