Research On The Flow Field And Perforation Characteristics Of The Abrasive Supercritical Carbon Dioxide Jet

As one of the non-aqueous fracturing methods,supercritical carbon dioxide?SC-CO₂?jet fracturing is regarded to be of great prospect in effectively developing the unconventional reservoirs including shale gas.Perforation into the formation rock through casing using the abrasive jet is indispensable for the hydro-jet fracturing technique.However,SC-CO₂ has low viscosity and high diffusivity,whether it can efficiently carry the abrasive particles to successfully form abrasive SC-CO₂ jet is still needed to be determined.Therefore,theoretical analysis,numerical simulations and lab experiments are carried out in this dissertation to investigate essential problems including the particle-carrying capacity of SC-CO₂ fluid and the flow field and the perforation performance of the abrasive SC-CO₂ jet.This research has great significance in forming SC-CO₂ jet fracturing technique and promoting the field operations.Coupling the equations of the computational fluid dynamics,thermodynamics of CO₂ and forces exerting on particles in jet fluid,numerical simulations are carried out on the process of particle acceleration in SC-CO₂ jet,obtaining the influences of fluid temperature and particle diameter on particle-carrying capacity.Results show that,under the same conditions,particle in SC-CO₂ obtains the higher jet velocity at nozzle exit.SC-CO₂ can carry particle to accelerate to the same level of jet velocity at different fluid temperature and particle diameter conditions,demonstrating the technical feasibility of the abrasive SC-CO₂ jet.Based on the simulations on the flow fields of abrasive jets of SC-CO₂,water and guar fracturing fluid,influences of parameters like ambient pressure,fluid temperature on two-phase velocity distributions out of jet nozzle are studied.Results show that,particle impact velocity in abrasive SC-CO₂ jet is higher than that in abrasive jets of water and guar fracturing fluid.Particle velocity drops slightly with the ambient pressure increasing,and the increases of the fluid temperature and particle diameter can both enlarge the particle impact velocity to the different extents.Numerical simulations are carried out to examine the particle velocity distribution under different nozzle structures,optimizing the nozzle structure parameters for preand post-mixed abrasive SC-CO₂ jet.The shooting on abrasive SC-CO₂ jet using the high-speed video camera indicates that,SC-CO₂ has higher particle-carrying capacity than gaseous CO₂,and the pre-mixed abrasive SC-CO₂ jet has larger and more stable jet structure than the post-mixed one,providing the critical design of jet nozzle for the abrasive SC-CO₂ jet perforation experiments.Using the self-developed abrasive feeding facility under pressure condition,experiments on characteristics of abrasive SC-CO₂ jet perforation and the scanning electron microscopy?SEM?analysis on micro-structure of the perforation hole are well conducted,studying the influences of jet parameters such as pressure difference,standoff distance and so forth,and the failure mechanism of different rock and aluminum targets.Results show that,abrasive SC-CO₂ jet can erode all samples made of marble,limestone,shale and aluminum to holes,demonstrating the working efficiency of the jet.Rocks are broken mainly in the mechanism of the high-speed particle impact,partially in the tensile failure mechanism from SC-CO₂ jet.