Study on Proppant Transport in Fractures of Supercritical Carbon Dioxide Fracturing

摘要

Supercritical carbon dioxide (SC-CO2) fracturing is a promising stimulation treatment in the unconventional oil and gas industry, which combines the utilization of greenhouse gas and the development of energy resources. Aiming to study the proppant transport in fractures, which is one of the most important issues in this technology, a generalized and pragmatic numerical method coupled with multiphase flows, physical property models of CO2, and heat and mass transfer in the formation rock has been established to calculate the proppant transport in fractures of SC-CO2 fracturing. Experimentally, experimental validation of the proppant transport was conducted, which verifies the accuracy of the simulation results. Combining experimental and simulation results, the flow condition of proppant-SC-CO2 two-phase flow was divided into a stationary layer, a rolling layer, a saltatory layer, a suspending layer, and a pure liquid/gas layer. Numerically, movement laws of proppant beds in fracture and influence factors on the distribution of proppant beds were analyzed. Proppant bed was formed near the inlet of the fracture at the initial stage of fracturing, and the height and length of the bed would increase concurrently until reaching a steady state. The equilibrium height has positive correlations with the injection temperature, proppants' volume fraction, density, and diameter and negative correlations with the injection pressure and displacement. Nevertheless, the equilibrium time has negative correlations with all these factors except pressure. Additionally, the proppant transports of SC-CO2 and water were compared, which indicated that water has better proppant-carrying capacity with lower proppant bed height and longer proppant bed length at the same fracturing time. The distribution of the proppant bed with the structure of a nozzle inlet is relatively homogeneous in the near-wellbore zone of the fracture compared to the cuboid structure, which could reduce the risk of fracture closure in the near-wellbore zone. Results obtained in this paper could provide references for SC-CO2 fracturing design which would promote the development of this technology.