Study On Steering Performance Of Viscoelastic Steering Acid

Nowadays low permeability, ultra-low permeability reservoirs have gradually become the main battlefield of oil and gas field development. Acidizing is one of the important measures for development and stimulation of low permeability reservoir. Recent years the viscoelastic surfactant-based acid has been proposed and used widely in the field to meet the purpose of uniform acidizing, which can significantly improve the acidizing effect. However, some theoretical issues about viscoelastic surfactant diverting acidizing have not yet been understood clearly in domestic and overseas, which restricted the better play of this technology. Based on current related research result about viscoelastic diverting acid and microstructure characterization methods of surfactant solutions, relationship between microstructure, viscoelastic properties and diverting properties of viscoelastic diverting acid was studied in this paper.The effects of acid concentrations, calcium ions, surfactant concentration and alcohol concentration on the rheological properties of viscoelastic diverting acid were investigated by the steady-state viscosity test, viscoelasticity test and thixotropy test. The experimental results and rheological parameters calculated through related model showed that there is little difference in viscosity at different acid concentrations, but with decrease of acid concentration, the elastic modulus G and structural parameter λ both increase first and then decrease with a maximum when acid concentration in the range of12%to15%. When the acid concentration was8%to10%, phase separation was observed and with a mutation on rheological properties of viscoelastic diverting acid. Both calcium ions and alcohol can promote the formation of a more stable structure, indicated by a significant increase in system elastic modulus and structural relaxation time τBy employing dynamic light scattering experiments and scanning transmission electron microscopy, changes in aggregates’size and microstructure of diverting acid solution were observed. With a decrease in acid concentration the average particle size of micelles increased first and then decreased. At high acid concentrations, the system was mainly composed of long wormlike micelles which intertwined and formed a network structure and with a lower acid concentration the mesh size between network structure became smaller, the structure of system gradually became homogeneous, continuous and dense. A synchronous change in the microstructure of system and the rheological properties at different acid concentrations was observed, which indicated that the changes in morphology of micellar aggregates is the underlying causes making the differences in elastic properties and structural strength. A same law was observed when changes the concentration of alcohol.The diverting property of viscoelastic diverting acid has been studied by using microporous membranes to simulate porous medium of reservoir. With decreasing acid concentration, the diverting ability of solution first increased and then decreased. By calculating elastic viscosity, it found that the diverting performance is not only related to the static viscosity, static viscoelasticity but also related to dynamic viscoelasticity and elastic viscosity when passing through a porous medium. Simulation results also show that the model which takes the tensile pressure drop into account which caused by the elastic viscosity is more closer to the real situation.