The Texture of Acidized Fracture Surfaces: Implications for Acid Fracture Conductivity

Abstract

Summary In an acid-fracturing treatment, fracture conductivity is created by differential etching of the fracture surface by the acid; without nonuniform dissolution along the fracture face, the fracture will close after pumping ceases, and little lasting conductivity will be created. Despite this critical role of differential etching in the creation of fracture conductivity, little is known about the texture of the fracture surface created during acid fracturing or about the dependence of this texture on the acidizing conditions. To study this important aspect of the acid-fracturing process, we developed a new surface profilometer to measure the surface profile of a rock sample accurately and rapidly and used the instrument to characterize fracture surfaces after acidizing. The profilometer measures the distance to the rock surface with a laser device that measures distance with an accuracy of 0.001 in. The rock sample is mounted on a servo-table that automatically moves the sample in selectable increments (typically, 0.025 in.). With this device, the surface of a standard API fracture-conductivity sample can be scanned in a few hours, and a digitized profile image can be obtained. This digital image is used to characterize the etched surface topography quantitatively. We have measured the etched-fracture-surface profile for a wide range of acidizing conditions. The etched-surface characteristics depend strongly on the acidizing conditions, including acid type and strength, velocity in the fracture, leakoff rate, and rock type. Results for typical acid-fracturing fluids and conditions are presented along with recommendations for fluid systems that create the smallest-scale differential etching.