Abstract

Stimulation of carbonate formations by acid dissolution of the rock has been an efficient and successful method of bimproving production in oil and gas wells. Hydrochloric acid is the normal fluid of choice. However, in high temperature applications corrosion issues limit usage, especially in chrome completions. Acetic acid has been used with some success and with adequate corrosion protection. But due to its low reactivity at higher temperatures, the efficiency with which a gallon of acid dissolves the formation is perceived as low. This perception comes from reaction efficiency of acetic acid reported in the literature ranging in values from 90% at 25 °C to 40% at 121 °C for 2 to 15 wt%, respectively. Acetic acid reaction on calcium carbonate is controlled by its small dissociation constant, 1.754E-05 at 25 °C (77 °F) and therefore is labeled a weak acid.

Experimental data presented here illustrates that taking advantage of the low concentration acetic acid's higher efficiency could give significant advantages to removal of carbonate rock under high temperature conditions. Dissolution of calcium carbonate using 5 molar acetic acid and dilution using fresh water has been evaluated at temperatures of 25 ° to 125 °C (77 ° to 257 °F) to determine what changes in reaction efficiency can be attained. With no shut-in, an 8.5 to 35% increase in efficiency was observed by decreasing the injection rate. This technique should be exceptionally useful in the petroleum industry to acid-stimulate wells with carbonate formation at high temperatures and containing chrome tubing. A further application of the technique could be removal of calcium carbonate scale under similar conditions.

Introduction

Well completions, cleanout and stimulation operations in wells with high bottomhole temperatures have for years beenconducted using organic acids because of their low corrosivity.1,2 In addition, more significant control of wormhole formation has been observed in matrix acid stimulation of carbonates.2,3 While hydrochloric acid is cheaper its reactivity is fast and complete organic acids with small dissociation constants react slower and not to completion.4,5 The slower reaction rates of the organic acids also allow for retardation and therefore deeper penetration into a reservoir by the acid.

Several studies into wormhole generation and comparisons of laboratory to field results have been conducted.2,3,6,7 Optimal injection rate for major wormhole generation was found to be lower for acetic acid, than hydrochloric acid.7 Also, it was found that larger wormholes could be generated using acetic acid.

In the treating of oil reservoirs with acids potential asphaltene sludging is possible. Rietjens et al. (1997), found that acetic acid tested with three different oils and using several strengths did not induce sludge in most cases.8 Even in the presence of 1000 ppm Fe(III). Another study demonstrated acetic acid stimulation fluid with 3000 ppm Fe(III) only produced a trace of precipitate.3

The reaction of acetic acid and other "weak: acids on calcite have been studied by many.8–13 Principle equations of this reaction are:

Equation (1) (Available in full paper)

Equation (2) (Available in full paper)

Equation (3) (Available in full paper)

Equation (4) (Available in full paper)

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