Modeling of Tubing Pickling in Conjunction With Acidizing Treatments

Abstract

Summary Pickling well tubulars to prevent pumping unwanted materials into the formation is an issue that should be decided on an individual-job basis. Acid pickling of tubing is a process of flow in a tube or an annulus with heterogeneous reactions occurring at the wall of the tube. The reacting solid species at the wall are primarily mill scale and other types of inorganic scale. The fluids injected for pickling are usually strong acid solutions [hydrochloric acid (HCl)] but also may include surfactants, organic solvents, and gelled solutions to aid in lifting solids from the wellbore. Thus, the process is a complex one involving several reactions and perhaps multiple stages of fluids. Tubing pickling is an essential part of well-stimulation treatments if the main acid is to be bullheaded. Standard design parameters for a pickle treatment depend on experience and personal judgment. It appears that the standard pickle-treatment design is overestimating the required volume of pickling acid. Field data indicated that excessive acid volumes were used for tubing pickling because large returns of unreacted (live) acid were recovered at the surface. Careful analysis of flowback samples obtained from several deep gas wells showed that only a fraction of the acid was consumed by mill scale and other tubing contaminants. In this paper, proposed mechanisms are presented to explain the behavior of acid contact with the tubing and a mathematical model is developed to predict acid consumption and dissolution of tubular contaminants. The model applies to the bullheading case where the acid is pumped down the tubing and then flowed back to the surface. The model considers the reaction of acid with mill scale (Fe3O4). The equations formulated are solved numerically to predict the concentrations of major chemical species in the well flowback samples. Such a model is extremely valuable in optimizing future pickling treatments. The acid volume needed for pickling operations can be reduced significantly, and other improvements can be made without extensive and costly field testing. Finally, recommendations are given to design acid-pickling treatments better.