Effect of Acidizing Additives on Formation Permeability During Matrix Treatments

Abstract

Matrix acidizing treatments are performed to increase the permeability of oil and gas formations and to remove various types of damage. Mineral acids, organic acids and chelating agents are used to stimulate carbonate formations and to remove damage. Sandstone formations are usually treated with mineral acids (usually containing some HCl and HF) to remove damaging solids or liquids. Usually, stimulation in sandstone is not achieved. While a reactive fluid must be present to dissolve the damage or to remove part of the formation, numerous "additives" can be used to aid the penetration of the fluid or to perform other important tasks. To enable the reactive fluid to be pumped from the surface through the tubing and into the formation, corrosion inhibitors usually will be required to protect the metal from attack by the fluid itself. Surfactants, solvents, iron control agents, non-emulsifiers and several other types of agents also may be present. There is increasing concern and some evidence (H. A. Nasr-El-Din, SPE 56712) that the additives adsorb onto the formation (or scale) and may actually be a cause of increased damage. Stimulation of carbonates frequently results in the formation of highly permeable wormholes, so adsorption of the additive may not be as important as damage caused in sandstone formation where damage removal is the primary goal. However, it is very possible that the additives may alter the surface kinetics during limestone acidizing, and thus change the degree of penetration or the efficiency of wormhole formation. To understand some of the effects of additives on acidizing operations, we have studied some commercial corrosion inhibitors, surfactants and solvents during treatments of simulated sandstone and carbonate formations. The goal of the tests was to determine if the additives are damaging the formations due to adsorption or precipitation or altering the removal of damage or the dissolution mechanisms. Two major techniques were used: linear core flood tests and rotating disk tests. Using these methods, we have determined the effects of selected additives of the effectiveness and kinetics of reaction of HCl and chelating agents on limestone and Berea sandstone. The effects of the additives depend on their individual chemical structure as well as the fluid and the formation being penetrated. In general, additives that are highly soluble in the fluid had less effect than agents that are insoluble or dispersible in the fluid. We have also shown that some additives can change the break-through volume during carbonate stimulation tests, thus altering the effectiveness of the treatment. In the paper, we will describe tests conducted at various temperatures, including tests run at 350°F to evaluate hydroxy chelating agent formulations14,15 for use in matrix stimulation of carbonates. Introduction Acidizing fluids include formulations based on hydrochloric acid (HCl), various hydrochloric acid and hydrofluoric acid (HF) mixtures (i.e., Mud Acids), organic acids and chelating agents. The major function of these fluids is to remove damage by dissolving scales and formation fines and to stimulate the formation. Various acidizing formulations also are used to treat carbonate and silicate formations and to remove scale from tubulars or surface equipment. By definition, these fluids are reactive; therefore, they will react with well tubulars or pumping equipment as well as with the formation. As a result, most of these reactive fluids require addition of a corrosion inhibitor to preferentially protect the metal surfaces from corrosive attack1.

Effect of Acid Additives on the Reaction of Stimulating Fluids During Acidizing Treatments

“…More recently, Dabbousi et al (1999) andNasr-El-Din et al (2002) showed that acidizing additives may affect dissolution of some types of scale or cause changes in the surface tension of the fluids. Frenier and Hill (2002) used linear core flood and rotating disk tests to investigate if the use of some additives such as corrosion inhibitors, surfactants, and solvents can cause damage to the formations due to adsorption or precipitation, alter the acid effectiveness in removal the damage, or change the dissolution mechanisms. The authors reached a general conclusion that acid additives that are highly soluble in the stimulation fluid, such as surfactants, have minimal effect on the performance of the fluid or on formation damage.…”

Reaction Kinetics of Gelled Acids With Calcite

“…[20][21][22][23][24][25][26][27][28][29][30][31][32] The dissolution rate was calculated from the slope of the first few points in plots of calcium concentration and reaction time. The initial surface area of the disk was also used to determine the dissolution rate.…”

“…This instrument has been extensively used to investigate the reaction of acids and chelating agents with carbonate rocks. [19][20][21][22][23][24][25][26][27][28][29][30][31][32] It has been also used to study mass and heat transfer into non-Newtonian fluids. [16][17][18] The reaction between acid and rock is a three-step process involving:…”

Influence of Acid-Fracture Fluid Properties on Acid-Etched Surfaces and Resulting Fracture Conductivity

“…While iron (III) in solution can reduce calcite dissolution rate at low rotational speeds as it is likely that iron (III) hydroxide forms a surface layer that can not be removed at low speeds, at high concentrations and high rotational speeds, it can increase significantly the dissolution rate of calcite. While some have observed that mutual solvent (10 vol% EGMBE) in acid solution can increase the dissolution rate significantly (Jordan et al 2002;Taylot et al 2004), others have not observed any noticeable changes (Frenier and Hill 2002). Corrosion inhibitor, a quaternary amine, has been shown to reduce calcite dissolution rate.…”