Novel Low Viscosity, Single Phase, Polymer-Free Delayed HCl Acid System for Stimulation of High Temperature Carbonate Reservoirs
Authors: Zakaria Ahmed , Singh Akshaya , Bhaduri Sumit 
Published: February 2022

Abstract
Retarded acid systems are highly sought out to allow for deep penetration of acid to improve the efficiency of acid stimulation treatments. This is important in high temperature carbonate reservoirs or in long intervals that shift injection rates away from optimum, causing near-wellbore (NWB) spending and inefficient stimulation. Acid retardation can be accomplished by viscosifying acids with polymers/gelling agents or emulsifying in diesel. However, these systems had their performance limitations and associated drawbacks. This paper presents a novel low-viscosity, single-phase, polymer-free delayed HCl acid system as an alternative for gelled and emulsified acids for stimulation of high temperature carbonate reservoirs. Novel concept of acid retardation without emulsifying the acid in diesel or gelling with linear/cross-linked polymer was introduced and evaluated.
Static carbonate dissolution tests were first conducted to optimize the formulation of the new acid system to achieve the desired retardation. The retardation performance was then further evaluated using reaction kinetics measurements and coreflow testing. The diffusion coefficient of the delayed acid was measured and compared to the 15% HCl at wide temperature range (75 to 300°F). Coreflow tests were conducted on Indiana limestone cores (6 in. length and 1.5 in. diameter) with wide permeability range from 1 to 1000 md, interstitial velocity from 0.2 to 10 cm/min, and temperatures up to 300°F. The delayed acid system was also evaluated in coreflow at both 15 and 28 wt% acid strengths. Computed tomography scanning was used to characterize the wormhole dissolution pattern.
At 275°F, diffusion coefficients measured from the reaction kinetics tests are 100 times lower for the delayed acid compared to HCl. Coreflow testing results showed that the delayed acid system requires significantly less acid pore volume to breakthrough compared to HCl acid at all injection rates. At low non-optimal injection rate of 0.5 ml/min and high HCl acid strength of 28 wt%, the delayed acid requires six times less acid pore volume to breakthrough compared to 28 wt% HCl and creates a dominant wormhole rather than face dissolution. In the comparison tests with the emulsified acid, the delayed acid penetrates 40% deeper in long Indiana limestone cores at 300°F. The results confirmed that the use of low viscosity delayed acid would prevent NWB spending, improve the acid penetration to access deeper reservoir area, and eventually lead to successful acid stimulation treatment even when pumping or completion limitations force low injection rate.
The low viscosity, single-phase, polymer-free delayed HCl acid system showed tunable reaction rate, high dissolving power, low corrosion rates, low friction pressures, and no residue left in the formation combined with no special operational requirements. Most importantly, it provides the industry with a novel alternative fluid system to the emulsified and gelled acids.