REACTION RATE VS. TRANSPORT LIMITED DISSOLUTION DURING CARBONATE ACIDIZING: APPLICATION OF NETWORK MODEL.

Abstract

Coreflood experiments have been performed and a two dimensional network model has been developed to study the flow, dissolution, and permeability increase in carbonates during acid stimulation treatments. The goal is to be able to predict the behaviour of the random flow channels, called wormholes, that form in the rock as a result of acid attack. Understanding this channeling phenomenon is essential in predicting acid penetration distances and the resulting injectivity or productivity increases during matrix acidizing, or in understanding acid leakoff behaviour during fracture acidizing. Until now, relatively little has been known about the wormholing phenomenon because of the stochastic nature of channel formation and the inherent difficulty in modelling the random behaviour. Model predictions and experimental coreflood results are compared for linear systems. The model predicts wormholing and the effects of acidizing parameters such as fluid velocity, mineral dissolution rate, and rock properties on channel structure and the corresponding rate of channel propagation. Experimental data and model prediction support the conclusion that wormholing is necessary for matrix treatment effectiveness, but does not occur for the extremes of high and low values of the Damkohler number for flow and reaction, which controls the process. The model thus accounts formore » the ineffectiveness of aqueous hydrochloric acid for matrix treatments in limestone due to rapid acid spending, and for the possible ineffectiveness of treatments in dolomite due to mineral dissolution rates that are too slow to promote channel formation.« less