Alternate Acid Blends for HPHT Applications

Abstract

Abstract Protecting completion and production equipment is of utmost concern during acidizing The higher the temperature, the more difficult it is to protect metal against corrosion, and the required inhibitor loadings increase with temperature, resulting in greater likelihood of formation damage. In addition, the protection times are reduced dramatically, which can potentially limit well stimulation treatments (for example, fluid volumes caused by pump time limitations). These problems become increasingly severe in formations with bottomhole temperatures greater than 250 F (120 C). Even if adequate corrosion protection can be achieved during stimulation, post-treatment production of chlorides associated with the injected HCl-based acids remains a problem. There is increasing concern when the wellbore contains high-alloy metals, such as stainless and duplex steels, which are susceptible to hydrogen embrittlement and chloride stress cracking. When combined with the possibility of erosion corrosion caused by high production rates, acidizing high-pressure, high-temperature (HPHT) wells poses risks. A combination of organic acids (acetic and formic) can be used instead of hydrochloric acid (HCl) to minimize corrosion problems in high-temperature applications. The blends are designed so that the dissolving power is equivalent to HCl with significantly reduced corrosion rates and the absence of Cl- ions. Some of the gelling agents developed for HCl yield higher viscosities in organic acids on an equivalent polymer-loading basis. The end result is an organically based, high-temperature acid system that uses existing technology and is technically and economically more attractive than an HCl-based system. The example application includes the use of an organic acid blend to effectively stimulate the 350 F (175 C) Arun limestone formation in Indonesia. A total of 17 large-scale acid fracs and a similar number of large matrix-acidizing treatments have been performed over the past 3 years. Rheology and corrosion data will be presented with the details of the treatment procedures and production responses. Introduction Corrosion is defined as the deterioration of a substance (usually a metal) caused by a reaction with its environment. The molecules of acids ionize in a water solution to release the hydrogen ion from the acid's constituent elements. The strength of an acid is proportional to the concentration of hydrogen ions present. The attack of acid on metal tubulars manifests itself through the dissociated hydrogen ions of the acid solution, which results in the oxidation and dissolution of iron at the anodic sites on the metal surface with the attendant reduction of hydrogen ions and generation of hydrogen at the cathodic sites. Acid corrosion can be minimized by the use of corrosion inhibitors. The following factors will influence the performance of commonly available inhibitors:–temperature acid strength–contact time–inhibitor concentration–concentration and compatibility of various acid additives Increases in any of these factors, except inhibitor concentration, will increase corrosion rates. Unfortunately, the relationships are not straightforward. For example, increasing the temperature by 50 F from 150 F to 200 F will cause a less proportional increase in corrosion compared to a similar incremental temperature increase from 250 F to 300 F. A 5% increase in HCl concentration from 10% to 15% will not yield the same effect on corrosion as would a similar 5% incremental increase from 15% to 20% HCl. Acid corrosion is also strongly influenced by the chemical composition of the steel. Therefore, metal chemistry is an important factor when acid corrosion of steel is considered. P. 523