Investigation On Supercritical Carbon Dioxide Fracturing Fluid System

Supercritical carbon dioxide fracturing technique attracts more attention as its unique advantages,especially when it applies to low-permeability and super-low-permeability oil and gas reservoirs.However,there are some disadvantages for traditional supercritical carbon dioxide fracturing fluid because of the low viscosity.The proppant carrying ability and filtration control ability of the fracturing fluid are too weak to fracturing.Therefore,it is important to do more research on high-viscosity supercritical carbon dioxide fracturing fluid.In this research,chemical additives were added into the system in order to obtain a novel supercritical carbon dioxide fracturing fluid system with better fracturing properties.Cosolvent and thickeners are crucial factors for supercritical carbon dioxide fracturing fluid systems.All studies were carried out under 42°C.Initially,we intended to obtain an optimum formula of supercritical carbon dioxide fracturing fluid system by doing a series of experiments.Through evaluating the solubility of the cosolvents and compatibility investigation,we selected some cosolvent-thickener candidates.By measuring the minimum miscible pressure of the cosolvent-thickener system,TH-1/CS-2 and TH-3/CS-4 systems were chosen.Further more,we obtained the following results by evaluating the minimum miscible pressure and viscosity measurements.The minimum miscible pressure of TH-1/CS-2 system was 8.0 MPa to 13.5 MPa,and the viscosity was between 0.19 mPa·s and 0.47mPa·s.The minimum miscible pressure of TH-3/CS-4 system was 12.2 MPa to 22.5 MPa,and the viscosity was between 0.50 mPa·s and 1.43 mPa·s.By analyzing these results,we concluded that,TH-3/CS-4 was the competitive cosolvent-thickener system for supercritical carbon dioxide fracturing fluid.It is necessary to investigate the viscosifying mechanism of the system,with the help of molecular dynamics simulation.By studying the density and solubility parameter of the supercritical carbon dioxide,we understood the initial properties of supercritical carbon dioxide.The density and solubility parameter of supercritical carbon dioxide increased with the increase of pressure.The solubility parameter increased linearly with the increase of density.Then,we carried out molecular dynamics simulation of the selected supercritical carbon dioxide fracturing fluid system.All experiments were carried out under 315 K and 20MPa.The concentration of CS-4 and TH-3 had no effect on density of the whole system.However,we realized from the simulation results that,higher CS-4 concentration meant bigger solubility parameter,and higher TH-3 concentration meant higher viscosity value.Thus,we concluded that,CS-4 and TH-3 can greatly affect the solubility and viscosity independently of the fracturing fluid system.Finally,we evaluated the properties of the selected supercritical carbon dioxide fracturing fluid system.The properties included proppant carrying ability,filtration control ability,core permeability damage ratio,and hard residue content.Compared with traditional supercritical carbon dioxide fracturing fluid,the proppant's settling velocity decreased obviously.It meant the investigated system obtained better proppant carrying ability.The filtration coefficient of the investigated system was 0.88 m/min^1/2,which was lower than traditional system filtration coefficient(3.38 m/min^1/2).The core damage ration for low-permeability cores were lower than 20%,which meant super-low core permeability damage ratio.There is no hard residue content of the investigated system.By assessing the properties of the obtained supercritical carbon dioxide fracturing fluid system,we found out that the selected system has excellent performances.Thus,the investigated supercritical carbon dioxide fracturing fluid system is an excellent fracturing fluid which can be used in low-permeability and super-low-permeability reservoirs.