Mohammed H. Al-khaldi

Diffusivity of Citric Acid During its Reaction With Calcite

The mass transfer process during the reaction of citric acid with calcite was investigated using a rotating disk apparatus. The effects of disk rotational speed, initial citric acid concentration and temperature on the effective diffusion coefficient of citric acid were examined. Using various citric acid concentrations (1 wt%, 2 wt%, 5 wt% and 7.5 wt%), the diffusion coefficient of citric acid was calculated at 25°C, 40°C and 50°C. The effective diffusion coefficient of citric acid was found to be a function of the interplay between the calcium citrate precipitation and the presence of the counter-calcium ions. At high-initial acid concentration (5 wt% and 7.5 wt%), the effects of calcium citrate precipitation and counter-calcium ions were significant and the calculated citric acid diffusion coefficients were not comparable with those obtained using the rotating disk. However, the effects of both the calcium citrate precipitation and the counter-calcium ions on the citric acid diffusivity were minimal at low-initial citric acid concentrations. The effect of temperature on the diffusion coefficient of citric acid at a constant citric acid concentration was found to follow Arrhenius law, and the activation energy was 37.9 kJ/mol.

Evaluation of Organic-Hydrofluoric Acid Mixtures for Sandstone Acidizing

Mud acid, which is a mixture of hydrochloric (HCl) and hydrofluoric (HF) acids, is typically used to stimulate sandstone formations. The success rate of mud acid is limited mainly because of its high spending rate, high corrosion rate, and incompatibility with HCl sensitive minerals; i.e., illite. Organic (acetic, formic and citric) HF acid mixtures, in comparison, have a retarded nature and a low corrosion rate, and are compatible with sensitive sandstone formations; therefore, they can be used as an effective stimulation fluid alternative to regular mud acid. In this study, dissolution tests of kaolinite, illite or chlorite in acetic, formic and citric with 3 wt% HF acid was conducted. A coreflood was conducted to investigate the ability of organic acids, when combined with HF acid, to remove carbonaceous and siliceous minerals from Berea sandstone plugs at 250 °F. The core effluent samples were analyzed using inductively coupled plasma (ICP). The core plugs were analyzed using the X-ray diffraction (XRD). All of the tested organic acids were effective in dissolving calcium during the coreflood preflush stage without any indication of precipitation; however, only citric acid extracted aluminum during the preflush, while formic and acetic acids were not effective in leaching aluminum. Additionally, low amounts of aluminum were detected in the effluent of acetic-HF during the main flush and severe damage to the core plug was noticed. Although formic-HF acid extracted a significant amount of calcium and aluminum, there was no change in permeability before or after the treatment. Citric-HF acid dissolved a significant amount of calcium and aluminum, and an apparent increase in permeability was subsequently observed. In addition, a higher permeability ratio was obtained with low HF acid concentrations. This work provides new insights into the applications and potential limitations of organic-HF acids.

Evaluation of Chelating-Hydrofluoric Systems

Different chelating agent-HF mixture systems have been used to stimulate deep hot sandstone formations. Typically, chelating agents are used as a substitute for HCl and organic acids in HF based systems to stimulate hot and exotic oil and gas sandstone formations. Apart from reducing the risk of corrosion, secondary and tertiary reactions, the system can be deployed in formations containing high clays and carbonates as well as iron and zeolite bearing sandstone formations. However, several fields’ cases indicated precipitation of aluminum-based scale following acid treatments that include these systems. The focus of this paper is to identify the type of precipitations that occur during the reaction of chelating-HF systems and determine the factors that affect these precipitations. Solutions of different ethylenediaminetetraacetic acid EDTA containing two ammonium bifluoride ABF concentrations of 0.5 and 1.0 wt% were examined in this study. Aluminum chloride or calcium chloride was added separately to each chelating-HF solution to contain various concentrations of aluminum or calcium. The filtered liquid was analyzed using inductively coupled plasma ICP, while formed solid precipitate was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The main type of precipitate in solutions containing aluminum was identified as AlF3. The amount of precipitate was found to be dependent on solution initial aluminum and free fluoride concentrations. In other words, the main factor that controlled the aluminum-fluoride precipitation was found to be F/Al ratio and above a critical F/Al value, AlF3 precipitates.