The damage mechanism of spent acid in acid fracturing and damage reduction measures in Daniudi gas field

In the process of acid fracturing, spent acid after acid-rock reaction will adsorb and block matrix pores, damaging reservoir, resulting in damage to the reservoir. In order to clarify the influence of spent acid damage on carbonate reservoir of Majiagou Formation in Daniudi Gasfield, the spent acid damage experiments on matrix, natural fracture and acid etched fracture were carried out through the configuration of field spent acid, and the damage mechanism was analyzed from micro and macro perspectives. In addition, the potential sediment test of acid rock chemical reaction of carbonate rock cores containing gypsum in Daniudi Gasfield has been carried out. The experimental results show that spent acid has a high damage rate to matrix. Spent acid enters the matrix in the form of pore clogging to contaminate the core, with a failure rate of about 30%. The spent acid remains on the surface of natural- fracture, and the reservoir with high permeability is less damaged by the spent acid. Spent acid damage rate is between 10 and 20% in the natural fracture. But acid etched fracture is almost unaffected by spent acid. Therefore, the damage of spent acid is mainly caused by blockage, and the adsorption damage is relatively low. On the other hand, Gypsum has no obvious damage in acidizing fracturing. Finally, in order to reduce the spent acid damage, acid optimization measures were taken. The optimized acid can reduce the damage of spent acid by less than 10%.