Abstract

The successful matrix stimulation of carbonate reservoirs requires that live acid penetrate past the formation damage. Effective acid penetration is facilitated either by transport through natural fractures or the formation of dissolution channels (wormholes) during the acidizing process. The optimal formation of wormholes and/or transport of live acid along natural fractures depends on the rate of reaction between the acid and the formation minerals, the rate of acid transport to the mineral surface, and the rate of acid convection along the wormhole and/or fracture. There are three prevailing theories in the literature that estimate the efficacy of acid stimulation treatments in carbonate formations. These theories predict the conditions at which wormhole formation occurs most efficiently and are based on the existence of:

  1. an optimum injection rate at a transition between reaction-rate limited and fluid-loss limited regimes,

  2. an optimum Peclet number, and

  3. an optimum Damköhler number.

The models are compared with a large set of experimental data representing a wide range of conditions encountered worldwide. The results show the conditions under which each theory is most appropriate. Parameters studied include injection rate, temperature, and fluid-mineral system. Validated models are then used to simulate skin evolution during actual matrix acidizing treatments. Results of the laboratory and field validations highlight gaps in the current technology and provide directions for further investigation.

Introduction

The injecting acid into a carbonate reservoir results in the formation of highly conductive flow channels commonly referred to as wormholes. These flow channels form when the dissolution is influenced by the rate of mass transfer and the rock is high soluble in the acid. (Typical carbonate formations are close to 100% soluble in many acids and chelants). Under the appropriate conditions, the nonuniform acid velocity field created by local formation heterogeneities leads to uneven rock dissolution and the formation of dissolution channels that may extend deep into the rock matrix. The structure of these dissolution channels have been experimentally observed to depend on operating conditions, including temperature, acid injection rate, formation properties, and reactant properties. The dissolution structures range from deep penetrating wormholes that effectively bypass formation damage to the compact dissolution of the wellbore wall resulting in nothing more than an increase in wellbore radius. For a carbonate matrix stimulation treatment to be successful, it is important to acidize under conditions that will lead to the formation of deep penetrating wormholes using minimal acid volumes.

Numerous carbonate dissolution models have been developed over the past 30 years to determine the optimum conditions for wormhole formation and to determine the rate of wormhole growth. The objective of this work is to review the existing theories and models of wormhole formation and to identify the conditions under which the models apply. Technical limitations for each model are also discussed.

Background

Wormhole formation and skin evolution. One of the main challenges in predicting the effectiveness of a carbonate stimulation treatment is accounting for the wide range of dissolution structures that can be formed and their impact on skin evolution. The structure of the dissolution channel is highly dependent upon the injection rate and fluid/mineral properties. The main types of dissolution structures include:

  1. Face dissolution,

  2. Conical wormholes,

  3. Dominant wormholes (long, narrow, relatively unbranched wormholes - represents optimum),

  4. Ramified wormholes, and

  5. Uniform dissolution.

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