TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract

Pickle treatments are commonly conducted to remove various contaminants from the wellbore prior to performing chemical treatments in various wells. Introduction of these contaminants into the formation during the treatment can cause loss of well performance, especially in tight formations. These treatments should be designed to remove various contaminants from well tubulars in an efficient and cost effective way.A pickle treatment was designed to clean well tubulars in several wells prior to performing acid fracturing in the carbonate formation. The pickle treatment consisted of several fluids such that each fluid removed one type of contaminants. This paper examines the effects of fluid composition, volume, and placement (bullheading versus coiled tubing) on the efficiency of pickling treatments. More than twenty pickle treatments performed on deep, sour, hot gas wells were examined.To assess pickle treatments in the field, spent acid was analyzed and the concentrations of key ions were measured. Sufficient measurements were made to obtain concentration profiles of chemical species in the spent pickle acid. These profiles were numerically integrated and the data were used to assess the outcome of each pickle treatment. In addition, particulate solids present in the spent acid were separated and analyzed using XRD and XRF techniques.Pickle treatments were conducted on more than twenty gas wells. Some of these wells were perforated, while others were not perforated. Un-perforated wells were pickled using 1.75" high-pressure coiled tubing. On the other hand, perforated wells were pickled using bullheading. The pickle treatments consisted of slugs of surfactants, organic solvents, hydrochloric acid and gelled fluids. Samples of all pumped fluids were collected and analyzed. In addition, well flow back samples were collected every two minutes and were also analyzed to determine their composition.Analysis of spent acids indicated that the major tubular contaminants are mill scale and pipe dope. Spent acid samples collected from deep sour gas wells contained up to nearly 105,500 mg/L of total iron. The efficiency of removing pipe dope was followed by tracing the concentration of zinc, the main acid-soluble constituent in the pipe dope, in the spent acid. The concentrations of acid and iron were used to determine acid reaction with the corrosion products present in the tubing. The concentration of the chloride ions was used to determine the dispersion and mixing behaviour of the acid slug.This paper presents the results obtained from analyzing more than 2000 samples obtained from twenty treatments. The results of this work have been used to enhance the efficiency of the pickle treatments and to significantly reduce the treatment cost. 4 Another main source of wellbore contaminants is iron scale. New low-carbon steel pipes (J-55, C-95 and L-80) are always covered with mill scale (magnetite, Fe 3 O 4 ). 7-10 Iron oxides dissolve in HCl and release ferric and ferrous ions into the acid. Ferric ions will precipitate once th...

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPickling 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 annulus with heterogeneous reactions occurring at the wall of the tube. The reacting solid species at the wall are primarily mill scale, pipe dope, and other scales of various types. The fluids injected for pickling are usually strong acid solutions (HCl), but may also include surfactants, organic solvents, and gelled solutions to aid in lifting solids loosened from the tubing wall. Thus, the process is a complex one involving several reactions with perhaps multiple stages of fluids.Tubing pickle is an essential part of well stimulation treatments if the main acid job 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 indicate that excessive acid volumes are used for tubing pickle because large returns of unreacted (live) acid are usually recovered on the surface. Careful analysis of flowback samples showed that only a fraction of the acid is being consumed by mill scale and other tubing contaminants.In this paper, proposed mechanisms to explain the behavior of acid contact with the tubing are presented and a mathematical model for predicting acid consumption and dissolution of tubular contaminants is developed. The model is developed for the bull-heading case where the pickling acid is pumped down the tubing, then flowed back to the surface. The model considers the reaction of acid with mill scale (Fe 3 O 4 ). The equations formulated are solved numerically to predict the concentrations of major chemical species as a function of position along the tubing and in the effluent from the well during flowback.Such a model can be extremely valuable in optimizing the application of future pickling treatments. Using this model, the acid volume needed for pickling operations can be reduced significantly and other improvements can be made without extensive, costly field testing. Finally, some recommendations are made to design optimum pickling treatments.

TX 75083-3836, U.S.A., fax 01-972-952-9435. ProposalBarite is a common weighting agent used for oil well applications. It is commonly used to provide adequate weight to control downhole pressures in drilling slurries. Since it is used for control of pressure, it is a common source of formation damage, especially in deep, hot formations where drilling fluid weights exceed 17 ppg. Although alternatives do exist for barite, (e.g., formate drilling fluids) they are significantly more expensive, and not available on a rapid turnaround basis as in emergency situations required by well control operations. Once damage has occurred, remediation is a very difficult proposition. Barite is not soluble in typical oilfield inorganic acids used in stimulation. The most common methods of remediation are fracture stimulation and/or deep perforations. The use of these methods can remediate the problem, but at high cost. Also, since the well productivity cannot be evaluated prior to stimulation operations, a poor quality well cannot be identified until a significant investment has been made.The most common chemicals currently in use to address this problem are "barite dissolvers". These agents are all strong metal ion chelators. Aminopolycarboxylic acids (e.g., EDTA) and similar reagents have been used with low success rates. 1 Recently, hydroxylaminopolycarboxylic acids were introduced, but to the best of the author's knowledge was not extensively used in the field. 2 This study investigates a new barite dissolver and compares its performance with other commercial dissolvers. The new material is a proprietary dissolver supplied in a liquid form. This study investigates the dissolving capacity, and evaluates the effectiveness of the dissolver by conducting coreflood tests. These tests were conducted using sandstone cores obtained from a deep gas well (300ºF). The effectiveness of the dissolver was followed by measuring the concentration of barium in the core effluent. Unlike other barite dissolvers, the new chemical is shown to be very effective in restoring the permeability of damaged cores. The new chemical does not require long soaking times, and does not induce fines migration due to chemical means.

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