Improved Success in Acid Stimulations with a New Organic-HF System
Authors: Shuchart C.E. , Gdanski R.D. 
Published: December 2007

Recent studies in sandstone acidizing have shown the importance of HCl in maintaining the solubility of the reaction products of HF acid. However, HCl can cause such damage as corrosion, crude oil sludging, and instability of formation minerals. These problems can become severe at elevated temperatures. HF fluids based on acetic acid and formic acid can successfully overcome these problems. Despite the successful application of the organic-based HF fluids, very little work has been done to understand and optimize these systems. Fluid reactivity studies, flow tests, and flowback analyses following field treatments were undertaken as a means of studying acetic-HF and formic-HF fluids. Severe, damaging precipitation of aluminum fluorides during the HF reactions was discovered with these fluid systems. This paper will discuss these results and the development and application of a new organic-HF fluid system that overcomes the precipitation problems and maintains all the advantages of acetic-HF and formic-HF fluids. Laboratory flow tests, returns analyses, and case histories of the new system will be also be presented.

Introduction

Over the years, the need for alternatives to HCl-based acidizing fluids has been realized. Common problems associated with HCl-based fluids include high reactivity, high corrosivity, sludging tendencies when the fluids contact crude oils, and the sensitivity of clay minerals and zeolites. These problems are aggravated at higher temperatures. Organic acids, such as acetic acid, formic acid, and combinations of the two have overcome many of these problems.

Ion Exchange and Clay Swelling. One problem with organic acids that has been overlooked until recently is ion exchange. Organic acids are weak acids that have a very low degree of ionization. Therefore, an organic acid by itself cannot ion-exchange or prevent clay swelling or fines migration. Pumping plain organic acids into a formation would be like treating it with fresh water. As a result, 5% NH4Cl must be added to all organic acid fluids used in sandstone formations to overcome these problems.

HCl Sensitivity of Clays. It has long been known that a select group of minerals will decompose in the presence of HCl or any strong acid. The most common minerals in this group are chlorite and zeolites. Their instability in strong acid is directly related to their structure. Alumina layers are attacked by the strong acid, which causes their structures to collapse. This collapse results in a residual silica gel mass that can be severely damaging. Therefore, the use of organic acids in place of HCl in acidizing formations containing these minerals is imperative.

Recent research has shown that all clays (with their layered structures) have a temperature above which they are no longer stable in HCl. Fig. 1 (Page 7) plots the stabilities of clays in HCl with respect to temperature. For example, a formation at 225 F containing 10% illite would be considered HCl-sensitive; therefore, organic acid-based fluids should be used instead of HCl-based fluids to prevent formation damage.

Aluminum Precipitation. Despite the successful application of the organic-based HF fluids, researchers have done little to understand and optimize these systems. The chemistry of HC/HF fluid systems, however, has recently been studied in much detail. This paper is an extension of the earlier work, focusing on organic-HF fluid systems. Fluid reactivity studies, flow tests, and flowback analyses following field treatments were performed to study the currently used acetic-HF and formic-HF fluids. A somewhat surprising observation from these tests was the nearly complete precipitation of aluminum fluoride complexes during the HF reactions. This paper will discuss these results and the development and application of a new organic-HF fluid system that overcomes the precipitation problems and maintains all the advantages of acetic-HF and formic-HF fluids. Laboratory flow tests, returns analyses, and case histories of the new system will be also be presented.

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