Synthesis And Properties Of Polymeric Amphoteric Surfactants For Cleaning Fracturing Fluid

Compared with conventional single stranded surfactants, Gemini surfactants have more unique surface-active properties. High surface activity leads surfactant molecules to absorb on gas-liquid interface more easily. The CMC values of Gemini surfactants are about one or two orders of magnitude lower than those of the corresponding monomeric surfactants, so the generation of the micelles is easier than the corresponding monomeric surfactants. Meanwhile, the molecules of Gemini surfactants contain multiple hydrophilic groups which make surfactant excellent solubility property at low temperature. In addition, the mixed system of Gemini surfactants and common surfactants will generate remarkable synergistic effect. Therefore, Gemini surfactants possess good prospects in the development and application of clean fracturing fluid, and become one of the research hotspot in the surfactant field.In the paper, oligomeric zwitterionic surfactant, containing three hydrophobic carbon chain, three quaternary-amine cationic and five carboxylate groups, was synthesized by using diethylenetriamine pentaacetic acid and sodium hydroxide as crudes, dodecyl bromide as the alkylating reagent, isopropanol as solvent, in rurn, through neutralization and quaternary amination reaction at80℃. The structures of final product were characterized by FTIR spectrum,1HNMR spectra, the results indicated that the final product was the desired product, sodium N, N’, N"-dodecyl-divinyl-triamine-pentacetate.The surface tension of sodium N, N’, N"-dodecyl-divinyl-triamine-pentacetate aqueous solution in different concentrations was analyzed by Wilhelmy type. The values of CMC and critical surface tension of sodium N, N’, N"-dodecyl-divinyl-triamine-pentacetate solution were1.10×10-4mol/L,32.60mN/m respectively. In addition, the maximum surface excess concentration and average minimum area per molecule of sodium N, N’, N"-dodecyl-divinyl-triamine-pentacetate were2.74×10-6mol/m2、0.61nm2, respectively. The solubility property of the surfactant was determined by using dissolution method in different temperature. The results indicated that the Krafft point of oligomeric zwitterionic surfactant was under0℃,that is, the surfactant had good solubility.The rheology properties of mixed surfactant systems consist of oligomeric zwitterionic surfactant, anionic surfactant SDS and inorganic salt KC1were measured by rheological method in different situations. With concentration of SDS increasing, the viscosity of system and the length of micelles increased first, following decreased. When the mass fraction of SDS was0.48, the viscosity value reached maximum. Moderate concentration of inorganic salt KC1could shield the electrostatic repulsion between surfactants which lead to growth of micelles and increase of viscosity of solution. However, excessive inorganic salt could break the micellar structure, and then viscosity decreased clearly. With the SDS and KC1adding, a series of shapes of micelles were observed by cryo-TEM freeze etching transmission electron microscopy, for example, formation of micelles, growth and entanglement of micelles, branching of micelles. And these explained the change of τR, η0, G0of micellar system on the mesoscopic. The effect of temperature on system rheological properties was obvious, the micellar length and viscosity of solution also decreased with temperature increasing. In addition, the viscosity value reached maximum when the pH value was7in the mixed surfactant systems. And then filtrated a set of clean fracturing fluid formula that the mass concentrations of oligomeric zwitterionic surfactant, anionic surfactant SDS and KC1were1.56wt%,1.44wt%,10wt%respectively, and the value of pH was7in the mixed solution.The properties of clean fracturing fluid were studied and shown that the clean fracturing fluid had good shear resistance, heat resistance and sand-carrying performance. The viscosity of system retained around53.6mPa·s in the88℃,170s-1, and the sedimentation rate of single sand was0.50mm/s. At the same time, the leak-off coefficient of clean fracturing fluid was4.6×10-4m/min1/2in the experiment condition, indicated that the filtration of fracturing fluid was small in the low permeability core and could meet the applied requirement. In addition, the clean fracturing fluid could be broken quickly, and the gelout time was less than1h. The surface and interface tension of gelout liquid were28.54mN/m,0.93mN/m respectively, and the damage rate of gelout liquid to core permeability was11%. It was indicated that the clean fracturing fluid could be applied to formation fracturing construction under88℃.