A novel retarded HCl acid system for HPHT carbonate acidizing applications: Experimental study

Retarded acid systems for high-pressure and high-temperature (HPHT) carbonate reservoirs are the key to provide a deep acid penetration and higher acid efficiency. The current study experimentally evaluates a new retarded HCl acid for HPHT carbonate acidizing applications. The acid retardation mechanism is attained by using a polymeric resin-based composition that restricts the proton mobility of the acid species. Ten Indiana limestone cores were used with the dimensions of 6 in (0.15 m) length and 1.5 in (0.038 m) diameter and average permeability of 9 mD (0.009 (Formula presented.)). Two sets of experiments were conducted, one at 250°F (394 K) and another at 300°F (422 K), with acid injection rates from 0.5–10 cm³/min (8.3E−9 to 1.67E−7 m³/s). The pore volume of acid required to breakthrough (PVBT) was monitored. Effluent samples were collected to track the pH and the dissolved material. Computed tomography (CT) was used to scan the core before and after the experiment to determine the wormhole propagation along with the core. The optimum injection rate at 250 and 300°F (394 and 422 K) were found to be 2.5 and 6 cm³/min (1.67E−8 and 1.25E−7 m³/s) with PVBT of 0.41 and 0.43 PV, respectively. The 3D CT scan showed an effective wormhole profile featuring a single dominant trajectory with few branches. These results are confirmed by calcium ion concentration profiles through inductively coupled plasma (ICP) analysis. The use of the new acid system will provide deep penetration with low injected acid volume. As a result, the stimulation job efficiency will improve.