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. We conducted a series of acid fracture conductivity tests using a protocol that mimics the fluxes in a hydraulic fracture, both in the main flow direction along the fracture, and in the fluid loss direction. In our tests, the injection rate into the fracture is much higher than in many previous tests, and the fluid loss flux is controlled to match field fluid loss rates. We studied three commonly used acid fracturing fluids—an acid viscosified with polymer, an emulsified acid system, and an acid viscosified with surfactants—at elevated temperatures of 200°F and 275°F. The acid fracture conductivity apparatus is similar to a standard API fracture conductivity cell, but with a capacity to hold core samples that are 3 in. long in the leakoff direction. The long cores allow for better control of leakoff as the acid creates wormholes into the core samples.
In these tests, acid was pumped through the fracture for contact times ranging from 15 to 60 minutes. After the fracture surfaces were carefully characterized with a surface profilometer, the fracture conductivity was measured at increments of closure stress, up to a maximum closure stress of 6,000 psi.
In this paper, we present the results obtained from a series of experiments with these fluids using Indiana limestone and dolomite core samples. Among the findings:
The fracture conductivity created did not show a general increase with acid contact time, and in fact decreased at higher contact times with some fluid systems. This suggests that optimal times of acid exposure in acid fracturing treatments exist.
There were large differences in the conductivity created with the three acid systems tested. At 200°F, the acids viscosified with polymer or surfactants created much higher conductivity than the emulsified acid system.
The laboratory-scale acid fracture conductivities measured in these experiments do not agree with the predictions of the Nierode-Kruk correlation.