George McMennamy, NOWSCO Services
Matrix acidizing is often undertaken to improve the permeability of reservoir rock. When excessive quantities of ferric (Fe+++) ions are taken into solution by matrix acidizing, iron deposition and permeability reductions can occur after acidization. Sources of iron include (1) iron oxides from tubular goods, (2) pipe scale and (3) iron in mineralogical form such as siderite, pyrrhotite, magnetite, hematite and pyrite. Preventing iron deposition in matrix acidizing is essential to a successful acid treatment. This paper details a relatively new technique for controlling the precipitation of ferric compounds in matrix acidizing by incorporating a reducing agent/chelator blend. This new method is compared to three older techniques commonly used in iron control: buffering the pH, chelating, and a combination of buffering and chelating. Using standard wet chemical methods, the sequestering capabilities of the various iron control methods are evaluated at various values of pH and temperature. The effect of calcium ions on iron-sequestering capacity is also demonstrated in the paper.