Matrix Treatment Employs New Acid System for Stimulation and Control of Fines Migration in Sandstone Formations

Summary A matrix acidizing system employing fluoboric acid (HBF4) has been developed to stimulate problem sandstone formations. The fluoboric acid hydrolyzes to generate hydrofluoric acid (HF), thus achieving deeper live-acid penetration. Laboratory tests also show that spent fluoboric acid reacts with undissolved clay to reduce cation exchange capacity greatly and render the clay insensitive to incompatible fluids. Introduction Sandstone matrix acidizing long has been used as a means of improving production of oil. and gas by removing formation damage and increasing permeability of the zone immediately around the wellbore. Laboratory and field studies1–4 have demonstrated the effectiveness of this type of treatment. In spite of widespread use, however, many formations do not respond satisfactorily to conventional HCl/HF treatments. This normally is attributed to rapid spending of HF near the wellbore. Some wells initially show good stimulation but later experience an unusually rapid decline in production rate. Such production declines commonly are observed in wells producing from both consolidated and unconsolidated sands. The declines usually are attributed to plugging by migratory clays and other fines. Formation plugging by migratory fines was demonstrated by Krueger et al.5 in both laboratory and field studies. Core test results revealed that high flow rates caused dislodgment of fines with resulting loss of permeability. Krueger also reported field studies in which the production decline rate was accelerated by flowing wells above an optimal rate. As the production rate was increased, a corresponding increase in inorganic solids content of the produced oil also was observed. These observations strongly support the theory of formation damage from fines migration. Migratory fines apparently are released first by exposure to strong acid and later by mechanical forces resulting from the increased flow of produced fluids. Various treatments and production techniques6 have been devised in an effort to control or minimize this problem. Krueger et al.5 and Templeton et al.7 described treating techniques in which wells, following stimulation, were returned to production at a gradually increasing rate to minimize fines migration. Various clay stabilizing agents8,9 also have been applied in an effort to control movement of fines. Also, delayed acting acidizing systems7,10 have been developed to provide deeper live-acid penetration and, thus, remove damaging fines some distance from the wellbore.