Laboratory Evaluation of Viscoelastic Surfactant Acid Diversion for Carbonate Reservoirs

Four alkyl propargyl bis(hydroxyethyl) ammonium surfactants with different lengths in hydrophobic tail (C8–C16) are prepared by direct alkylation of diethanolamine with alkyl bromides. The surfactants exhibit excellent surface properties. The critical micelle concentration (CMC) values of the surfactants 2c and 2d are found to be 7.63×10-4 and 1.23×10-4 mol/L, and the corresponding surface tension (γCMC) values are 31.83 and 31.69 mN/m, respectively. Furthermore, all of the synthesized propargyl quaternary ammonium salts are found to be very effective in inhibiting the corrosion of N80 steel (IE% = 92–98%). They can be used at a relatively high temperature (≥90°C) and a low concentration (0.039 wt.%). In addition, the apparent viscosity of surfactant 2d is studied at various temperatures, and it can be used as a viscoelastic surfactant up to 100°C. The synthesized surfactants possess multiple functions and have a very good compatibility with VES systems. They can be potentially used in various oil field applications such as enhanced oil recovery, fracturing, acid diversion, and well stimulation.

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A New Cationic Polymer System That Improves Acid Diversion in Heterogeneous Carbonate Reservoirs

SPE Journal ◽

10.2118/194647-pa ◽

2020 ◽

Vol 25 (05) ◽

pp. 2281-2295

Author(s):

Abhishek Sarmah ◽

Ahmed Farid Ibrahim ◽

Hisham Nasr-El-Din ◽

Jennifer Jackson

Keyword(s):

Rheological Properties ◽

Experimental Studies ◽

Polymer System ◽

Cationic Polymer ◽

Carbonate Reservoirs ◽

Acid System ◽

Anionic Polymers ◽

The Matrix ◽

Acid Diversion ◽

Indiana Limestone

Summary In-situ gelled acids are used for acid diversion in heterogeneous carbonate reservoirs. However, most of the gelled systems are based on anionic polymers that are difficult to clean up after the acid treatments. Residual polymer deposition leads to formation damage by blocking pore throats in the matrix. This work evaluates a new cationic-polymer acid system with self-breaking ability for application as an acid diverter in carbonate reservoirs. Experimental studies have been conducted to examine the rheological properties of these polymer-based acid systems. The apparent viscosities of the live and the partially neutralized acids at pH from 0 to 5 were measured against the shear rate (0 to 1000 s−1). The effects of salinity and temperature (80 to 250°F) on the rheological properties of the acid system were also studied. The viscoelastic properties of the gelled acid system were evaluated using an oscillatory rheometer. Dynamic sweep tests were used to determine the elastic (G′) and viscous (G″) moduli of the system. Single-coreflood experiments were conducted on Indiana limestone cores to study the nature of diversion caused by the polymer-acid system. The effect of permeability contrast on the process of diversion was investigated by conducting dual-coreflood experiments on Indiana limestone cores that had permeability contrasts of 1.5 to 20. Computed tomography (CT) scans were conducted to study wormhole propagation after acid injection for both single and dual cores. The live acid system displayed a non-Newtonian shear-thinning behavior with the viscosity declining as temperature increased. For 5 wt% hydrochloric acid (HCl) and 20 gal/t polymer content at 10 s−1, the viscosity decreased from 230 to 40 cp as the temperature increased from 88 to 250°F. Acid-spending tests demonstrated that the acid generated a gel with improved viscosity of 260 cp (at 250°F and 10 s−1) after it reached a pH of 2. The highly viscous gel plugged the wormhole and forced the acid that followed to the next higher-permeability zone. The viscosity of the gel continued to increase until it broke down to 69 cp (at 250°F and 10 s−1) at a pH of 4.8, which indicates a self-breaking system and more thorough cleanup potential. Coreflood studies indicated that the wormhole and the diversion process are dependent on the temperature and the flow rate. There was no indication of any damage caused by the system. The injected acid pore volume to breakthrough (PVBT) decreased from 2.2 to 1.4 when the temperature increased from 150 to 250°F. The strong elastic nature of the gel (G′ = 3.976 Pa at 1 Hz) formed by the partially neutralized acid system proves its suitability as a candidate for use as a diverting agent. This new acid-polymer system has significant promise for use in acid diversion to improve stimulation of carbonate reservoirs.

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