Thomas Welton - Optimization of Surfactant-based Fluids for Acid Diversion

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      Publication Details (including relevant citation   information):

      Nasr-El-Din, Hisham A., Van Domelen, Mary Susan, Sierra,   Leopoldo, Welton, Thomas Donovan -

      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 lab16 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: (1) study the rheological properties of the   surfactant gelling agent with various common acid additives, (2)   assess the impact of the surfactant gelling agent on acid   propagation in a core study, and (3) validate 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.

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