New Model of Acid-Fracture Conductivity Based on Deformation of Surface Asperities
- Ming Gong (U. of Texas) | Stephan Lacote (U. of Texas) | A.D. Hill (U. of Texas)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 1999
- Document Type
- Journal Paper
- 206 - 214
- 1999. Society of Petroleum Engineers
- 4.3.4 Scale, 5.2 Reservoir Fluid Dynamics, 3.2.4 Acidising, 3.3.1 Production Logging, 5.8.7 Carbonate Reservoir, 1.8 Formation Damage, 3 Production and Well Operations
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Acid-fracture conductivity is affected by the aperture and contact area of the fracture under closure stress. This, in turn, depends on the surface asperities created by the acidizing process and the mechanical strength of these asperities. Acid contact time, acid leakoff, rock mechanical properties, and formation heterogeneity all affect the creation of hydraulic conductivity of an acid fracture.
We have conducted a systematic experimental study to investigate the mechanisms of the creation of hydraulic conductivity in acid fractures, including characterization of the surface roughness created by acid etching, investigation of the damage to the rock compressive strength by acid, and measurement of hydraulic conductivity under closure stress. Experimental data show that longer acid contact results in rougher fracture surfaces and, in turn, higher hydraulic conductivity. However, acidizing also reduces the rock compressive strength causing the surface asperities to easily deform under stress.
Based on our experimental results, a new fracture deformation model was derived which considers both the surface roughness and the rock mechanical properties. The fracture closure under stress is modeled as plastic deformation of asperities. Finally, a cubic law is used to calculate the fracture conductivity. The prediction of acid-fracture conductivity using this model with appropriate parameters shows good agreement with the experimental data.
Acid fracturing is a well-stimulation method commonly used in carbonate reservoirs. In the process, an HCl solution, sometimes viscosified or emulsified, is injected into the formation above the fracture pressure to create a fracture or to open existing natural fractures. Acid etches the fracture faces unevenly, leaving a conductive pathway for reservoir fluids to flow into the wellbore. The key to successful acid fracturing is the achievement of acid penetration and the creation of sufficient fracture conductivity. Much research has been done to study the acid penetration in acid fracturing. However, the hydraulic conductivity created by acid etching is not well understood. Although there is an empirical correlation for evaluating acid-fracture conductivity reported by Nierode and Kruk1 over 20 years ago, there is no theoretical model available to predict the hydraulic conductivity of acid fractures.
Acid etching is a stochastic process and the resulting hydraulic mechanisms of acid fractures are complex. The conductivity is affected by both the aperture and the contact area of the fracture under closure stress. The damage of the rock strength at the fracture surfaces by acid adds complexity to the prediction of hydraulic conductivity of acid fractures. The leakoff of acid into the formation through the fracture faces makes the situation even more complex. Acid contact time, acid leakoff, rock mechanical properties, and formation heterogeneity all affect the creation of hydraulic conductivity of an acid fracture.
This work explores the mechanisms of hydraulic conductivity of acid fractures in two ways. The first is a systematic experimental study of the creation of acid-fracture conductivity, including characterization of surface roughness created by acid etching, investigation of the damage of rock compressive strength by acidizing, and measurement of hydraulic conductivity under closure stress. Experimental data have shown that longer acid contact results in rougher fracture surfaces and, in turn, higher hydraulic conductivity.
The second focus of this work is the mathematical modeling of acid-fracture conductivity. Several different theoretical models for fracture conductivity have been reviewed and examined. Based on experimental results, a new fracture deformation model was derived with a consideration of both the surface roughness and the rock mechanical properties. The roughness of acid etched surfaces as well as the rock strength have been correlated to acidizing conditions. The fracture closure under stress is modeled with the plastic deformation of asperities. Finally, a cubic law is used to calculate the fracture conductivity.
A total of 55 acid fracturing experiments with Indiana limestone core samples have been conducted in this experimental study. 2 For one complete acid-fracture conductivity experiment, a series of measurements or tests were conducted, including (1) acid fracturing experiment, (2) hardness test, (3) surface roughness measurement, and (4) fracture conductivity experiment. The major parameters varied in the acid-fracture conductivity experiments were the effect of acid contact time and acid leakoff rate.
Acid Fracturing Experiment.
In our experiments, we used two pieces of rectangular core samples of size 2 in. wide by 1 in. thick by 2.5 in. long to compose an artificial fracture of width 0.1 in. All the core samples were cut from one block of outcrop Indiana limestone rock to assure the core samples having properties as close as possible. The average permeability of the rock samples is about 7 md and the porosity is about 18%.
During acidizing experiments, to eliminate the gravity effect on acid etching, the acid cell was placed in such an orientation that the fracture was vertical and acid flow was upwards. Acid of 15% HCl flowed along the artificial fracture face, while the leakoff acid perpendicularly penetrated the fracture faces and went out of the cell through the leakoff lines. A pair of flow distributors were placed at both ends of the fracture to assure the acid flows uniformly through the fracture. To keep CO2 in solution, a back pressure of 1,000 psi was applied at the downstream end. The leakoff rate was controlled by setting an appropriate back pressure, around 1,000 psi, on the acid leakoff lines.
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