Numerous sets of well returns after HF acidizing treatments were analyzed to better understand the chemistry involved in HF acidizing. This understanding has led to more effective and efficient fluid designs. Returned samples were analyzed with various analytical techniques, including X-ray diffraction, inductively coupled plasma (ICP), and 19F nuclear magnetic resonance (NMR) spectroscopy. The combination of ICP and l9F NMR determined fluoride distribution on silicon and aluminum in the samples and the extent of HF fluid reactions.

The bottomhole static temperature (BHST) of the treated formation dominates the HF fluids’ extent of reaction in sandstone reservoirs. Analyses showed significant amounts of silicon (as silicon fluoride complexes) in returns from low-temperature formations (75°F), indicating that the secondary reaction did not go to completion. At 135°F, small amounts of silicon fluorides were present in the return samples, depending on the residence time of the fluid in the formation. At higher temperatures (150 to 200°F), the Si content of the samples was low, and no silicon fluorides returned from the wells. The overall F/Al ratios in the return samples ranged from 0.5 to 1.6, depending on the BHST, concentration of HF, concentration of HCl in the HF fluid, and contact time. Along with a better understanding of HF chemistry, the analyses presented numerous problems and concerns during the treatments, such as brine incompatibilities, complete consumption of the acid through the HF reactions, and reactions with carbonates. In most cases, these problems can easily be avoided in subsequent treatments. Close analysis of HF acidizing returns indicate that improved fluid designs can provide more effective and efficient stimulation treatments.

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