Study on reservoir damage during acidizing for high-temperature and ultra-deep tight sandstone

Acidizing treatment for conventional sandstone reservoir is a common method to relieve skin damage near the wells. However, this method is not suitable in ultra-deep and high-temperature tight sandstone reservoir, which usually causes greater damage. Therefore, experimental studies on the damage mechanism of low-permeability sandstone reservoir were performed to obtain the main factors causing damage in this paper. First, the reaction mechanism of clay minerals with hydrochloric acid was investigated by corrosion experiment. Then, based on the core-flooding experimental setup, the damage mechanisms of pre-flush acidizing and mud acidizing were also studied under the condition of high-temperature. Finally, chelate acid instead of hydrochloric acid is selected as preflush to prevent and control the migration damage of acidified particles. The results show that: (1) The order of reaction speed of hydrochloric acid solution with four clay minerals is chlorite > montmorillonite > illite > kaolinite; after the reaction of four clay minerals with hydrochloric acid solution at 150 °C, the complete mineral structure was damaged to varying degrees, and it was easy to produce fragmentary or broken granular solids. (2) During mud acidizing for tight sandstone, some acidizing secondary products with low solubility will be produced. The secondary products such as sodium fluorosilicate, sodium fluoroaluminate, potassium fluorosilicate, calcium fluorosilicate and calcium fluoride were deduced by SEM and EDS analysis. During displacement, these particles moved and blocked the pore throat, resulting in permeability decline and reservoir damage. (3) With a low rate of dissolution of clay minerals, chelate acid can be used as pad-acid to reduce the crystal structure damage of clay minerals. This research provides a reference for the study of acidizing damage in Tarim oilfield and other ultra-deep and high temperature tight sandstone reservoirs.