For over 50 years, mixtures of hydrochloric and hydrofluoric acids have been used for the removal of near-wellbore damage in sandstone formations. Such mixtures dissolve many siliceous minerals, including clays and quartz fines, the materials most frequently associated with particulate plugging of the formation pores. Unfortunately, the dissolution of these minerals is not a simple process and various chemical reactions can result in the generation of voluminous solid precipitates or colloidal gels. The appearance of such precipitates in the near-wellbore, can ultimately cause further formation damage and negate the benefit of the acid treatment.
Other byproducts can be formed in sandstones containing carbonate minerals that react with acid and fluoride ions to produce the very insoluble calcium fluoride, CaF2, which is another potential source of formation damage. For this reason, traditional HCl:HF matrix treatments in sandstone formations are always preceded by a preflush, usually consisting of HCl to dissolve these carbonates.
This approach is not always successful and adds to the complexity of the operation. The situation is worse in multistage treatments, which traditionally involve many repeat stages of preflush, main HF-stage, overflush and diverter. In such situations, it is difficult to ensure that the correct acid stage is always entering the appropriate zone and encountering the appropriate mineralogy. The result can be poor zonal coverage, poor damage removal, creation of unexpected damage due to acid/rock incompatibilities and, ultimately, poor stimulation results.
This paper presents the results of a new acidising technique that eliminates the need for preflushes, across a wide range of mineralogies. It describes the background to this approach and reviews the results of laboratory testing and field treatments. It concludes with a matrix acidising methodology that can improve logistics, reduce cost and improve results while, simultaneously, making treatments easier to implement and control, at the field level.