| Hydraulic fracturing techonology is the most important stimulation treatment for the development of tight oil and gas resources.However,during the stimulation treatment,the fracturing fluid will invade the reservoir matrix,damage the permeability,and reduce the stimulation effect.Therefore,this paper studied the damage of guar gum fracturing fluid to tight reservoir matrix,investigated the measures to reduce the permeability damage caused by liquid phase,and prepared the cleanup additives and the clay stabilizer with low damage rate,which can provide guidelines for developing the tight oil and gas resources with high efficiency.The tight sandstone cores taken from a block in Xinjiang Oilfield,with the permeability of 0.10×10-3?m2(within the range of 0.08×10-3?m2-0.17×10-3?m2),0.05×10-3?m2(within the range of 0.035×10-3?m2-0.065×10-3?m2),and 0.01×10-3?m2(within the range of0.007×10-3?m2-0.020×10-3?m2)were taken as the research object.The damage of fracturing fluid to tight reservoirs were studied by a combination of physical simulation experiments and nuclear magnetic resonance analysis.The results showed that the damage rates caused by the solid phase were less than the damage rates caused by the liquid phase,meaning that the liquid phase damage was the main damage caused by the intrusion of the fracturing fluid into the tight reservoir.Therefore,it is proposed that the reduction of tight reservoir permeability damage caused by fracturing fluid should focus on reducing water lock damage and water sensitivity damage.For the water locking damage of tight reservoir,cores with the permeability of0.10×10-3?m2,0.05×10-3?m2,and 0.01×10-3?m2 were employed to study the influence of the interfacial tension of the cleanup additive on the flowback rate of the invading fluid and the damage rate caused by water locking effect.It was found that the interfacial tension should be less than 0.85m N/m?0.26m N/m,and 0.092m N/m,when the water locking damage rate was less than 10%.Therefore,it is suggested that for the tight reservoirs with the permeability less than 0.05×10-3?m2,the interfacial tension of the cleanup additive should be less than0.10m N/m.Guided by this result,the cleanup additives based on sodium perfluornonenoxy benzene sulfonate and alkanolamide were developed.The water locking damage rate was reduced by 44.37%,for the cores with the permeability of 0.01×10-3?m2.Through the study of the interface expansion rheology of the cleanup additives and the interface assembly behavior,the mechanisms of the reduction in oil-water interfacial tension of the cleanup addtives were proved as that the synergistic effect of the compound system can increase the adsorption density and the adsorption speed of surfactant onto the interface.Clay stabilizers with high relative molecular mass may block the channels for oil and gas migration in tight reservoirs and cause permeability damage,although clay stabilizers can inhibit clay swelling and migration.In order to find out the matching relationship between the relative molecular mass of clay stabilizer and the permeability of tight reservoir,poly(dimethyldiallyl ammonium chloride)and poly(ethylene imine)were taken as research objects,and the effect of polyer-type clay stabilizer with different relative molecular mass on cores with the permeability of 0.10×10-3?m2,0.05×10-3?m2,and 0.01×10-3?m2was studied.The results showed that in order to not cause core permeability damage due to clogging of the clay stabilizer,the relative molecular mass of the clay stabilizer should be less than 10000,5000 and 2000,respectively.For the water sensitivity damage of tight reservoir,the double quaternary ammonium salt with low relative molecular mass(less than 1000)was synthesized.The water sensitivity damage rate was reduced by 50.77%for the cores with the permeability of 0.01×10-3?m2.By means of TOC,FT-IR,SEM,TGA,XRD,and zeta potential tests,the anti-swelling mechanisms of double quaternary ammonium salt were studied.It can be firmly adsorbed on the clay surface under the combined action of electrostatic interaction and hydrogen bonding.Its hydrophobic segments convering the clay surface enhances its hydrophobicity,prevents the adsorption of water molecules on the clay surface,reduces the water content between the clay layers,and thereby weakens the hydration effect of the clay.The positively charged double quaternary ammonium salt molecules on the clay surface can reduce the charge of the surface of the clay particles,reduce the repulsion between the clay sheets,compress the hydration layer,and therefore,the clay was inhibited from swelling and maintains the clay particles in a stable state.Compared with the original fracturing fluid,the water phase damage rate of the fracturing fluid constructed of the double quaternary ammonium and the developed cleanup additives was reduced by 47.61%. |
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