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

Abstract

Abstract 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. Introduction Acid fracturing is a well stimulation process in which acid dissolution along the face of the hydraulically induced fracture is expected to create lasting conductivity after fracture closure. However, conductivity after fracture closure is created by acid only if the fracture face is non-uniformly etched by the acid, so that parts of the fracture face that have not been etched deeply serve as pillars to maintain open flow pathways when the fracture closes. At the scale at which acid fracture conductivity is measured in the laboratory, the texture of the fracture face should have a dominant influence on the resulting fracture conductivity, at least at low closure stresses - if the fracture faces are smooth, only a narrow slit will remain when the fracture closes and conductivity will be low, while if the fracture surfaces are very rough, large pathways though the fracture will be propped open by the large surface asperities, and conductivity will be high. As the closure stress is increased, surface features along the fracture faces may be crushed, and eventually, at high closure stress, the lasting fracture conductivity may depend more on the rock strength than on the initial etching pattern. In this paper, we present an experimental methodology to carefully characterize acid-etched rock surfaces, and then relate the fracture surface features to the measured fracture conductivity. The preliminary results presented here show how statistical properties of the surface roughness distribution are related to the fracture conductivity. One of the first attempts to analyze how surface topography is related to fracture conductivity was presented by Ruffet et al.1 They presented an experimental methodology to characterize the etched surfaces quantitatively and evaluate its relationship with the acid injection conditions. In their approach, the surface profile was measured using a mechanical profilometer after each etching experiment; these digital data were used to calculate the statistical measurements of the data distribution and the linear and absolute roughness values; finally, they use a global roughness parameter that encapsulates all of them for comparing different treatment conditions. In addition, they estimate the mechanical behavior of the surface under closure stress, using digitized profile data to calculate specific topographic descriptors which were used to calculate the final fracture conductivity.