New HF acid system produces significant benefits in Nigerian sandstones

Abstract

This paper details the results of several matrix acidizing treatments carried out in Nigerian sandstones, using a new Hydrofluoric acid system. This system, which contains a phosphonic acid complex, reduces the reaction rate of HF acid on clays, whilst at the same time keeping the reaction products in solution. The acid system was previously described by Di Lullo and Rae in SPE 37015. The case histories included in this paper are treatments carried out on several formations in Nigeria. They include remedial treatments carried out on zones which had experienced detrimental results due to poorly-designed conventional HF acid stimulations. The new HF acid system increases the effectiveness of the treatment in two main ways. Firstly, it forms a thin layer of an aluminum silicate phosphonate compound, which is resistant to HF acid, over the clay. This layer diverts the HF acid away from the clays, increasing the tendency to react with the silica. Quartz dissolution is also improved and acid penetration is greatly increased. This action also has the benefit of reducing the production of insoluble compounds formed as a result of the HF acid/clay reaction. The other main benefit of the system is that the phosphoric acid complex acts to chelate metallic ions, keeping previously insoluble reaction products in solution. Secondary benefits include reduced inhibitor loading and safer handling, as the pH of the fluid never approaches the low pH of the conventional systems.