Acid re-fracturing: Is it a good practice?

The success of acid fracturing depends on the conductivity created and retained under closure stress in addition to the length of conductive fracture. Majority of acid fracturing treatments show a sharp decline in conductivity with increasing closure stress with almost no significant conductivity after a short production time. As a result, many wells are re-fractured in order to restore back to the original productivity after the initial fracture. However, the success of these re-fracture treatments has been diverse with respect to extent of stimulation. The effect of re-fracturing on acid fractured wells has not been studied and the conditions that result in success or failure of re-fracturing operations are not understood. An experimental study was conducted to investigate the effect of re-fracturing on already acid fractured cores exposed to closure stress. Indiana limestone cores were acidized with a typical acid system of 15 wt% HC1 acid viscosified with a polymer under typical field conditions. After the first acidizing process, conductivity measurements were conducted on acid-etched core faces up to a certain closure stress. While the fracture was kept under the closure stress, a re-fracturing treatment was conducted under the same conditions as the initial acidizing. The re-etched fractures were once again placed under different levels of closure stress and conductivity measurements taken at each stress. Experiments were conducted under different conditions of leak-off, polymer concentration and closure stress after the first acidizing in order to determine influence of these parameters on the re-fracturing conductivity. Acid re-fracturing enhanced fracture face etching, while significantly increased fracture conductivity under closure stress. However, leak-off, polymer concentration, and closure stress did influence the degree of success of acid re-fracturing. The re-fractured sample with leak-off and lower polymer concentration resulted in the most enhanced fracture conductivity in comparison to the initial acid fracturing process.