By contrast, acid-oil emulsions for matrix acidizing are designed to work close to the borehole and have lower treatment volumes than those for acid fracturing techniques. Acid-oil emulsions for matrix acidizing of carbonate formations consist of an internal HCl phase and an external oil phase. Hydrogen ion transport from the acid droplets to the rock surface takes place by Brownian diffusion— which dramatically slows the acid reaction rate.7 Laboratory data show that when HCl droplets are suspended in diesel oil, the reaction rate can be retarded by more than an order of magnitude (right).8
In addition to the slow reaction rate
3. Fredd CN and Fogler HS: “Optimum Conditions for Wormhole Formation in Carbonate Porous Media: Influence of Transport and Reaction,” SPE Journal4, no. 3 (September 1999): 196−205.
Panga MKR, Ziauddin M and Balakotaiah V: “Two-Scale Continuum Model for Simulation of Wormholes in Carbonate Acidization,” AIChE Journal51, no. 12 (December 2005): 3231−3248.
4. Damaging particulates may include native clays and carbonates or material from drilling and workovers. Damage may also occur from other mechanisms including clay swelling, scale, organic deposits, wettability changes and bacterial growth.
5. Van Domelen MS and Jennings AR Jr: “Alternate Acid Blends for HPHT Applications,” paper SPE 30419, presented at the SPE Offshore Europe Conference, Aberdeen, September 5−8, 1995.
6. Zaeff G, Sievert C, Bustos O, Galt A, Stief D, Temple L and Rodriguez V: “Recent Acid-Fracturing Practices on Strawn Formation in Terrell County, Texas,” paper SPE 107978, presented at the SPE Annual Technical Conference and Exhibition, Anaheim, California, USA, November 11−14, 2007.
7. Brownian diffusion or motion is the random movement of particles suspended in a liquid or gas.
8. Navarette RC, Holmes BA, McConnell SB and Linton DE: “Laboratory, Theoretical and Field Studies of Emulsified Acid Treatments in High-Temperature Carbonate Formations,” SPE Production & Facilities15, no. 2 (May 2000): 96−106.
Winter 2008/2009
Laboratory Testing Testing new treatments and techniques in the laboratory offers many advantages including simplicity, cost and avoidance of possible problems in the field. Good laboratory data will confirm treatment models and indicate the right path for successful field operations. Proper laboratory testing for acidizing techniques can optimize treatment volumes and pinpoint potential problem areas as well as confirm theoretical underpinnings. A strong case in point is the use of emulsified acids in matrix acidizing of carbonate formations at higher temperatures. One way to address the problem of fast reaction rates at high temperatures is to use acid-oil emulsions to retard the reaction rate. These emulsions have been applied in both acid fracturing and matrix acidizing of carbonates. In acid fracturing, the emulsions help enhance and enlarge conductive pathways far from the borehole. Acid fracturing typically employs chemical and mechanical diversion techniques to ensure that the treatment flows to its intended location.6
HF + mineral + HCl
AIFx + H2SiF6
H2SiF6 + mineral + HCl AIFx + mineral
silica gel + AIFx AIFy + silica gel ; x > y
Distance from wellbore
> Sandstone acidizing reactions. When sandstone formations are treated with HF and HCl, three sets of reactions occur. Close to the wellbore, the primary reaction of the acids with the minerals forms aluminum and silica fluorides. These reactions rapidly dissolve the minerals and do not yield precipitates. Farther from the wellbore, these primary products undergo slower secondary reactions to form silica gel, which can precipitate. Finally, at a somewhat greater distance from the injection zone, a tertiary set of reactions can occur, forming additional silica gel precipitate. The kinetics of the secondary and tertiary precipitation reactions become exponentially more rapid at higher temperatures and may cause sandstone acidizing treatments to fail.
Diesel Emulsifier, HCl
corrosion inhibitor, H2S scavenger
20 Reservoir face 19 18 17 16 15 250 300 350
> Emulsions. Acid-oil emulsions decrease reaction rates by limiting access of the HCl droplets to the reservoir face. Each droplet contains HCl plus other components such as emulsifiers, corrosion inhibitors and hydrogen sulfide [H2S] scavengers (top). The extent to which the emulsion retards the reaction rate can be expressed as the retardation factor, FR. This factor is a function of the ratio of the reaction rate with HCl to the reaction rate of the emulsion. Laboratory core data on carbonates using 15% and 28% HCl in stabilized emulsions show that reaction rates can be retarded by factors of 15 to 19 times in the temperature range 250 to 350°F [121 to 177°C] (bottom). (Retardation data adapted from Navarette et al, reference 8.)
HCl,%
15 28
55
Retardation factor, FR
Primary Secondary Tertiary
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