There are a number of sandstone reservoirs in which more than 50% of the matrix is composed of clay and feldspar minerals. Typically, these reservoirs are subject to fines migration and respond poorly to conventional matrix acid stimulation treatments. There are numerous challenges when treating these formations: the removal and/or stabilization of the mobile fines in the pore spaces without destabilizing the clays in the matrix or the matrix itself; the need to stimulate the formation some distance away from the wellbore, and, equally importantly, to minimize reaction products precipitating in the matrix; and the very low critical velocities that can lead to plugging while injecting the treatment. In many conventional acid treatments, after an initially good response to the treatment, the production falls to levels similar to or lower than before the treatment. A common compromise is to empirically adjust the strength of a HF/HCl acid system used to treat a particular formation, so as to delay the onset of renewed fines migration after the treatment for as long as possible, at the expense of optimizing productivity. In many cases this results in making the treatments uneconomic.
To meet theses challenges a new fluoroboric acid system has been developed. The basic chemistry used is similar to that of a retarded HF acid previously described in the literature as clay acid (Thomas and Crowe 1978).. However, unlike the retarded HF acid, the new fluid uses organic acid as a chelant and is effectively a blend of organic/fluoroboric acid and hence an organic clay acid. The fluoroboric acid is generated by the addition of hydrofluoric and boric acid. By adjusting the initial concentration and ratio of hydrofluoric and boric acid, it is possible to optimize the stimulation effect of the treatment in a particular formation and prevent future fines migration. A key is the initial concentration of free hydrofluoric acid and the available hydrofluoric acid from hydrolysis of the fluoroboric acid with respect to the clay mineralogy and temperature. The concentration of the organic acid, the chelant, is also adjusted based on an analysis of the effluent during core flow testing, to minimize precipitation.
Prior to customizing the organic clay acid system, treatments were performed in low temperature (< 140°F) reservoirs, with 30% kaolinite along with zeolite and chlorite present in the formation matrix. While there was a noted stimulation effect and fines stabilization, the initial post-treatment productivity fell short of that seen using an organic mud acid. In the case of organic mud acid, however, the production declined rapidly, indicating renewed fines migration. This led to a reformulation of the organic clay acid for use as the main treating fluid, eliminating the need for HF preflushes. The initial productivity of wells treated using the reformulated organic clay acid were higher than that obtained using an organic mud acid and remained stable, indicating effective fines migration control.
In contrast to what might be expected it has been observed during the testing that it is not always the "weakest" treating fluids that are the least damaging, especially in formations with low critical velocities. There is an apparent balance between the tendency for a fluid to move fines in the matrix and to dissolve them, with very low dissolution rates increasing the probability of plugging the formation.
Since 2003, more than 120 successful treatments have been performed using a customized organic clay acid as the main treating fluid to stimulate a variety of reservoirs previously considered untreatable or difficult to treat. The temperature of these reservoirs ranges from as low as 105°F to as high as 250°F, and the clay/feldspar content in the matrix often exceeded 40%. The treatments were greatly helped by the use of a geochemical simulator with which to optimize the acid formulations, with respect to both clay content and temperature.