Experiments on solubility of different acidizing fluids to Syrian limestone

Acidizing stimulation treatment always plays a very important role in enhancement of the production of oil and gas wells. Whether the characteristics of the chosen acid fluids used in the acidizing treatment meet the quality of formation determines success of the treatment. With development for more than half century, it has already evolved from conventional acids including normal hydrochloric acid and mud acid to retarded acids including multicomponent acid fluid, emulsified acid fluid, gelled acid fluid and surface cross-linked acid fluid. However, many of the unconventional oil and gas reservoirs are buried very deep, which leads to the reaction temperature between acid and formation rock becomes higher. This can fasten the reaction rate of acid and rock, decrease the effective distance of live acid, and lower the efficient acid fluids. The final result is the stimulation treatment cannot remove the damage of near well-bore, increase the production of oil and gas wells and reach the aim of deep-penetration. To solve the problems above, many researchers made character evaluation of only 1 to 2 acid fluids and the temperature in the experiments is too limited, which is not enough to many unconventional reservoirs. Besides, to retarded acids, exploring the reaction mechanism from the perspective of molecular is essential. Based on SY/T 6526-2002 of “Method of Testing the Rate of Dynamic Reaction between Hydrochloric Acid and Carbonate”,several experiments were conducted on the solubility of hydrochloric acid, organic acid (hydrochloric acid combined with acetic acid), gelled acid, emulsified acid and surface cross-linked acid to Syrian limestone powder by using acid - core reaction device under the conditions of high pressure and temperature in laboratory. The results show that，(1) The solubility of five acidizing fluids to limestone is above 90% in 2 hours. (2) The solubility of hydrochloric acid and organic acid to limestone is stronger than other three kinds of retarded acid. (3) Although the speed of dissolution of emulsified acid and surface cross-linked acid is slow, correspondingly low solubility under 95℃, due to destruction of stability of emulsified acid under the condition of higher temperature of 150℃, both the rate of dissolution and the solubility are highly enhanced. (4) In temporary condition, the inhibiting effect of “multiphase isolation” for H+ is stronger than high viscosity, and the more complex the linking between macromolecules is, the severer constraining effect for H+ will be. (5) The increase of temperature obviously speeds up the rate of dissolution, and has a great effect on the solubility of gelled acid, emulsified acid and surface cross-linked acid, but not at all for normal hydrochloric acid and organic acid. We obtained several different acidizing fluids’ reaction rates and solubility values at different times under different temperatures, which would be helpful for the optimization of acidizing fluid systems in oilfields.