The success of carbonate stimulation treatments is highly dependent upon the formation of conductive flow channels, or wormholes. Success requires wormhole formation during matrix acidizing treatments and lack thereof during fracture acidizing treatments. The structure of the wormhole channels, which varies significantly with flow conditions and acid/mineral properties, ultimately controls the effectiveness of a stimulation treatment. This paper describes a dynamic model of wormhole formation that accounts for wormhole structure based on the Damköhler number. It addresses the issue of scaling laboratory data to the field by including the effects of fluid loss and wormhole competition. The model predicts skin evolution based on basic input parameters and is used to demonstrate matrix acidizing strategies for optimum skin reduction with several acid systems.
The results demonstrate that, under typical treatment conditions, conventional matrix treatments with straight HCl cause face dissolution and provide little reduction of skin. Under the same conditions, alternative fluids such as weak acids and emulsified HCl create dominant wormhole structures that penetrate deep into the formation. While these fluids are more effective than straight HCl (especially at high temperatures and low injection rates), additional injection strategies can be used to increase the depth of penetration and, in turn, improve skin reduction. Injection strategies such as increasing the injection rate or decreasing the rate of dissolution (by changing the fluid properties) promote deeper penetration of wormholes. These injection strategies rely on maintaining an optimum Damköhler number for effective wormhole formation during matrix stimulation treatments. The model also applies to the selection of fluid properties to control wormhole formation during fracture acidizing treatments.