The standard description of HF-acidizing chemistry clearly demonstrates a primary and secondary reaction of HF with alumino-silicates. Field experience has taught our industry that possible precipitation during the secondary reaction can adversely affect treatment success, especially in formations with high K-feldspar content or temperatures above 300 F. Recent work has also reported the existence of a third, or tertiary reaction of HF with alumino-silicates.
This paper reports the determination of the rate law and kinetics for the primary reaction of HF on sand, clay, and feldspar over a broad temperature range. The rate law for the primary reaction of HF with sand had no HCl dependence and was second-order in HF concentration. The rate law for the primary reaction of HF with clay and feldspar was assumed to also have no HCl dependence and was adequately described by a first-order in HF concentration. In addition, a mass transfer equation for reactive fluids flowing through porous sandstone media is proposed. These findings were made possible by recently applied experimental techniques including 19F NMR Spectroscopy, fractional pore-volume flow experiments and an accurate knowledge of the HF stoichiometry. Laboratory findings have been verified through detailed analyses of flowback samples from field treatments.
Fundamental research into the detailed chemistry of HF acidizing has produced critical information that renders much earlier geochemical and engineering modeling incomplete. Geochemical models that rely on global thermodynamic completeness of reactions ignore the effects of reaction kinetics and have been unable to reproduce the observed chemical species. Engineering models have assumed mass transfer-controlled kinetics in the absence of any real information about reaction rate laws and surface-controlled kinetics. In addition, some of these latter models have required empirically adjustable parameters to reproduce short-core effluent analyses and are not based on realistic chemistry.
Now that the chemistry of HF acidizing is more accurately understood, reaction-rate laws and kinetics are required so that engineering models can be developed that will help improve treatment design and performance. Therefore, experiments were designed that allowed us to determine relevant reaction-rate laws. However, with the discovery of essentially "three reactions" of HF, the experiments had to be designed to isolate individual reactions. Recent work has documented the rate laws for the secondary and tertiary reactions and the importance of acid decomposition of clays. This paper documents the experimental determination of the rate law for the primary reaction and its consequences on treatment design.
Berea Core Flow Tests. Flow tests were conducted with the equipment and method reported earlier. The Hassler sleeve was loaded with a Berea core 12 in. long and 1.5 in. in diameter. The general procedure for the tests was to flow a preflush of HCl to remove the carbonates from the core. The reactive fluid of interest was flowed next, followed by an overflush of HCl and final displacement of the fluids from the core with NaCl brine. The reactive fluid was a mixture of HCl and HF that would allow chemical theories to be tested. The reactive fluid ranged from about 20 to 120 pore volumes (PV) in size.
Analytical. Liquids collected from the column flow tests were analyzed for the various ions in solution. Inductively coupled plasma (ICP) emission spectroscopy was used to analyze for Na, K, Ca, Mg, Fe, Si and Al. 19F NMR spectroscopy of the samples provided the fluoride distribution on Si and Al, which was used to calculate the F/Al molar ratio. Cl was determined by AgNO3 titration, and acidity was determined either by pH measurement, titration with NaOH to a pH of about 3, or by ionic balance.