The transport and reaction of fluids in porous media results in unique pore growth and channel evolution as the media are dissolved. This often leads to the formation of highly conductive flow channels, commonly referred to as wormholes. The objective of this work is to predict the influence of transport and reaction on the structure of the wormhole channels. An experimental and theoretical investigation of a variety of fluid systems, including strong acids, weak acids, and chelating agents, provides a wide range of conditions for studying wormhole formation. A generalized description of the dissolution phenomenon is introduced, and a common dependence on the Damkohler number is demonstrated. The Damkohler number is shown to dictate the type of wormhole structure formed by systems with various degrees of transport and reaction limitations. An optimum Damkohler number for channel formation is observed at a value of approximately 0.29 for all of the fluid systems investigated. The stochastic nature of the dissolution phenomenon is described using network models. Results from a 2-D network model and a 3-D physically representative network model agree qualitatively with experimental results and substantiate the existence of an optimum Damkohler number.

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Influence of Transport and Reaction on Wormhole Formation in Porous Media

Statistical and continuum models of fluid-solid reactions in porous media

The dissolution of calcite in CO2-saturated solutions at 25°C and 1 atmosphere total pressure

Chemical dissolution of a porous medium by a reactive fluid—I. Model for the “wormholing” phenomenon

Temperature dependence of calcite dissolution kinetics between 1 and 62°C at pH 2.7 to 8.4 in aqueous solutions

A kinetic model for the reaction of hydrochloric acid with limestone has been determined. Reaction order and rate constant for this model were calculated from experiments where acid reacted with a single calcium carbonate plate. Experiments were performed so that acid flow past the plate and mass transfer rate to the rock surface could be calculated theoretically. The resulting model, therefore, accurately represents the acid reaction process at the rock surface and is independent of mass-transfer rate. Combination of this model with existing theory allows prediction of acid reaction during acid-fracturing operations. A model for acid reaction in wormholes created during matrix acidization treatments is presented along with data for reaction of hydrochloric, formic, and acetic acids in a wormhole. Factors limiting stimulation in acid fracturing or matrix acidizing treatments are discussed. (27 refs.)

Characteristics of Acid Reaction in Limestone Formations

A procedure is developed for predicting changes in the porosity distribution in a sandstone resulting from reaction with hydrofluoric acid. This procedure is based on a theory for slow heterogeneous reactions in a porous solid where the solid matrix is consumed in the reaction process. Reaction rate data for use in this theory are obtained from experiments where acid is injected through short cores and effluent concentration measured using a fluoride specific ion electrode. This rate is found to be first order in hydrofluoric acid concentration. Variations in rate with temperature and quantity of rock dissolved are shown.

B.B. Williams

Papers

Characteristics of Acid Reaction in Limestone Formations

Hydrofluoric Acid Reaction with a Porous Sandstone