| After solving the acid crosslinking technology, conventional guar gum CO2 foam fracturing fluid foam stability is greatly improved to further enhance the carrying capacity and meet the needs of fracturing to the greatest degree. However, the use of cross-linking technology can not completely avoid the damage caused by the residue, besides, it is difficult to further reduce the construction friction and the shear resistance is also very difficult to improve. Even though residue damage and hear resistance are completely solved, there is still a serious shortage of temperature resistance. At the same time, the use of high concentration of VES increases the construction cost, which ultimately limits the application of the system. Thus, to establish a clean CO2 foam fracturing fluid, first of all, it is necessary to study the factors that affect the stability of CO2 foam fracturing fluid.In this paper, the viscoelastic properties of CO2 foam fracturing fluid film were tested by the method of small stress response period. We analysed the membrane viscoelastic (elastic modulus G, the viscous modulus G ", apparent viscosity eta) and foam stability from both the micro and macro perspectives. The results showed that foam stability "foam half-life" did not increase with the increacing of CO2 foam fracturing fluid film apparent viscosity "?". The foam stability of VES-CO2 foam with small apparent viscosity was significantly better than that of the HPAM-CO2 foam with larger apparent viscosity. The influence of membrane viscoelasticity on foam stability was more regular than the apparent viscosity and the effect of Liquid storage and the effect of Liquid storage modulus G'is more direct. The stronger the film structure, the more effective against the drain and stable foam. Based on the above resluts, we put forward a theory of the clean CO2 foam fracturing fluid, which is based on the combination of elastic-based supramolecular polymer fracturing fluid and CO2 foam fracturing fluidCombining the requirements of foam stability for liquid film elasticity and demand of the water solubility and viscosity of the thickening agent for fracturing, we design and synthesis of supramolecular polymer as the stabilizing agent of CO2 foam fracturing fluid, which can turn a water solution into a fluid structure. The experimental results showed that the solution based on the supramolecular stabilization agent is mainly elastic with good solubility and good viscosity. In the presence of suitable foaming agents, the foam stabilizing agent can significantly increase the elastic modulus of G's solution and the corresponding foam stability at high temperature was also enhanced.Through the investigation and evaluation of indoor performance, we found that he polymer CO2 foam fracturing fluid shows good temperature resistance with the maximum temperature of 140? and it can gel break liquid without residue.We conducted a field test successfully in Yanchang oilfield, the first application of supramolecular polymer CO2 foam fracturing fluid in China was realized. Site data showed the excellent performance of the foam fracturing fluid, which caused little damage to shale reservoirs and fully met the shale gas fracturing construction requirements. Gas production began after 80 cubic meters of liquid discharge and 3 mm nozzle pressure was greater than 11 MPa, meanwhile unimpeded gas reached 100 thousand cubic meters per day which contained a small amount of water.The theoretical research and practical results showed that the stable clean supramolecular polymer CO2 foam fracturing fluid is feasible and the obtained supramolecular foam stabilizer can be compatible with CO2. The stablished CO2 foam fracturing fluid shows good temperature resistance stability and hardly have any residue. These results provide a theoretical and practical basis for further research and development of clean CO2 foam fracturing fluid which can meet more stringent requirements. |
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