Hydrofluoric acid (HF)-sensitive reservoirs that could not be hydraulically fractured or stimulated effectively have been a challenge to the petroleum industry. Experience shows that when such formations are stimulated with conventional acid systems based on HF or hydrochloric acid (HCl), production performance declines shortly after the treatments. Treatments with an organophosphonic acid complex have been performed in seven wells including oil producers, water injectors, and gas-injection wells in the Niger delta.