Foamed Acid, A New Concept In Fracture Acidizing
Authors: Scherubel Gary A. , Crowe Curtis W. 
Published: December 2007

Leakoff control is the key to effective stimulation of carbonate reservoirs by acid fracturing. Converting the acid to a stable foam is one method of greatly improving control of fluid loss to the formation. This method is particularly useful at high temperatures since most of the commonly used fluid-loss additives are destroyed by acid at elevated temperatures. Foaming the acid not only provides fluid-loss control but retards acid reaction rate.

Acids are easily converted to a stable foam by adding a foaming agent and injecting gases at the surface. For high temperature applications, foam stabilizers can be added to the acid to enhance foam stability.

This paper presents data showing the effect of foam quality, permeability and temperature on leakoff rate in limestone cores. Recommendations are presented for improving the effectiveness of foamed acid treatments. Also, field results are presented to show the increased effectiveness presented to show the increased effectiveness of stimulation treatments using foamed acid.

Introduction

One of the most difficult problems encountered in well stimulation is the control of fluid loss during acid fracturing treatments. Most commonly used fluid-loss additives are either destroyed by the acid or rendered ineffective at elevated temperatures. The constant and rapid acid erosion of the fracture faces further camplicates the problem by preventing filter cake formation.

Foams present a new approach for achieving acid fluid-loss control. Previous laboratory studies have demonstrated the effectiveness of foam as a fracturing fluid, emphasising its fluid-loss-control properties. These same properties have also been properties. These same properties have also been attributed to foamed acid; however no substantiating data have yet been presented This paper presents laboratory data describing fluid-loss-control properties of foamed acid against limestone and illustrates the difference observed between acid and nonreactive foamed fluids.

ACID FRACTURING

Leakoff control during acid fracturing treatments has proved to be of prime importance in achieving optimum stimulation results. Although a number of authors have proposed techniques to control acid proposed techniques to control acid fluid-loss, new and more efficient methods are constantly being sought. Most conventional fluid-loss additives control leakoff by depositing a low permeability filter cake against the fracture face. Fluid loss is thus reduced by a wall-building mechanism.

More recently, foams have been used to achieve fluid-loss control in acid fracturing treatments. One of the advantages of foam is its quality of being a clean fluid, void of particulate fluid-loss additives which may have potential for producing formation damage. Also, since producing formation damage. Also, since foams are not wall-building fluids, leakoff control is not affected by fracture face erosion.

The flow of foams through porous-media has been investigated by several porous-media has been investigated by several authors. It has been demonstrated by Bernard and Holm that the effective permeability of a porous media is greatly permeability of a porous media is greatly reduced in the presence of foam.