Thomas Welton - High-Viscosity-Yield Acid Systems for High-Temperature Stimulation

Document created by Thomas Welton on Feb 10, 2017
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  Welton, Thomas D., Van Domelen, Mary S. -

  Abstract: Summary This paper discusses the   development of a unique in-situ crosslinkable acid system that   uses a blend of hydrochloric acid (Hcl)/formic acid as the base   acid and a synthetic polymer gelling agent. The ability to   in-situ crosslink an organic acid blend is novel. In addition, an   unexpected result of the fluid development was the discovery of   its unique rheological properties. Historically, both gelled and   in-situ-crosslinked acids have been used for fluid-loss control   during fracture acidizing and for diversion in matrix treatments   in carbonate formations. Various synthetic polymers are used to   gel the acid. Past research indicates that ~20 cp base-gel   viscosity is required as the first step in fluid-loss control.   In-situ crosslinking allows very high viscosities to be generated   as the acid spends. The crosslinked gel creates a permeability   barrier and subsequent fluid stages are diverted to other   sections of the zone. When the acid fully spends, the gel breaks,   giving a low-viscosity fluid. HCl is the most common base acid   used for carbonate stimulation. Combinations of HCl and organic   acids have been used because of their high dissolving power and   relatively low rates of corrosion at elevated temperatures. In   extreme cases, combinations of organic acids are used. While   HCl/formic-acid blends have been used in the past, the unique   rheological properties of these blends have not been fully   explored. The chemistry and rheology of gelled and in-situ   crosslinked HCl/formic-acid blends equivalent to 28% HCl will be   described and compared with traditional gelled acid and in-situ   crosslinked acid. Introduction The stimulation of carbonate   reservoirs is often achieved through the use of fracture or   matrix acidizing. For maximum benefit, the acid system must be   properly matched with the formation characteristics as well as   the associated completion and production equipment. With higher   temperatures or acid strengths, the difficulty in inhibiting   corrosion increases along with the likelihood of formation damage   because of the inhibitor. High-alloy steels have been steadily   gaining in popularity for use in high-temperature reservoirs that   contain corrosion fluids such as carbon dioxide (CO2), hydrogen   sulfide (H2S), or corrosive brines (Murali 1984a, 1984b, 1984c;   McDermott and Martin 1992). In the petroleum industry, these   high-alloy steels or corrosion-resistant alloys (CRAs) are   commonly chromium alloys, such as 13Cr and the newer super 13Cr   (Canyard et al. 1998; Sakamoto and Maruyama 1996; Asahi et al.   1996). One drawback to 13Cr and duplex CRAs is that they are   highly susceptible to corrosion by mineral acids such as Hcl   (Nasr-El-Din et al. 2003; Nasr-El-Din et al 2002a; Crolet 1983;   Garber and Kantour 1984; Kolts and Cory 1984). One potential   solution to this problem is to use organic acids. Organic acids   have been extensively used in the acid stimulation of hydrocarbon   reservoirs (Harris 1961; Scheuerman 1988; Wehunt et al. 1993;   Fredd and Fogler 1998; Shuchart and Gdanski 1996; Coulter and   Jennings 1997; Nasr-El-Din et al. 1997; da Motta et al 1998;   Huang et al. 2000a, 200b; Wang et al. 2000; Nasr-El-Din et al.   2001; Frenier 1989; Hashem et al. 1999; van Domelen and Jennings   1995; Smith et al. 1970; Chatelain et al. 1976). The use of a   combination of organic and inorganic acids dates back to 1978   (Dill and Keeney 1978). More recently, Nasr-El-Din and coworkers   studied the rates of reactivity by rotating disc method   (Nasr-El-Din et al. 2002b). Organic-acid systems may be more   attractive than HCl systems because of their significantly lower   corrosion rates and extended reaction times. Acetic acid is   available in concentrations up to 100%, while formic acid is   available in 70 to 90% concentrations. For field use, however,   acetic solutions are normally diluted to 15% or less. At   concentrations greater than 15%, one of the reaction products,   calcium acetate, can precipitate because of its limited   solubility, depending on temperature. Similarly, the   concentration of formic acid is normally limited to 10 to 11%   because of the limited solubility of calcium formate. Gelling   agents are often used in fracture acidizing to increase the live   acid-penetration distance and to help control fluid loss. Gelling   agents can also be used in wellbore cleanouts in both sandstone   and limestone formations to help transport fines out of the   wellbore. Next, the efficiency of matrix-acidizing treatments can   be enhanced with viscosified acids (Paccaloni et al. 1993; Hill   and Rossen 1994; Jones et al. 1996). Commonly used   high-temperature, acid-gelling agents are copolymers consisting   of various ratios of acrylamide, acrylamidomethylpropane sulfonic   acid, quaternized dimethylaminoethylacrylate and quaternized   dimethylaminoethylmethacrylate. The ratios of these monomers in   the polymer will control the viscosity of the polymer on a   per-pound basis, the capability and nature of the crosslink, the   viscosity profile as a function of temperature and the   upper-temperature limit of the gelled-acid fluid (Chatterji and   Borchardt 1981; Norman et al. 1981).

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