Numerical Study On Proppant Transport With Supercritical Carbon Dioxide In Fractures

Supercritial CO₂?SC-CO₂?fracturing,as a kind of waterless fracturing method,is an effective treatment in the unconventional oil and gas industry.Due to its characteristics of lower viscosity and density,the abilities and rules of proppant transport with SC-CO₂ in fractures have not been figured out yet.Moreover,SC-CO₂ fracturing is likely to generate complex fracture which makes it more difficult to carry proppant.Therefore,proppant transport with SC-CO₂ in different geometric fractures including plannar fracture,T shape fracture and crossing shape fracture are studied in this paper.The main research works and achievements are as follows:A numerical model for solving the two phase flow in the plannar fracture was developed,which took the interactions of proppant particles and the variation of fluid properties into consideration.The results indicate that the transporting capability of slick water is better than that of SC-CO₂,since the viscosity of SC-CO₂ is lower than that of slick water.And with the increae of the injection temperature,the height of sand bed increases gradually,and the distance between the leading edge of sand bed and the entrance of fracture decreases.In addition,with the increase of fracture outlet pressure,the height of sand bank decreases gradually.Numerical simulations were carried out to assess the impact of the properties of proppant and fracturing fluid on sand-carrying in fracture.The results show using proppant whose density and diameter are respectively less than 1540kg/m³ and 0.25mm as well as sand-carrying fluid whose sand ratio is lower than 8%and injection rate is higher than 2kg/s are beneficial to form uniform sand bed.Moreover an empirical model for predicting the non-dimensional equilibrium height of sand bed is developed.There is a certain function relationship among the non-dimensional equilibrium height,Reynolds number,Archimedes number and sand ratio.Three types of fracture including planar fracture,T shape fracture and crossing shape fracture were modeling to study the transport rules of SC-CO₂ fracturing.The results show for T shape fracture case,turbulent flow regime might develop at fracture junction which can make proppant propagate to natural fractures.For crossing shape fracture,turbulent flow takes place behind the fracture junction,which causes little sand dune to form within downstream of induced fracture.Additionally,the thinner width of natural fracture as well as the larger intersection angle of the crossing shape fracture,the more difficult for proppant to enter natural fracture.To summary,the paper revealed the unique characteristic of proppant transport with SC-CO₂ and the laws of sand-carrying of SC-CO₂ fracturing in plannar and complex fractures.Also an empirical model for predicting the non-dimensional equilibrium height of sand bed was developed.Research results are expected to provide a theoretical basis for the application of SC-CO₂ fracturing.In the meanwhile,it will provide references for the parametric optimization and technical adaptability of SC-CO₂ fracturing.