Chelating Agents as Effective Matrix Stimulation Fluids for Carbonate Formations
Authors: Fredd C.N. , Fogler H.S. 
Published: December 2007

Abstract
The success of conventional matrix acidizing treatments with hydrochloric acid is often limited due to rapid acid spending at low injection rates. Previous studies have demonstrated the effectiveness of ethylenediaminetetraacetic acid (EDTA) as an alternative to HCl for stimulating carbonate formations. This work extends the study to include other chelating agents of the aminopolycarboxylic acid group. Results show that 1,2-cyclohexanediaminetetraacetic acid (CDTA) and diethylenetriaminepentaacetic acid (DTPA) effectively wormhole in limestone, even when injected at moderate pH values and at low flow rates where only face dissolution would occur with HCl. Rotating disk experiments have demonstrated that the dissolution of calcite by chelating agents is not necessarily limited by reactants transport to the surface. Therefore, we have derived a modified Damkohler number that includes the effects of reactant transport, reversible surface reactions, and products transport. The wormhole structure and permeability response depend on this modified Damkohler. In addition, there exists an optimum modified Damkohler number at which a single dominant wormhole channel is obtained and the pore volumes to breakthrough is minimized. This optimum Damkohler number occurs at approximately 0.17 for all of the fluids investigated.

Introduction
Matrix acidizing treatments often require low injection rates to prevent fracturing the formation rock or are required in heterogeneous formations with zones of low-conductivity (which need stimulation the most) that accept acid at low rates. It is at these low injection rates that the problem of rapid acid spending severely limits the acid penetration distance.
The injection of hydrochloric acid into carbonate formations at low rates results in face dissolution, or complete dissolution of the carbonate matrix near the wellbore. This face dissolution consumes large volumes of acid and provides negligible increases in the conductivity of the formation.
Various acid systems such as oil external microemulsions containing HCl and foamed acids (nitrogen gas and aqueous HCl) have been shown to stimulate carbonate formations at lower injection rates. However, strong acids such as HCl destabilize asphaltene particles in crude oil and cause the formation of asphaltic sludge and rigid film emulsions. This common problem is even more severe when ferric ions are present. A variety of acid additives (anti-sludging agents, corrosion inhibitors, and iron reducing agents) have been used to prevent the sludging problem. However, their effectiveness is limited by the need to obtain a compatible combination of additives and a lack of understanding of the complex chemistries involved in the precipitation reactions. These limitations demonstrate the need for alternative stimulation fluids that combine the ability to stimulate at low injection rates with fluid properties that are not conducive to asphaltic sludge precipitation or corrosion problems.
Previous work in our laboratories has demonstrated that the chelating agent ethylenediaminetetraacetic acid (EDTA) can effectively wormhole in limestone, even when injected at moderate or non-acidic pH values (4 to 13) and at low flow rates where HCl is ineffective. The dissolution mechanism involves chelation of calcium ions and does not require conventional acid attack. This ability to stimulate under non-acidic conditions combined with the ability to chelate metal ions provide other benefits of using EDTA. It has been shown that EDTA does not induce the precipitation of asphaltic sludge from crude oil, even in the presence of 3000 ppm of ferric iron. In addition, corrosion is negligible for alkaline solutions of EDTA below 204 C (with possible exceptions when copper, tin, and aluminum are present). Therefore, EDTA provides the properties necessary for a matrix stimulation fluid (wormholes formed in carbonates at low injection rates) while not requiring additives to control corrosion or asphaltic sludge precipitation.
The success of EDTA as an alternative stimulation fluid for carbonate formations has led to further investigation of chelating agents of the aminopolycarboxylic acid family. P. 23