Successful Sandstone Acid Design Case Histories: Exceptions to Conventional Wisdom

Abstract

Abstract The most common purpose of sandstone matrix acidizing is removal of siliceous particles such as formation clay, feldspar, and quartz fines that are restricting near wellbore permeability, plugging perforations or gravel packs. This is accomplished with injection of acid formulations containing hydrofluoric (HF) acid or its precursors, as HF is the only common acid that dissolves siliceous particles sufficiently. Such plugging particles may be naturally occurring or may have been introduced into the formation during well operations. The formation damage such particles can cause may occur during any well operation, including drilling, completion, workover, stimulation, and production. HF acid reaction in sandstones is rapid, therefore only matrix formation damage very near the wellbore can be expected to be treated effectively. Nevertheless, removal of near-wellbore formation damage can result in several-fold increases in well productivity. However, the risk of failure is fairly high. Beginning with the groundbreaking work of McLeod1, sandstone acidizing guidelines based on mineralogy and permeability considerations have been developed and expanded in recent years, sometimes in the interest of increasing the frequency of treatment success. Selected acid types and concentrations, in particular, are now often based on such guidelines. HF acid guidelines can be useful in providing a starting point in the treatment design process. However, indiscriminate reliance on them can still lead to disappointing results. Reliance on such guidelines alone violates their original intent, and diminishes their meaning. This paper presents case studies in which successful sandstone acidizing treatment designs were unexpectedly out of step with conventional guidelines which did, however, provide meaningful starting points in acid selection in some cases. Successful treatment procedures were instead developed from diagnosis of formation damage contributors, evaluation of production and stimulation histories, and a philosophy of flexibility and continuous improvement in stimulation procedures and execution practices. Case examples include well and field-wide treatments conducted in North America, South America, and the Far East.