Abstract
In highly heterogeneous carbonate reservoirs, several acid systems have been used to enhance acid distribution during matrix acidizing. Gelled acids are commonly used to improve the wellbore coverage. However, the injection of such acids at low rates and high temperature has a negative effect on spending rate, initiates filtercake formation which inhibits the wormhole growth, can cause formation damage and has high corrosion rates. This paper presents an innovative formulation for emulsifying the gelled acid (polymer assisted emulsified acid) for better acid treatments at high temperatures.
The objective of this work is to investigate the behavior and the performance of a new innovative acid system: polymer assisted emulsified acid as a self-diverting acid by conducting viscosity measurements, reaction rate experiments, thorough coreflood study, and acid diversion experiments. The system was 15 wt% gelled acid emulsified in diesel with 70:30 acid/diesel volume ratio.
Coreflood experiments were conducted at 230°F at different injection rates on low and high permeability Indiana limestone. Also, a 0.5 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 internal phase of the emulsified acid from 0 to 1.5 vol% enhanced significantly the viscosity of the emulsified acid and reduced the diffusion coefficient by one order of magnitude.
Coreflood results showed that the polymer assisted emulsified is an effective wormholing fluid at low injection rate while maintaining the high viscosity of the acid system for zonal coverage. Also, it was shown that the emulsion/polymer retention is the main source for permeability damage. However, flowback with mutual solvent removes any remaining damage and permeability enhancement is achieved
Acid diversion experiments were presented that show the self-diverting ability of the polymer assisted emulsified acid into the low permeability cores.
