Abstract
Carbonate matrix stimulation of tight and high temperature formations is challenging. The use of retarded acids is inevitable to allow deep acid penetration in those cases. Recently, polymer free delayed acid (PFDA) systems were introduced as a low friction acid system with superior performance. This study presents a comparison between the two delayed acids under linear flow (lab scale) and radial flow (field scale).
Linear core flow experiments were conducted using limestone and dolomite cores at temperatures from 200°F to 300°F. 15%, 20%, and 28% HCl based delayed acid systems were tested. 6 in. and 20 in. cores were used in those tests. The information from the linear experiments for both systems were utilized to build a model for those acid systems. An upscaling scheme based on changing the flow area and the wormholes number as a function of acid penetration was realized. The upscaled model was calibrated using radial experiments and in agreement with field observations.
The linear experiments have revealed that in limestone both acid systems provide PVBT values less than 1, even at injection rates as low as 0.5 cc/min. The 20 in. core experiments have shown that PFDA has better performance than emulsified acid at both low and high injection rates. The upscaled model has showed that injecting only 30 gal/ft. of PFDA or emulsified acid at low injection rate (0.1 gal/min.ft) can produce 9.3 ft. and 7.8 ft. wormholes, respectively.
PFDA with dolomite experiments have shown PVBT values less than 1 PVBT while emulsified acid with dolomite experiments have shown values in the range of 2 PVBT. The radial model has shown that injecting 30 gal/ft. of emulsified acid at 0.1 gal/min. ft. can generate ~4 ft. wormhole (-2.8 skin). On the other hand, injecting PFDA in dolomite formation at the same conditions can yield 9.3 ft. wormhole (-3.64 skin). This value agrees with the field results of treating tight dolomite formation with PFDA.
This paper presents an experimentally validated model for PFDA and emulsified acid. The upscaled model agrees with field treatments and allows comparison between the two systems at the well scale. PFDA outperform emulsified acid in treating carbonate formations.
