HP/HT Matrix Acidizing Treatments of Carbonate Rocks Using A New Retarded HCl Acid System
Authors: Ibrahim Ahmed Farid , Nasr-El-Din Hisham , Jiang Li 
Published: January 2020

Matrix acidizing has long been used to improve the gas and oil wells productivity by creating high permeability channels (wormhole) that bypass the damage. The performance of carbonate reservoir acidizing depends on the selected acid or acid mixtures. HCl has by far been the most common acid species used in carbonate acidizing. High corrosion and reaction rates of HCl especially under high-pressure/high-temperature (HP/HT) conditions limits its applications. Organic acids, such as methanesulfonic acid (MSA), can be used as an alternative to HCl. However, MSA is expensive and some shortcomings are associated with the organic acids such as product solubility, stability, biodegradability. Therefore, this study proposes a new thermally stable retarded HCl acid system that offers lower reaction and corrosion rates. The retardation effect is achieved by using a cost-effective polymeric resin-based composition that restricts the proton mobility of the acid species and, hence, reduces its reaction rate with any substrate. This adjust effect results in an apparent single-phase formulation that can be deployed with straightforward operational protocols on a commercial scale.

The new retarded HCl acid system was used to acidize 10 Indiana limestone cores (6 in. length and 1.5 in. diameter) with permeabilities ranging from 6 to 11 md. coreflood experiments were conducted at 0.5 to 10 cm3/min to construct the acid efficiency curves at 250 and 300°F. The injected acid volume to reach breakthrough (PVBT)) was recorded. Effluent samples were collected and analyzed for pH, calcium concentration, and unconsumed acid concentration. ImageJ software was used to analyze Computed Tomography (CT) scans to build image for the created wormholes.

The preliminary results show that, with proven thermal stability up to 350°F, this single-phase acidizing agent has demonstrated a customizable retardation factor of the acid's reaction rates with carbonate. The coreflood test results involving Indiana limestone acquired at 250 and 300°F, respectively, show unique breakthrough features. At 250°F, the optimum injection rate was ca. 1 cm3/min and it requires 0.34 pore volume to break through the core. At 300°F, the optimum injection rate shifted to ca. 7.5 cm3/min and it requires 0.45 pore volumes to breakthrough the core. The complimentary 3D CT scan exhibits an effective wormhole profile featuring mostly a single dominate trajectory with few sidetracks. These results are confirmed by inductively-coupled plasma (ICP) analysis of the calcium ion concentration profile, in which only minimal etching out of the core is needed to establish the conductive wormhole network through the core.

Major advantages of the new acid system include: (1) deployablility to hotter zones that are otherwise beyond the reach of raw mineral acids; (2) deeper wormholes to enhanced well productivity; and (3) cost-effectiveness because of its remarkable breakthrough efficiency in carbonate reservoir compared to conventional acidizing systems.