Tubing pickle treatments can eliminate formation damage that is caused by introducing millscale, pipe dope and other tubular contaminants into the formation. Two of acid pumping techniques implemented to perform the pickling treatments are bullheading and coiled tubing. In the bullheading method, the acid is pumped down the tubing, then the spent acid is lifted to the surface using reservoir pressure. Coiled tubing is another method where the acid is pumped down the coiled tubing, and flowed up through the tubing/coiled tubing annulus.
This paper examines the advantages and disadvantages of the two commonly used techniques for tubing pickling: bullheading and coiled tubing. The efficiency of each method is analyzed based on extensive analysis of field samples. In addition, the paper also addresses the effects of acid volume, concentration, and pumping flowrate on the efficiency of removing mill scale from the wellbore.
Field data indicated that tubing pickle is a must. Extremely high levels of total iron (up to 105,000 mg/l) were noted in the spent acid. The relationship between dissolved iron and acid concentration profile depended on the mode of acid pumping. In the case of bullheading, the iron peak coincides with the tail of the acid peak, whereas it coincided with the leading edge in the case of coiled tubing.