Influence of Acid-Fracture Fluid Properties on Acid-Etched Surfaces and Resulting Fracture Conductivity

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.

Large-Scale Acid Fracturing Based on a Large-Scale Conductivity Apparatus

“…Although most of the acid-fracturing experiments mentioned considering the leak-off of acids, their results did not verify the wormhole development and breakthrough into the cores. 10 , 11 , 24 , 29 − 31 Based on the previous findings, the leak-off of acids was not explored in this study; however, it should be investigated in future studies.…”

“…To deeply investigate the relationship between acid etching and conductivity in large-scale experiments, the results of this study were compared with the findings of other small-scale experiments. ,− The physical and scanned morphologies in small-scale experiments had deeper channels in the fracture where an initial acid–rock reaction occurred compared with the other locations. The etched grooves were more obvious in the vicinity of the inlet.…”

“…Different conductivity results in previous studies were selected for comparison: #1 comprised SET2 4 and SET3 7 cores used by Malagon, #2 comprised the samples by Pournik, and #3 comprised the Indiana limestone data from Hill . All the data were from the experiments on gelled acid etching under similar experimental conditions.…”

“… 7 The measurement method of acid-etched fracture conductivity was introduced in 1968, 8 when Nierode and Kruk put forward an empirical equation for estimation based on their experiments. 9 Laser scanning technology was used to get the accurate morphology after etching, 10 , 11 revealing various mechanisms of an acid–rock reaction. 12 , 13 Meanwhile, innovative testing techniques and further investigations were used to study the factors influencing the acid-etching conductivity.…”

“…The measurement method of acid-etched fracture conductivity was introduced in 1968, when Nierode and Kruk put forward an empirical equation for estimation based on their experiments . Laser scanning technology was used to get the accurate morphology after etching, , revealing various mechanisms of an acid–rock reaction. , Meanwhile, innovative testing techniques and further investigations were used to study the factors influencing the acid-etching conductivity. − Cores of different dimensions were used in experiments on acid-etched fracture conductivity: the columnar cores used in the classical N-K experiment were 25.4 mm in diameter and 50.8–76.2 mm in length; , the core used by Ruffet was 100 mm in diameter and 400 mm in length . Antelo and Navarrete in their small-scale core experiment, using a modified API (American Petroleum Institute, which sets some standards for the oil industry) conductivity cell , but with a small size, proposed that representing an accurate heterogeneity of the reservoirs is difficult using small-scale apparatuses.…”