Abstract

In this paper, the results of a fundamental study on the mechanisms behind acid wormholing in carbonate formations are presented. In the wormholing process, the chemistry of the acid-rock reaction is combined with the physics of fluid flow and these two processes are the subject of study in this paper. Acid spending is studied by modeling the wormhole as a cylindrical pore and numerically solving the convection-diffusion equations. A finite acid-rock reaction rate is assumed, allowing calculation and study of spending profiles in both the diffusion-controlled and the reaction-controlled regime. The wormhole growth rate and the geometry of the final wormhole pattern are determined by the combined effect of acid spending and fluid flow. Flow properties such as fluid loss from wormhole to formation and fluid distribution are studied through numerical calculations. It is shown how wormhole growth properties are affected by the length and distance of neighboring wormholes. The effect of injection rate and diffusion is studied with a simple model. This model explains several experimentally observed phenomena, such as the existence of an optimum injection rate and a lower wormhole efficiency at higher rates.

Introduction

When acid is injected into a carbonate formation, highly conductive channels or "wormholes" are usually formed. The success of acid treatments in carbonates both above and below fracturing pressure depends heavily on the characteristics of the wormhole pattern. Wormholing has a negative effect on acid-fracturing treatments because it will increase fluid (acid) loss from fracture to formation. In matrix acid treatments, wormholes are favorable because they can bypass the damaged near wellbore region and decrease the skin. Wormholing is the result of two, physically distinct, but intrinsically connected processes:

  1. acid reaction and acid spending in the rock pores and in wormholes

  2. fluid loss from wormhole to formation and fluid distribution in a multiple wormhole geometry

A proper understanding of the combined effect of these processes is essential for the design of acid fracturing and matrix acid treatments. In this paper, the basic chemistry and physics underlying the wormholing mechanism are studied. Acid reaction and spending in a wormhole, and associated wormhole growth, is largely a chemical problem. The characteristics of the acid spending process determine whether wormholing occurs and how much reactive acid is available for length growth at the wormhole tip. In this work, acid spending is studied in two ways:

  1. modeling the wormhole as a cylindrical pore

  2. numerically solving the mass balance equations that connect acid transport by convective diffusion and acid reaction at the pore wall

Models based on acid spending in a pore have been used before in relation to wormholing. However, the mathematical problem was simplified because an infinite acid-rock reaction rate and diffusion-controlled spending was assumed. This assumption excludes the study of situations in which the acid spending rate is (partly) reaction-rate controlled, such as low-temperature dolomites or low-reactivity acids. In this work a finite acid-rock reaction rate is assumed, given by the power relation J=krateCn. If a finite reaction rate is used, the transition from a reaction-controlled spending rate to a diffusion-controlled spending rate can be studied. With this model, wormholing can be explained on a qualitative level. P. 233^

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