Application of Novel Polymer Assisted Emulsified Acid System Improves the Efficiency of Carbonate Acidizing

Abstract

In highly heterogeneous carbonate reservoirs, several acid systems have been 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 it initiates filter cake formation which inhibits the wormhole growth. On 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 address 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 thorough coreflood study and acid diversion experiments. The system was 15 wt% gelled acid emulsified in diesel with a 70:30 acid/diesel volume ratio.Coreflood experiments using Indiana limestone were conducted at 230°F at different injection rates and the core samples were imaged using a computed-tomography (CT) scan technique after each coreflood experiment. 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 cm 3 /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.

A Review on Use of Emulsified Acids for Matrix Acidizing in Carbonate Reservoirs

“…Water in oil emulsions provide better retardation due to the low diffusivity created by the oil phase around acid droplets, which allows for minimum acid volume contact with t h e r o c k − t u b u l a r s u r f a c e . T h u s , m o s t r esearch 18,26,[29][30][31][32][33][34][35][36][37]39,40,[42][43][44][45][46][47][48][49][50][51]53 has adapted W/O emulsions due to their desirable retardation effect and corrosion inhibition in comparison to O/W emulsions, 27,28 as depicted in Figure 5.…”

“…Coarse emulsions have until now been considered more cost efficient in matrix acidizing operations due to having low amount of surfactants and the ability to hold large volume of acid in contrast to microemulsions, which require large volume of surfactants for stability. Hence, very few publications − created microemulsions, whereas research studies ,− created coarse emulsions, as illustrated in Figure .…”

“…Beyond critical micelle concentration, droplet size does not decrease; however, in the case of pickering agents, this can lead to increased stability with a relatively lower amount than that used for surfactant. Moreover, macroemulsions are favored for matrix acidizing over microemulsions, which provide sufficient stability with ample acid volume. − The use of a pickering emulsion and a hybrid emulsifier (combination pickering particles and surfactants or polymers) greatly minimizes the emulsifier concentration while forming a strong structural network, which provides enhanced stability. ,, …”

“…To validate that the emulsion formed is water in oil, an electrical conductivity test is performed, which would show near-zero electrical conductivity if oil is the external phase. ,,, If there is slight conductivity, then the emulsion is stirred for an additional 10–20 min maximum to ensure proper homogenization of the phases. , This also highlights the resistance to instability by the colloid due to increase in oil percentage and viscosity. As the oil percentage increases, the emulsion viscosity increases, thereby enhancing the severity of emulsification.…”

Synthesis and Characterization of Surfactant for Retarding Acid–Rock Reaction Rate in Acid Fracturing

“…and Chan, 2004), and the fracturing fluid technique has received considerable attention. Subsequently, other fracturing fluid techniques were greatly developed, for example, hydraulic fracturing fluids (Zhang et al, 2018a;Zhou et al, 2019), oil-based fracturing fluid (Zhang et al, 2018b), emulsified fracturing fluid (Buijse and van Domelen, 2000;Sayed et al, 2012;Zakaria and Nasr-EI-Din, 2015), foam fracturing fluid (Sayed and Al-Muntasheri, 2016;Dehdari et al, 2020;Qu et al, 2020), thickening fracturing fluid (Liu and Li, 2016;Cai et al, 2018), alcohol-based hydraulic fracturing (Marrugo-Hernandez et al, 2018), and surfactant fracturing fluid (Zhang et al, 2018c;Lu et al, 2019;Mejia et al, 2019;Tangirala and Sheng, 2019;Zhang et al, 2019). Although hydraulic fracturing fluid has been widely used, it still represents the poor stability to shear resistance and serious filtration loss.…”