In this paper, we will describe the development and testing of hydroxyaminocarboxylic acid chelating agents (HACA) as components of matrix acidizing formulations for stimulating carbonate formations. The pioneering work of Fredd and Fogler1 demonstrated that organic acids and chelating agents have unique attributes that lend themselves to efficient production of wormholes in many acidizing situations. We have applied the techniques for determining transport and reaction kinetics to stimulation fluids containing the hydroxy amino chelating agents. Linear core flood tests were used to study wormhole formation. Rotating disk tests with marble samples were employed to elucidate the kinetics and mechanisms of the dissolution processes over a temperature range of 20 °C to > 100 °C (65 to >200 °F).

Both HACA chemicals studied, that is, hydroxyethylethylenediaminetriacetic acid (HEDTA) and hydroxyethyliminodiacetic acid (HEIDA) produced wormholes in limestone cores when tested at 150 °F. However the efficiency and capacities differ. Because these chemicals have high solubility in the acidic pH range, it was possible to test acidic (< pH 3.5) formulations. These solutions have properties of both mineral acids and complexing agents. The rotating disk tests allowed us to measure the diffusion coefficients as well as the surface reaction rate coefficients, at different temperatures. These parameters were used to determine the efficiency of wormhole formation as a function of temperature, by an overall reaction rate vs. pump rate relationship; i.e., the Damköhler number.

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