A Novel Application of Closed-Fracture Acidizing

Abstract

Summary The success of acid fracturing depends on the conductivity created and retained under closure stress in addition to the length of conductive fracture. The 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 refractured to restore them to the original productivity after the initial fracture. However, the success of these refracture treatments has been diverse with respect to extent of stimulation. A new approach to overcome productivity decline of acid-fractured wells is to perform closed-fracture acidizing (CFA) at the appropriate time in the life of the well to keep fractures conductive. An experimental study was conducted to investigate the effect of CFA on already-acid-fractured cores exposed to a certain level of closure stress. Indiana limestone cores were acidized with a typical acid system of 15 wt% hydrochloric acid (HCl) 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 CFA 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 were taken at each stress. Experiments were conducted under different conditions of leakoff, polymer concentration, and closure stress after the first acidizing in order to determine the influence of these parameters on the refracturing conductivity. In all cases, CFA enhanced fracture-face etching, while it significantly increased fracture conductivity under closure stress. However, leakoff, polymer concentration, and closure stress did influence the degree of success of CFA. Leakoff allowance and lower polymer concentration resulted in the most enhanced fracture conductivity in comparison to the initial acid-fracturing process.

Case Study of Successfully Staged Acid Fracturing on the Ultra-Deep Horizontal Well for the Qixia Fm HTHP Tight Carbonate Gas Reservoir in China

“…The main acids are conventional acid (CA), gelled acid (GA), in situ gelled acid, self-diverting acid (SDA), surface and in situ cross-linked acid, solid acid, and organic acid (Taylor and Nasr-El-Din, 2002;Bybee 2006;Nasr-El-Din et al, 2007;Ding et al, 2010;Gomaa and Nasr-El-Din, 2010;Liu et al, 2013). The acid frac techniques are acid frac with the aforementioned single and combined acid system, such as pad-acid-fracturing (PAF), multistage alternating injection acid frac, propped acid frac, and closed fracture acidizing (CFA) (Liu and Zhao, 1998;Nelson et al, 1998;Yi et al, 2010;Pournik et al, 2011). However, acid frac of ultra-deep high temperature and high pressure (HTHP) fractured-vuggy carbonate reservoirs is challenged by rapid acid rock reaction rate caused by limited acid-etched fracture length, high closure stress caused by limited acid-etched fracture conductivity, and natural developed fissure caused uncontrolled acid leak off and limited dynamic hydraulic fracture (Xue et al, 2019;Luo et al, 2020;Peng et al, 2019a;Li et al, 2019).…”

Research on a temporary plugging agent based on polymer gel for reservoir acidification

“…Generally, solid-phase invasion, fines migration, scale formation, and water sensitivity (Chen and Dai 2005;Wu et al 2007;Buchsteiner 1993;Sadeghazad and Beiranvand 2003) can cause pollution and plugging near the wellbore which has adverse effects on oil production. Acidizing treatment is one of the most important techniques that has been utilized in attempting to remove the pollution and recover formation permeability (King 1986;Wahib et al 2010;Buijse et al 2004;Pournik et al 2011). Acidizing treatment involves an injection of acidizing solution, such as HF, HCl, or HBF 4 into the formation which can react with clay particles, carbonate, and FeS, and thus, the permeability near the wellbore is recovered.…”