Application of capsule acid system used in deep acidizing in offshore oilfields in Bohai Bay Basin

In response to a series of issues with conventional acid fluid systems, such as high transportation risk, rapid acid-rock reaction and difficulty in achieving deep penetration, research was conducted on a capsule acid system used in deep acidizing. A capsule acid system, which remains in a solid state under normal temperature and pressure conditions while slowly releases hydrogen ions with changes in temperature and pH after entering the formation, was prepared. The interior of the capsule acid consists of a inner core composed of primary hydrogen-producing agent (hydrochloric acid and aminosulfonic acid) and primary fluorine-producing agent (ammonium hydrogen fluoride). This core is covered by a temperature-sensitive coating made of acrylic ester polymer, and the outermost layer is a pH-responsive coating made of acrylamide polymer. The preparation method of the coating material was introduced, and its solubility and mechanical strength were evaluated. The optimal synthesis conditions for the coating material were determined. Moreover, the particle size distribution, effective content and hydrogen ion release capacity of the capsule acid were measured. A capsule acid liquid system suitable for the formation characteristics of Block S in offshore oilfields in Bohai Bay Basin was developed. Laboratory experiments and field applications show that the capsule acid responds precisely to temperature and pH values, and exhibits strong hydrogen ion release capabilities, showing favorable inhibition of corrosion. This system presents excellent properties including slow-release, anti-swelling, iron stabilization, drainage assisting and blockage removal. The average effective acidification period can reach 200 days, effectively addressing the issue of short effective periods in acidizing construction in offshore oilfields. This technology provides a safe and effective means for increasing production and efficiency in offshore oilfields.