A Novel Polymer-Assisted Emulsified-Acid System Improves the Efficiency of Carbonate Matrix Acidizing

Abstract

Summary In highly heterogeneous carbonate reservoirs, several acid systems were used to enhance acid diversion during matrix-acidizing treatments. Viscosified acid with polymer increases the viscosity of the acid system to improve the wellbore coverage. However, the injection of such acid at low rates had a negative effect on the spending rate and starts filter-cake formation, which inhibits the wormhole growth. On the other hand, relatively low-viscosity emulsified acid is diffusion-retarded, which makes it an effective wormholing agent at the low injection rates that occur, for example, in low-permeability or damaged formations. None of the studies in the literature addresses an acid system that uses both advantages. The objective of this work was to investigate the behavior and the performance of a new acid system, polymer-assisted emulsified acid, as a self-diverting acid by conducting viscosity measurements through coreflood study and acid-diversion experiments. The system was 15 wt% hydrochloric acid (HCl)-gelled acid emulsified in diesel with a 70:30 acid/diesel volume ratio. Coreflood experiments with Indiana limestone were conducted at 230°F at different injection rates, and the core samples were imaged with a computed-tomography (CT) scan technique after each coreflood experiment. Also, 0.5 pore volumes (PV) of the polymer-assisted emulsified acid was injected to assess the effect of the acid on the permeability of the cores before breakthrough. Finally, two acid-diversion experiments at 1 cm3/min were conducted into pairs of low- and high-permeability cores to test the effect of polymer concentration in the acid internal phase on diversion. The viscosity measurements and acid-diffusivity measurements showed that increasing the polymer concentration in the acid internal phase of the emulsified acid from 0 to 1.5 vol% significantly enhanced the viscosity of the emulsified acid and reduced the diffusion coefficient by one order of magnitude. Coreflood results showed that the polymer-assisted emulsified acid was an effective wormholing fluid at low injection rates while maintaining the high viscosity of the acid system for zonal coverage. Also, it was shown that the emulsion/polymer retention was the main source for permeability damage. However, flowback with mutual solvent removed any remaining damage, and permeability enhancement was achieved. Acid-diversion experiments are presented that show the self-diverting ability of the polymer-assisted emulsified acid into the low-permeability cores.