Abstract
Stimulating offshore sandstone formations with depleted reservoir pressure is a challenge. The conventional practices for sandstone acidizing are to pump pre- and post-flush hydrochloric acid to minimize precipitates from the main hydrofluoric acid reacting with the carbonates, and to flow back the well immediately after the acid is pumped. However, in offshore wells, limited deck space for chemical mixing equipment and depleted reservoir conditions necessitated a retarded single-stage HCl/HF acidizing approach with nitrogen.
Because cores were not available, solubility testing was performed using cutting samples with normal HCl/HF acid and retarded single-stage HCl/HF acid. The retarded single-stage HCl/HF acid outperformed the conventional HCl/HF acid in these tests. Because of the challenge in treating long perforated intervals with various permeability zones and depleted reservoir conditions, the operator chose to pump retarded single-stage HCl/HF acid with nitrogen as a diverting and lifting agent. The retarded single-stage HCl/HF acid system eliminated the requirement of pre- and post-flush HCl acid stages, reducing the treatment complexity and overall treatment time.
The successful single-stage HCl/HF acid treatment resulted in a 225% increase in oil production and a 416% decrease in water cut. The decreased drawdown around the wellbore helped improve the oil-water ratio and deliver a lower water cut.
This paper summarizes the challenges of testing, designing and pumping the single-stage sandstone acid system in one well offshore Vietnam.
