Acid Re-fracturing: Is it a Good Practice?

Abstract

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% HCl 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 refractured sample with leak-off and lower polymer concentration resulted in the most enhanced fracture conductivity in comparison to the initial acid fracturing process. Introduction Acid fracturing is a well stimulation process in which acid dissolution along the face of the hydraulically induced fracture is expected to create lasting conductivity after fracture closure. However, conductivity after fracture closure requires that the fracture face is non-uniformly etched by the acid while the strength of the rock is still maintained at high levels to withstand the closure stress. While the etched pattern has a dominant influence on the resulting fracture conductivity at low closure stresses, conductivity is more dependent upon the strength of fracture face asperities as the surface features along the fracture faces are crushed under higher closure stresses.

State of the World’s Volunteerism Report 2018

“…150 Women and girls are also disproportionately vulnerable to crises, including natural disasters, and typically suffer higher fatality rates during disasters than other groups. 151 This vulnerability is often linked to cultural and behavioural limitations on their mobility, along with socially prescribed norms, roles and obligations for care taking. 152,153 Yet women are not inherently vulnerable to disasters, and their risk of dying during disasters can be greatly reduced when social norms reflect greater gender equity (box 2.5).…”

“…151 This vulnerability is often linked to cultural and behavioural limitations on their mobility, along with socially prescribed norms, roles and obligations for care taking. 152,153 Yet women are not inherently vulnerable to disasters, and their risk of dying during disasters can be greatly reduced when social norms reflect greater gender equity (box 2.5). 154 On the positive side, there is some evidence that shocks and stresses can also alter gender roles and norms -for example, when women take on traditionally male governance or economic roles during conflict.…”