Evaluation of Retarded HF Acid Systems

Abstract

Stimulation of sandstone reservoirs usually involves the use of a hydrofluoric-based acid (HF). Hydrofluoric acid reacts rapidly with clay minerals and slowly with sand particles. Stimulation of sandstone formations is a challenging task because it involves several chemical and physical interactions of the acid with the formation. Some of these reactions may result in formation damage. Retarded HF (RHF) acids are less reactive with the rock and normally result in deep acid penetration into the formation. Three RHF acids that are based on boric acid (H3BO3), aluminum chloride (AlCl3), and a phosphonic acid were examined. Several tests were conducted to evaluate interactions between RHF acids and sandstone rocks. Static solubility tests were performed using sand particles and clay minerals at 25 and 75°C. Chemical analysis of spent acids was extensively used in this study. Acidizing sandstone reservoirs using mud acid is a complex process where dissolution and precipitation occur simultaneously. The process is even more complicated when RHF acids are used. The results of this study indicate that the composition of spent acid strongly depends on the retarded system used. Dissolution of clays is generally reduced when a form of RHF is employed. However, increasing the soaking time caused precipitation in all RHF acids examined. Some of the RHF acids precipitate materials that are not encountered when regular mud acid is used. This paper addresses several chemical interactions that have not been addressed previously in the literature. Introduction Sandstone formations consist of four main categories of minerals: silica (quartz), feldspars, clays, and carbonates. Mud acid is a mixture of hydrochloric acid (HCl) and hydrofluoric acid (HF). This mixture can be prepared at different HCl:HF mass ratios to prevent the formation of damaging precipitates.1 The goal of retarded HF acids is to decrease the reaction rate of HF with alumino-silicates (clays and feldspars) to achieve a sufficient acid penetration into the formation and remove deep damage. A through literature review indicates that there are at least three main retarded HF systems. A brief summary of each acid system is given below. The first system (BRHF) is based on fluoboric acid (HBF4).2 Fluoboric acid can be generated by the reaction of boric acid (H3BO3) with HF (Eqs. 1 & 2). Fast Reaction:Equation (1) Slow Reaction:Equation (2) As HF spends on siliceous minerals, HBF4 hydrolyzes to regenerate HF (Eq. 2).3–4 The second system (ALRHF) is based on aluminum chloride (AlCl3).5 Aluminum chloride reacts with HF to form aluminum fluoride species (Eq. 3).Equation (3) As HF spends on siliceous minerals, AlF4 - hydrolyzes to regenerate HF (Eq. 4).Equation (4) The third system (PRHF) is based on using a phosphonic acid complex that contains five hydrogen atoms. This acid reacts with ammonium bifluoride to produce an ammonium phosphonate salt and HF. The fluoride ions are provided by the ionization of dissolved ammonium bifluoride. To form HF, hydronium and fluoride ions combine. However, the hydronium ion concentration is low because of the weak nature of the phosphonic acid. Therefore, equilibrium is created by the buffering action of the weak organic acid and ammonium phosphonate salt.6 The objectives of this study are to:examine the solubility of sand and clay minerals in the three retarded HF systems, andcompare these systems with full strength regular mud acid (RMHF).

Sandstone matrix acidizing knowledge and future development

“…Shuchart and Gdanski (1996), (Thomas et al 2001(Thomas et al , 2002 and Al-Dahlan et al (2001) discovered that the most likely limitations of mud acid are rapid spending due to fast reaction, which results in consequent precipitations of reaction products followed by secondary and tertiary reactions (Li 2004). This limits the acid penetration in the formation especially at elevated temperatures.…”

Corrosion inhibition of N80 steel in 10% HCl + 8% HBF4solution

“…In 1933, Halliburton first injected mud acid (a mixed acid by hydrochloric acid and hydrofluoric acid [HF]) into the well, which was to improve the oil production (Gidley, 1979). Since then, various types of acid have been developed to improve the acidizing effect of sandstone reservoirs (Zou et al , 2014; Li et al , 2015; Li et al , 2016; Shafiq et al , 2017), especially the fluoroboric acid (HBF 4 ) system (8-15 per cent HCl + 6-12 per cent HBF 4 ) which has been wide applied in the oilfield (Al-Dahlan et al , 2001; Shafiq and Shuker, 2013; Nasr-El-Din and Zhou, 2013). In acidizing process of petroleum oil wells, HBF 4 system is forced into the well through N80 steel tubing to open up near-bore channels in the formation and, hence, to increase the flow of oil (Finšgar and Jackson, 2014).…”

Stimulation of High Temperature SAGD Producer Wells Using a Novel Chelating Agent (GLDA) and Subsequent Geochemical Modeling Using PHREEQC

“…For deep damage a retarded HF acid is usually recommended (Gdanski 1985;Thomas and Crowe 1978;Al-Dahlan et al 2001). Regular mud acid with 12:3 HCl:HF ratio are the most popular and traditional acid used for sandstone for years.…”