| With the development of global oil and gas resources to ultra-high temperature reservoirs,fracturing fluid is supposed to have excellent properties,such as salt-resistantance,especially heat-resistantance.However,there is no available fluid to be used to develop the ultra-high temperature reservoirs,which restricts the utilization of deep oil and gas resources greatly.In order to solve this technical challenge,a quaternary polymer,named as NFT,was created by the copolymerization of DM-12,AMBS,AM and AA in water.The optimum conditions of NFT were obtained by a single variable method.It was researched that the optimum synthesis conditions for NFT were as follows:the total monomer concentration was 28 wt%;pH value of the solution was 7.0;the initiator concentration was 0.07 wt%;the reaction temperature was 60℃;the mole ratio of AM,AA,AMBS and MD-12 was 79.0%:19.8%:0.8%:0.4%.NFT is a non-crystalline polymer with good thermal stability by XRD,DTG and TGA.In addition,PEI,a non-toxic and environmentally friendly organic crosslinking agent,was used to crosslink with NFT at high temperature.The activation energy of the reaction of NFT with PEI is less than the PAM with PEI,which can satisfy the requirement that thicken reacts with crosslinker in short-time.In other word,it is proved that PEI was feasible in fracturing fluids.After obtained the formula,the performances of fracturing fluids were evaluated.First,the shear recovery test has verified that this non-Newtonian fluid had excellent shear recovery properties at 180℃,which showed that the fracturing fluid is beneficial to carry the sands and form long crack.Under the conditions of 180℃,170 s-1,the viscosity of the fracturing fluid maintained at about 100mPa·s after 120 min.while the viscosity of the fracturing fluid can reach 80 mPa·s after 120 min,under the conditions of 200℃,170 s-1.In order to further measure the temperature resistance of the fracturing fluid it was sheared at higher temperature.First,under the condition of 220℃,170 s-1,the viscosity of the fracturing fluid reaches 24 mPa·s after 120 min,indicating that the thermal damage of fracturing fluid and the repair effect of polyethyleneimine to NFT were dynamic equilibrium at this time.Then,it was remeasured at 220℃,100 s-1,the viscosity of the fracturing fluid maintained at about 120mPa·s after 120 min.Besides,under the conditions of 240℃,100 s-1,the viscosity of the fracturing fluid reaches 124 mPa·s after 120 min,showing that the fracturing fluid has an excellent temperature resistance at 240℃,100 s-1.Meanwhile,when the temperature was raised to 260℃,the viscosity maintained at about 35 mPa·s.Similarly,an obvious conclusion can be drawn that the thermal damage of fracturing fluid and the repair effect of polyethyleneimine to NFT were dynamic equilibrium at 110 s-1 and 260℃.The maximum temperature of the fracturing fluid was up to.220℃ after sheared 120 minutes under 170 s-1,and up to 260℃ after sheared 120 minutes under 100s-1.There is a strong spatial network structure by experiments of viscoelasticity and thixotropy.When encapsulated APS was added about 0.04%,the viscosity of the breaker is 2.4 mPa·s in 2h,which is transparent and no obvious residue.The settling rate of proppants and filtration rate of the fracturing fluid were 1.401 min/cm and 0.2637×10-5 at 180 ℃.In addition,the permeability damage rates of the fracturing fluid were 19.4%,18.2%and 18.8%at 25 ℃,60 0C and 90 ℃,respectively,which is less than the recommendation of industry standard(30%).Therefore,the fracturing fluid has great guiding significance for promoting the fracturing technique development of unconventional low permeability reservoirs,especially the ultra-high temperature reservoirs. |
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