Abstract
While experimental studies have shown acid type significantly influences resulting fracture conductivity, there has been limited work on how fluid properties relate to etched fracture faces and hence the resulting conductivity. The effects of acid solutions injected into hydraulic fractures created in carbonate formations can be assessed at the laboratory scale in acid fracture conductivity tests that mimic the conditions in an actual acid fracture treatment.
Many different acid systems are currently applied in acid fracturing treatments with various degrees of success. However, there is no clear understanding of the mechanisms that lead to success of a treatment. It is clear that acid properties influence and shape the success of an acid fracturing treatment. In order to develop a better methodology for design of acid fracturing treatments, the effect of acid fluid properties on the resulting conductivity and etching must be determined. A series of acid fracture conductivity tests was conducted using four commonly used acid fracturing fluids—gelled, in-situ gelled, emulsified, surfactant-based acid. Detailed rheological properties were measured in order to explain trends noted with conductivity data.
Acid system influences the degree of etching and the etching pattern due to differences in chemical and physical properties of acid systems. Under our experimental conditions, viscoelastic acid generated the greatest degree of etching and best etching pattern. Majority of experiments showed differences in conductivity among acid systems tested with most optimal acid system depending on the closure stress. While viscoelastic acid generated the highest conductivity at low closure stress, emulsified acid resulted in the largest retained conductivity at higher loads for our experimental conditions. Furthermore, effluent analysis on both the leakoff and fracture flow showed that most of the fracture face etching is the result of leaked acid into the formation with minimal etching from the fracture flow acid.
