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

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      Welton, Thomas Donovan, Van Domelen, Mary Susan -

      Abstract: Abstract This paper discusses the   development of a unique in-situ crosslinkable acid system that   uses a blend of 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 utilized 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   due to 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.[1-4] 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.[5-7] One drawback to 13Cr and duplex CRAs is that   they are highly susceptible to corrosion by mineral acids such as   hydrochloric acid (HCl).[8-12] One potential solution to this   problem is to use organic acids. Organic acids have been   extensively utilized in the acid stimulation of hydrocarbon   reservoirs.[13-29] The use of a combination of organic and   inorganic acids dates back to 1978.[30] More recently,   Nasr-El-Din and coworkers studied the rates of reactivity by   rotating disc method.[31] Organic acid systems may be more   attractive than HCl systems due to 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 15% 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.[32-34] Commonly used   high-temperature, acid-gelling agents are copolymers consisting   of various ratios of acrylamide (A), acrylamidomethylpropane   sulfonic acid (AMPS), quaternized dimethylaminoethylacrylate   (DMAEA-Q), and quaternized dimethylaminoethylmethacrylate   (TMAEMA-Q). The ratios of these monomers in the polymer will   control the viscosity of the polymer on a per-lb 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.[35-36]

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