Optimization of Surfactant-based Fluids for Acid Diversion

Abstract

Abstract This paper examines the use of surfactant gels during acid injection and describes the optimization of these fluids. Unlike available viscoelastic surfactants used today in the field, this surfactant is cationic at low pH values. If used in live acids, the fluid has relatively low viscosity when pumped. However, once the acid is spent the surfactant molecules significantly increase the fluid viscosity. To further enhance diversion, the acidic fluid can also be foamed. Alternately, brine gelled with surfactants can be foamed and utilized for diversion. Rheological measurements were conducted on Hastelloy fitted rotational viscometers at temperatures from 70 to 300°F. The effects of surfactant concentration and acid additives on the apparent viscosity of various surfactant-based fluids were investigated in detail. The surfactant was found to be stable and compatible with most acid additives. Some corrosion inhibitors adversely affected the apparent viscosity of surfactant solutions. The apparent viscosity increased with salt concentration. The apparent viscosity of the surfactant solutions can be predicted using Carreau-Yasuda model. Coreflood tests indicated that the surfactant delayed acid breakthrough in carbonate cores. Acceptable corrosion rates were obtained when this surfactant was added to the acid. The performance of this surfactant was validated with field trials. The surfactant was used in more than fifty field treatments. It was used with up to 28 wt% HCl as in-situ acid diverter. It was also used to enhance the stability and apparent viscosity of foams used for acid diversion in power water injectors. Introduction Surfactant gels have been used in acidizing, frac-packing, and traditional hydraulic fracturing since the late 1970's.1,3 During acidizing treatments of carbonate reservoirs the acid will enter the most permeable or least damaged zones. The acid will form highly conductive channels or "wormholes"4,5 Most of the fluid will flow into the path of least resistance leaving large portions of the formation untreated. Therefore, diversion is critical to the successful matrix treatments of the entire interval of interest. This diversion can be accomplished through mechanical, chemical means or both.6–12 Many chemical means exist including polymer gels, foams, oil- soluble resins, rock salt, and surfactant gels.6 Surfactant gels have recently become an increasingly popular choice for viscosifying acidic fluids.13–15 Extensive laboratory testing has been published to better understand how viscoelastic surfactants work in the field. It has been found that the rheological properties are affected by numerous factors such as surfactant concentration, temperature, acid concentration, acid reaction products, solvents, salt concentrations, and other additives such as corrosion inhibitors.16,17 Surfactants gels can be prepared using anionic, cationic, or amphoteric surfactants.13 Amphoteric surfactants have been the subject of several lab 16 and fields studies.7,10,11 The present study will focus on cationic viscoelastic surfactants. There are several advantages of the using the cationic surfactant tested in the for carbonate acidizing. The surfactant will reduce the volume of surfactant needed because it has a high activity (nearly 75 wt%); whereas amphoterics generally have a much lower activity (30–40 wt%). The surfactant does not require co-surfactants or additives to perform adequately. It is safe because methanol is not needed as an additive for performance. Finally, this surfactant, being cationic, will propagate in carbonate formations with minimum losses due to adsorption on the rock surface. The objectives of this study are to:study the rheological properties of the surfactant gelling agent with various common acid additives,assess the impact of the surfactant gelling agent on acid propagation in a core study, andvalidate the surfactant gelling agent with field trials. Experimental Studies Materials Hydrochloric acid (HCl) was obtained from Fisher Scientific with an assay of 36.2 to 37.5 wt% HCl. The concentration of HCl in each bottle was measured by titration with 0.1 N NaOH using an auto-titrator (Mettler DL70 ES). Calcium carbonate powder, sodium, calcium and magnesium chloride salts were ACS grade chemicals. The viscoelastic surfactant was nearly 75 wt% active. It is a cationic surfactant based upon quaternary ammonium chemistry.