The Texture of Acidized Fracture Surfaces: Implications for Acid Fracture Conductivity
- Camilo Malagon Nieto (Schlumberger) | Maysam Pournik (Texas A&M University) | Alfred D. Hill (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- August 2008
- Document Type
- Journal Paper
- 343 - 352
- 2008. Society of Petroleum Engineers
- 3.2.4 Acidising, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.5.2 Core Analysis, 3.3.1 Production Logging, 4.2 Pipelines, Flowlines and Risers, 3 Production and Well Operations, 1.6.9 Coring, Fishing, 4.2.3 Materials and Corrosion, 4.3.4 Scale, 4.1.2 Separation and Treating, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation
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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.
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 nonuniformly 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 particularly rough, large pathways throughout 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 rock strength than on the initial etching pattern.
In this paper, we present an experimental methodology to characterize acid-etched rock surfaces carefully 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.
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