Study On The PH-controlled Viscosity-variable-diverting-acid

Acidizing technique is one of important increasing production and injection method forcarbonate reservoir. In conventional acidizing in heterogeneous reservoirs, the acid first flowsin the high permeable formation that its permeablity further increases, which increases thedifference of vertical permeability. The steering aciding thechnique can solve this problem.The viscosity-variable-diverting-acid technique is developed in recent years based onviscoelastic surfactant and it has the following properties, such as self-steering, slow reactionrate, and harmless for reservoir. Based on the synthesized VES-TCJ in the laboratory, thispeper investigated the influence of HCl concentration, surfactant concentration(VES-TCJ)and shearing rate on the apparent viscosity of fresh acid system. It is confirmed that theapparent viscosity is less than25mPa·s under the condition that the HCl concentration is20%and the VES concentration is5%, so the system has very good pumping ability. Insimulation reaction of acid and carbonate rocks in formation, we prepared gel forming acidand investigated the influence of pH, VES-TCJ concentration, HCl concentration,temperature, shearing rate, Fe3+, inorganic ions and the main additives in acid systems onrheological properties of gel forming acid. The experiment shows that gel forming acid hasthe max apparent viscosity331.57mPa·s at pH=2.5, temperature equal to25℃and shearingrate equal to170s-1, and realize the steering and shunt in high permeable formation. The gelforming acid has good temperature resistance and dilution shearing feature, and its viscositydepends on VES-TCJ and Ca2+concentration. The Fe3+concentration is should tightlycontrolled in the aciding process. If Fe3+concentration higher than3000mg/L, it will have astrong on viscosity of gel forming acid and even the system loss the steering and shunt ability.The main additives in acid systems have little influence on its viscosity. The other inorganicions also have influence on the rheological properties of gel forming acid. Mg2+can greatlyincrease its viscosity, but the temperature resistance is poor. Na+and NH4+both also slightly increase its viscosity. The viscosity-variable-diverting-acid with above these inorganic saltsall have the dilution shearing feature. In addition, the break character ofviscosity-variable-diverting-acid is investigated by using distilled water, kerosene and diesel,respectively. The results show that the acid viscosity is less than5mPa·s and realize gelbreaking when the ratio of gel forming acid and distilled water is1:20without gel breaker,and the solution after gel breaking gel is clear and colorless and non-residue. It also realizegel breaking when the ratio of gel forming acid and kerosene or diesel is1:3under hightemperature and great shearing stress, and the gel breaking result of kerosene is better thandiesel.