Acidizing Sandstone Formations With Fluoboric Acid

Summary The continuing search for an effective deep-penetrating sandstone acidizing system recently has focused on fluoboric acid (HBF4). Because this acid first must hydrolyze in water to produce HF, it was believed that deeper radial penetration of live acid could be achieved during matrix acidizing operations. However, a thorough experimental study has shown that typical formation temperatures will cause fluoboric acid to spend at a rapid rate similar to that of conventional hydrofluoric acid (HF), thereby severely limiting its usefulness in most sandstone formations. Furthermore, the same silica reprecipitation potential associated with HF acidizing is also inherent in the HBF4 system. Introduction It has been recognized for several years that traditional 12% HCl/3% HF systems are effective only at removing silicate damage within 1 ft (0.3 m) of the wellbore.1,2 Thus, fines and clays beyond this radius, although possibly altered, are still present and potentially mobile upon production. This ability of fines to migrate has been established in the literature.3–6 Furthermore, it has been shown6 that fines can be entrained from an outer radius of more than 5 ft (1.5 m) in a high-rate well flowing water. These observations have fueled the desire for a slower-acting sandstone acid that can remove damaging fines before they can migrate and plug. The major obstacle to deep penetration of conventional HF/HCl mixtures is their rapid reaction rate at formation temperatures with clays and grain cementing material. It has been shown that HF will spend on clays within minutes at formation temperatures.7 Therefore, to achieve net removal of material beyond 1 to 1.5 ft (0.30 to 0.45 m) would require excessive treatment volumes, which would have the undesirable effects of unconsolidating formation materials8 and undermining cement integrity. What is desired is an acid system that does not spend rapidly on formation material, thereby affording deep penetration without turning the region adjacent to the wellbore to unconsolidated sand. Various acidizing systems have been developed over the past few years with this purpose in mind.9–11 Although many of these systems have attained limited success, the problem is not considered solved entirely. Most recently,12 HBF4 has drawn attention as a possible deep-penetrating sandstone stimulation acid because it has been observed to hydrolyze slowly at room temperature to produce HF. The purpose of this work was to determine whether HBF4 would provide any damage removal advantages over conventional HF under formation conditions. In relation to this, the effect of temperature was of particular interest because experience has shown the HF reaction to be very sensitive to this factor.7 Our conclusions concerning HBF4 applicability rest on data obtained from the following experiments.When data from stirred reactions of HBF4 and HF with clay were compared, little difference was observed at typical formation temperatures.Permeability response and effluent chemical analysis data from Berea coreflood experiments failed to give evidence for increased penetration distance or different chemical response of the core.Exposure of formation core material from an interval actually producing hydrocarbons yielded data similar to those obtained with Berea, indicating that no deep penetration advantage could be expected.