In an acid fracturing treatment, fracture conductivity is created by differential etching of the fracture surface by the acid – without non-uniform dissolution along the fracture face, the fracture will close after pumping ceases, and little lasting conductivity is created. In spite of this critical role of differential etching on the creation of fracture conductivity, little is known about the texture of the fracture surface created during acid fracturing, and 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 for accurately and rapidly measuring the surface profile of a rock sample, and have used it to measure 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 that are 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 quantitatively characterize the etched surface topography.
We have measured the etched fracture surface profile for a wide range of acidizing conditions. The etched surface characteristics depend very strongly on the acidizing conditions, including acid type and strength, solution apparent viscosity, velocity in the fracture, and leakoff rate, and rock type. Results for typical acid fracturing fluids and conditions are presented as well as recommendations for fluid systems that create the most small scale differential etching.