Optimizing HCl-Formic Acid Mixtures For High Temperature Stimulation
Authors: Dill Ray Walter , Keeney Bill Ray 
Published: December 2007

Abstract
Formic and acetic acids have been used in conjunction with stimulation and cleanout treatments for many years. They have found particular application because of their low corrosiveness on metals and compatibility with crude oils. The organic acids have been used either by themselves or in mixtures containing hydrochloric acid. The mixtures have shown the ability to develop etching of carbonate fractures that is usually representative of acid solutions containing much higher hydrochloric acid concentrations.
Recent laboratory investigations have shown that mixtures of formic acid and hydrochloric acid may have particular application as stimulation fluids in very high temperature environments. While it has been known that formic acid, or compounds that form formic acid through chemical reaction or degradation, can reduce the corrosiveness of HCl, there has been no systematic study to optimize such a system. This paper shows that the low corrosivity of HCl-formic paper shows that the low corrosivity of HCl-formic mixtures can be optimized and is a function of the ratio of HCl to formic acid and highly dependent on the corrosion inhibitor selected for use. It will be shown that mixtures containing as high as 10% HCl can be prepared which maintain low corrosivity at temperatures up to 400 deg. F without the use of inhibitor intensifiers. This low degree of corrosivity can be maintained with reduced concentrations of organic corrosion inhibitor.
This investigation resulted in the development of a retarding system that appears to function synergistically in the presence of formic acid. The reduced reaction rate on carbonates appears to be due to a change in the process controlling reaction kinetics. The retarding system changes the reaction kinetics from a diffusion controlled process to a process that is controlled by the surface reaction process that is controlled by the surface reaction rate. This approach to retardation allows deeper penetration of acid into fracture systems before the penetration of acid into fracture systems before the acid completely spends, and appears to be effective at temperatures to at least 400 deg. F.
The acid system resulting from this investigation has best application in treating wells with bottom-hole temperatures between 250 deg. and 400 deg. F. The mixed acid system can be used to prepare fluids for the following applications:perforating and completion,clay removal with HF/HCl/formic,breakdown fluid ahead of fracturing;retarded acid for fracture acidizing, andscale removal.

Introduction
Retarded acid systems are considered necessary to stimulate effectively the production of many high temperature formations. Retarded acids are used to achieve an acid penetration distance and a conductive fracture length that approaches the drainage radius of the well.
Chemically retarded and emulsified acid systems utilizing hydrochloric acid have provided adequate retardation at temperatures up to 93-3 deg. C (200 deg. F) and in many applications up to 121.1 deg. C (250 deg. F). Slower reacting and less corrosive acid systems are desirable to stimulate production in wells having temperatures above 121.1 deg. C (250 deg. F).
Formic and acetic acid have been used in conjunction with well completions, cleanout, and stimulation treatments for many years because of their low corrosivity on metals. The organic acids have been used alone, but an important application has been in mixtures with hydrochloric acid.
Organic acids, if reacted individually or as a hydrochloric-organic acid mixture, do not react generally to completion on carbonate formations. A previous investigation with acetic acid and formic previous investigation with acetic acid and formic acid shows 42% and 86% conversion to the calcium salts, respectively." Another investigation with hydrochloric-acetic acid mixtures shows that the acetic acid portion is converted to about the same extent. Fig. 1 shows data obtained with a 7 1/2 HCl-10% formic acid mixture. These data show that 86% of the formic acid in the mixture was converted to calcium formate at a temperature of 121.1 deg. C (250 deg. F).