Study On Dissolution And Thickening Characteristics Of PVAc Polymers In Supercritical CO₂

With the improvement of oil and gas exploration technology and the increase of resource demand,shale gas resources have received increasing attention.China's shale gas resources have great potential,and the development of shale gas has great strategic significance for China's energy and economic development.Normally,the porosity and permeability of shale reservoirs are low,so horizontal well development and fracturing technology become the key point for shale gas development.Hydraulic fracturing has the disadvantages of water resource waste,water pollution,reservoir damage,etc.,while supercritical carbon dioxide fracturing technology uses anhydrous supercritical carbon dioxide as the fracturing fluid.After fracturing,carbon dioxide becomes completely released from the formation and has almost no damage to the reservoir.Therefore,it has broad application prospects in the development of low permeability and water sensitive reservoirs.However,the shortcomings of the supercritical carbon dioxide fracturing system with low viscosity and poor sand carrying capacity limit the promotion and application of this system.Therefore,it is urgent to carry out theoretical research on supercritical carbon dioxide thickening,which provides an important theoretical basis for the application of the system.This paper uses of polyvinyl acetate(PVAc)to increase the viscosity of supercritical carbon dioxide systems to enhance their ability to carry sand.By studying the dissolution and thicking properties of PVAc in supercritical carbon dioxide,the effects of polymer concentration and cosolvent on the dissolution and thicking properties were investigated.Then,molecular dynamics simulation technology was used to analyze the influence mechanism of various factors,and the block group was introduced to modify the PVAc molecules to obtain a system with better solubility and thicking effect.At the same time,the key factors affecting the dissolution and thickening of such polymers in supercritical carbon dioxide are analyzed.The results show that the addition of ethanol,acetic acid and ethyl acetate can reduce the phase transition pressure of carbon dioxide + PVAc system,and the reduction of phase transition pressure for ethanol is the most obvious under the same conditions,followed by acetic acid.The viscosity of carbon dioxide + PVAc binary system and carbon dioxide +PVAc + ethanol/acetic acid ternary system increased with the increase of pressure,and the thicking effect of ethanol and acetic acid was similar,but ethanol had lower phase transition pressure.The results of molecular dynamics simulations show that the LA-LB interaction between PVAc and carbon dioxide plays a leading role in the dissolution of PVAc in the carbon dioxide system.Both ethanol and acetic acid produce strong hydrogen bonding with PVAc.The presence of this action can enhance the interaction between PVAc and the solvent and promote the dissolution of PVAc in the solvent.By adding EO blocks,PO blocks to the PVAc,or hydrolyzing the PVAc chain can further improved the solubility of the polymer chain in carbon dioxide.