Fracture Conductivity Using Emulsified Acids: Effects of Emulsifier Concentration and Acid Volume Fraction
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None uniform acid etching of the face of the fracture is the primary cause of fracture conductivity in acid fracturing. The retained conductivity of the fracture after the acidizing depends on many factors some pertaining to the rock texture, some to the acidizing conditions and others to the treating fluids. Previous research focused on the texture of rock and acidizing conditions such as flow rates, temperature and contact times. These parameters along with the type of acid used proved to affect the fracture conductivity as was shown in previous research. This paper focuses on the fracture conductivity after acidizing it with the emulsified acids. The objective of this work was to study the effect of droplet size, acid volume fraction and emulsifier concentration on the fracture conductivity. Indiana limestone rocks were acidized with different formulations of emulsified acid. A 15 wt% HC1 in diesel emulsion with 0.5, 0.6 and 0.7 acid volume fractions and with emulsifier concentrations of 1, 5 and 10 gpt were used in the tests. The tests were run using a modified API conductivity cell at 200oF and 1,000 psi. Each test was run for 30 minutes at a flow rate of 1 Litter per minute. The experimental setup provided close approximation to the field conditions. The standard fracture conductivity measurements were done after each experiment and the etched fracture surfaces were characterized using a laser profilometer. The results of the experiments showed that channeling and large-scale features on the etched surfaces were the dominant factors for fracture conductivity for the course emulsions. Higher acid volume fractions weakened the rock and caused the fracture to loss its conductivity faster at high closure pressures. When emulsions with smaller droplet sizes were used, the acid penetrated deeper inside the rock and caused small-scale features on the surface rather than channeling. This feature formed a mesh-like surface that was conductive even at high closure pressures. These new findings can be used to optimize the use of emulsified acid in the field. Copyright 2008, International Prtroleum Technology Confrence.
