Characteristics of Acid Reaction in Limestone Formations
Authors: Nierode D.E. , Williams B.B. 
Published: December 2007

Abstract
A kinetic model for the reaction of Hydrochloric acid with limestone bas 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 then discussed.

Introduction
To predict the stimulation ratio resulting from acid fracturing or matrix acidizing treatments it is necessary to know the rate of acid reaction under field conditions. In acid fracturing treatments, for example, reaction occurs as acid flows through a narrow fracture. Reaction in a matrix treatment occurs during flow through wormholes (channels of roughly circular cross-section) created by acid reaction. In both treatments, a large amount of mixing occurs during flow through the fracture or channel as a result of tortuosity and wall roughness.
Reaction rate can be obtained from experiments, or predicted by theoretical calculations that accurately model field conditions. In general a theoretical procedure is preferred since it can be used without recourse to laboratory testing. Acid-reaction-rate data have been reported from a number of experiments intended to simulate acid reaction in field treatments. Tests most often used are:
(1) the static reaction rate test, in which a cube of limestone is contacted with unstirred acid at a known ratio of rock surface area to acid volume;
(2) flow experiments, where acid is forced to flow between parallel plates of limestone; and
(3) dynamic tests, whine limestone specimens are rotated through an agitated acid solution.
In general, these tests model some aspects of the reaction process, such as area to volume ratio, or acid flow velocity, but do not accurately model all field conditions.
To obtain an accurate mathematical model for field treatments, assuming fracture or wormhole geometry is known, it is necessary to characterize acid reaction kinetics at the limestone surface, rate of acid transfer to the surface, and rate of fluid loss from the fracture or wormhole. (Each of these processes is shown schematically in Fig. 1.) processes is shown schematically in Fig. 1.) Reaction kinetics are independent of the geometry in which reaction occurs; therefore, once kinetics have been determined for a given acid-rock system field treatments can be simulated by prediction of the rate of acid transfer to the surface and fluid loss to the formation. Unfortunately, experiments reported to dare do not allow determination of a kinetic model.