Experimental Study On Pipe Flow Characteristics And Filtration Properties Of CO₂ Fracturing Fluid

The high efficient hydraulic fracturing technique for low permeability oil and gas reservoirs is not only a way to improve the production of oil and gas fields,but also the core technology for the economical exploitation of unconventional petroleum resources.It has been playing an important role in improving China's oil and gas production and accelerating the development of energy economy in western China.Therefore,it is widely used in the development of unconventional oil and gas fields around the world.Among the factors that affect the results of fracturing,fracturing fluid system and its performance is of the most important,and the foam fracturing fluid is widely used in fracturing fluid system at home and abroad,accounting for about 30%of field applications,and gas or oil wells using CO₂ foam fracturing technology,respectively,accounted for 90%and 30%,which has the advantages of easy discharge,non-residue,well sand-carrying and breaker free.Additionally,in the process of fracturing and the subsequent filtration,a large scale of CO₂ storage and utilization is also an important technology of achieving low-carbon environment.In this dissertation,based on a large-scale high temperature and high pressure foam fracturing fluid experiment system,the rheological properties and heat transfer characteristics of a novel CO₂ foam fracturing fluid system are studied experimentally.The effective viscosity,rheological parameters,convective heat transfer coefficient varying with the temperature,pressure,foam quality,shear rate are analyzed.The main conclusions are as follows:?1?The effective viscosity of CO₂ carboxymethyl cross-linked guanidine fracturing fluid is significantly higher than that of the foam fracturing fluid,the effective viscosity of two systems decrease with the increase of the shear rate,and the decrease rate is lower than the high shear rate region in the low shear rate region;In non-foaming state,the effective viscosity of the fracturing fluid and CO₂ mixture decreases with the increase of the foam quality.In the foaming state,the effective viscosity of the fracturing fluid and CO₂ mixture system shows the change law of the first increase and decrease with the increase of the foam quality.?2?In the foaming and non-foaming state,the rheological coefficient k*of the cross-linked guanidine fracturing fluid + CO₂ system was significantly higher than that of the fracturing fluid + CO₂ system.In the absence of foaming,the rheological index n' of the "carboxymethyl cross-linked guanidine fracturing fluid + CO₂" system increases with the increase of the foam quality,while the rheological coefficient k'decreases.In the case of foaming state,the rheological index n' decreases with the increase of foam quality,while the rheological coefficient k' increases,however,the CO₂ fracturing fluid system was opposite.With the increase of temperature,the rheological index n' of the "carboxymethyl crosslinked guanidine fracturing fluid +CO₂" system will increase and then decrease,and the rheological coefficient k' will decrease with the increase of temperature.?3?Regarding to the two fracturing fluids:1)the frictional resistance coefficient decreases with the increase of the flow rate;2)the frictional resistance coefficient of two fracturing fluids decrease with the increase of the CO₂ volume,and the magnitude of the varying of frictional resistance decreases at high flow rate.The frictional resistance coefficient of the CO₂ foam fracturing fluid in foaming state showed a tendency to decrease first and then decrease with the increase of the foam quality;3)in the foaming and non-foaming stages,the frictional resistance coefficient of the fluid decreases with the increase of the temperature,and the friction coefficient of the CO₂ foam fracturing fluid increases slightly with temperature at the critical temperature?31.2 ??,with the increase in the quality of the bubble,the trend also increased.?4?The researches on the heat transfer characteristics of two fracturing fluids suggested that:1)the influence of temperature on the heat transfer characteristics of the fracturing fluid and the CO₂ mixture system is small.With the increase of temperature,the heat transfer coefficient of two fracturing fluid systems showed a general increasing trend;2)when the quality of the foam is increased,the convective heat transfer coefficient of the fracturing fluid tends to decrease;3)with the increase of the shear rate,the interface quantity of the two-phase fluid and the collision probability of the bubble raises sharply,which is favorable to the heat transfer enhancement,which leads to the increase of the convective heat transfer coefficient.?5?The study on the filtration characteristics of fracturing fluid mixed system and pure CO₂ showed that,the filtration coefficient of liquid CO₂ and supercritical CO₂ increases with the increase of temperature.For the fracturing fluid and CO₂ mixed system in the non-foaming stage,increasing the foam quality will increase the fracturing loss of fracturing fluid,For the foamed fracturing fluid at the time of foaming,with the increase of the mass of the foam,the foam quantity of the fluid increases,and the stable foam structure is gradually formed,and the filter loss coefficient is reduced,and the damage of the foam fracturing fluid to the formation core And the core damage rate is between 16%and 21%.For the carboxymethyl cross-linked guanidine fracturing fluid + CO₂ mixed system,the filtration coefficient of foaming is slightly lower than that of the non-foaming,And the reduction coefficient of the filter loss coefficient is less than that of the fracturing fluid + CO₂ system.The damage of carboxymethylguanidine + CO₂ system fracturing fluid on the formation of the core is more than the foam fracturing fluid,and showed the rate of 26%to 41%,respectively.