Study On The Construction Of Thickened Supercritical CO₂ Fracturing Fluid And Its Effects On Reservoir Rocks

With the development of unconventional hydrocarbons resources in China,dry scCO₂fracturing technique has obtained more and more attention from the various organizations due to its unique technical features and performance advantages.However,some drawbacks（low viscosity,low proppant carrying ability,high filtration volume）of scCO₂ fracturing fluid hinders the field application of this new technique.Meanwhile,studies on the effects of scCO₂ fracturing fluid on reservoir formations are seldom investigated.To shed some light on these interaction mechanisms will certainly instruct the application of scCO₂ fracturing fluid.At 52℃,different thickeners and cosolvents for scCO₂ are screened and optimized to construct a superior scCO₂ fracturing fluid with CO₂.After some primary experiments,two combinations are chosen as the potential candidates for scCO₂ fracturing fluid.And the candidates are 12-hydroxy stearic acid/ethanol/CO₂ and polymethyl siloxane/kerosene/CO₂.For the 12-hydroxy stearic acid/ethanol/CO₂ system,the solubility pressure of the system varies from 7.6 MPa to 23.8 MPa and the viscosity of this system are between 0.03 mPa·s and0.73 mPa·s.For the polymethyl siloxane/kerosene/CO₂ system,the solubility pressure of the system varies from 22.8 MPa to 31.8 MPa and the viscosity of this system are between 0.54mPa·s and 4.67 mPa·s.Obviously,the polymethyl siloxane/kerosene combination has better CO₂ thickening ability.Compared with scCO₂ under the same conditions,the viscosity of the thickened scCO₂ increases 54.26 folds.And the formula of 5.0 wt.%polymethyl siloxane+10.0 wt.%kerosene+85.0 wt.%scCO₂ is chosen as the objective of all follow-up investigations.Shearing investigation of scCO₂ fracturing fluid shows that the system is shearing-thinning.And the viscosity of the system reduces dramatically with the increase of shearing rate.At shearing rate of 2148.6 s^-1,the shearing viscosity of the system is 1.61 mPa·s,while the value goes down to 0.14 mPa·s at 27225.7 s^-1.High shearing rate breaks the structure in the fluid which increases the fluid viscosity and the viscosity of the fluid is largely decided by the plastic viscosity.The proppant carrying ability of the system doesn’t impressively improve with a rise of around 40%.Meanwhile,with the increase of sand ratios,the proppant carrying ability shows a declining trend.The filtration of low-permeability sands shows that pure scCO₂ has a high filtration factor in low-permeability sands at a value of 3.89cm·min^-0.5 and the filtration curve of scCO₂ is a straight line.The same experiment is also conducted on the cores of shale and limestone.However,the filtration curve of scCO₂fracturing fluid is close to a parabola and the equilibrium value of filtration factor reduces to1.93 cm·min^-0.5.From regained permeability experiments of scCO₂ fracturing fluid,high permeability damage of scCO₂ fracturing fluid on tight cores is observed.At the core permeability of around 1.50 mD,the permeability damage ratio is 40.06%.Meanwhile,with the further reduction of core permeability,the ratio dramatically increases.The wettability alternation investigation reveals that the scCO₂ fracturing fluid can transfer the water-wet surface into neutral wetting which benefits the flow of oil and gas.SEM microscopic investigation discovers numerous thickener particles adsorbed on the surface of the core pores and channels.Because of Cassie effect,the water contact angle of low-permeability sands and shale increases from 60.73°,72.80°,and 82.33°to 103.86°,99.40°,and 93.53°respectively.Clay swelling tests show no visible swelling of clay mineral were observed after scCO₂ fracturing fluid treatment.Further XRD investigation also conforms the result as the peak position of clay mineral doesn’t move.And the calculated interlayer spacings of untreated and treated clay mineral are almost identical.Carbonate corrosion experiment shows scCO₂ fracturing fluid has corrosion effect on carbonate minerals.The mass corrosion ratio increases with the treated time.While the carbonate minerals are being corroded,the porosity and permeability of the carbonate cores also increase which is the result of enlarged pores and connected micro fractures caused by carbonate minerals corrosion.