Coiled tubing is usually used to conduct acid pickle treatments. The purpose of the treatment is to prevent pumping damaging materials into the formation prior to the main acid job. In this case, the acid is pumped down the coiled tubing, and then flowed up through the tubing-coiled tubing annulus. Pickling is a process of flow with heterogeneous reactions occurring with mill scale (Fe3O4) and other contaminants at both the inner wall of the production tubing and the outer surface of the coiled tubing. Pickling is an essential part of well stimulation treatments if the main treatment fluids (acidizing, fracturing, gravel packing, etc.) are to be bullheaded. However, it appears that traditional pickle practices are overestimating the required acid volume and/or concentration. Field data indicated that excessive acid volumes are used for tubing pickle because large returns of unreacted acid are usually recovered on the surface.
In this paper, mechanisms to explain the behavior of acid contact with the tubing are presented and a model for predicting acid consumption and dissolution of tubular contaminants is developed. The model includes reactions of acid with mill scale. A system of non-linear partial differential equations is developed and the equations are solved numerically to predict the concentrations of major species as a function of axial position along the tubing and in the effluent from the well during flowback.
Field application included pickling of low-carbon steel tubing (11,900 ft of 5.5-inch C-95) using coiled tubing. A slug of 5,000 gals of 20 wt% HCl with additives was used. Samples were collected from the treated well during the flowback of pickling treatment. The model was used to predict the concentrations of various species in the well flowback samples. Model predictions for acid, chloride ion, and total iron concentrations were in good agreement with field results. In addition, the model proved to be a valuable tool in optimizing future pickling treatments.
Scientists and engineers working on chemical treatments to enhance well performance usually focus on reservoir characteristics, fluid placement, fluid compatibility, and thermal stability. Less attention, however, is given to potential formation damage that might occur if the contaminants present in the well tubulars invade the formation.
Several studies were conducted to identify the type and amount of contaminants present in well tubulars.[1–4] Pipe dope and mill scale were identified as main contaminants present in the production tubing.[5–8] Invasion of these materials into the target zone can cause severe formation damage. Therefore, it is always recommended to minimize the amount of pipe dope used and to clean well tubulars prior to any chemical treatments, especially if these chemicals will be bullheaded. The types of fluids used in pickling treatments depend on the type of contaminants. Xylene and similar organic solvents are used to dissolve the organic portion of pipe dope. Hydrochloric acid is used to remove acid-soluble material present in the pipe dope (mainly zinc), and mill scale. Selection of acid additives depends on temperature, type of tubular, and the fluids that will come in contact with the acid. It should be noted that environmentally friendly pickling fluids were introduced over the last few years, with positive field results. [9,10]