Mechanical and Hydraulic Behavior of Acid Fractures - Experimental Studies and Mathematical Modeling




Gong, Ming

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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 a 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. There is an empirical correlation available to evaluate acid fracture conductivity, which was reported by Nierode and Kruk over 20 years ago. Acid etching is a stochastic process and the resulting hydraulic mechanisms of acid fractures are complex. The conductivity is affected by the aperture and 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 fracture 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. To study the effect of rock mechanical properties on the creation of hydraulic conductivity of acid fracture, important mechanical properties of the rock sample have been carefully measured. In order to understand the damage of rock strength by acid, the microstructures at the grain scale of core samples have been examined. Experimental data have shown that longer acid contact results in rougher fracture surface and, in tum, 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 our 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. The prediction of acid fracture conductivity using this model with appropriate parameters shows excellent agreement with experimental data.


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