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Abstract

This paper describes laboratory investigation of alternate solvent systems and their application to the removal of organic deposits. Aromatic solvents, such as toluene and xylene, have been popular remedies for near-wellbore cleanup operations. Stricter regulations governing disposal, volatile-emission limits, and flammability/handling concerns have made the use of toluene and xylene less attractive. In an effort to limit environmental and personal exposure hazards, alternate solvents have been investigated.

Theoretical solvency parameter comparisons linked with a data base search was used to initially screen alternate solvent systems. Quantitative paraffin and pipe-dope solvency tests, along with asphaltene stability titrations, were used to qualify potential solvent candidates. As a result of this work, several potential solvent candidates. As a result of this work, several alternative solvents have been found which fall outside the regulatory restrictions for xylene and toluene, while maintaining effective removal of paraffin, asphaltene and pipe-dope deposits.

Introduction

Changes in temperature, pressure drop, pH, C:H ratio, and gas/fluid ratio during crude oil production can cause paraffin and asphaltenes to deposit in the formation, wellbore tubing, surface flowlines, and storage tanks. Organic formation damage causes two negative consequences with conventional acid treatments:

  1. The damage is bypassed due to immiscibility with the acid system.

  2. The water zone is preferentially acidized.

Solvent soaks and continuous solvent preferentially acidized. Solvent soaks and continuous solvent treatments are popular methods for removing acid-insoluble formation damage prior to acidizing, oil-external emulsion blocks, and residual pipe dope prior to perforating and gravel packing. packing. Aromatic solvents, such as toluene and xylene, have been popular remedies for near-wellbore cleanup operations. Stricter popular remedies for near-wellbore cleanup operations. Stricter regulations governing disposal, volatile emission limits, and flammability/handling concerns have made the use of toluene and xylene less attractive. In an effort to limit environmental and personal exposure hazards, alternate solvents have been investigated.

Background
Solvents

Most operators consider toluene and xylene to be the best solvents for organic deposit removal. A number of other solvents and formulations have also been used, including aromatic distillates, Stoddard solvent (a petroleum naptha mixture commonly used for dry cleaning), natural extracts, hydrolyzable aprotic halogenated organic solvents such as allyl chloride, and water-soluble thiocarbamates, thiocarbonates and xanthates which settle to the bottom of a well and decompose into CS. Aromatic solvents have been combined with alkylaryl sulfonic acids for asphaltene removal. Aliphatic or cyclic amines have been shown to enhance asphaltene solubility.

Cosolvents are often used in many solvent formulations for water wettability but are generally viewed as enhancements to solvent compatibility and penetration rather than enhancement of organic solvency. Higher temperatures enhance paraffin solvency for most solvents but have no effect on asphaltene solvency.

Environmental Concerns

Although toluene and xylene are considered to be two of the best solvents for organic deposit removal, their use is sometimes restricted due to environmental concerns and stricter governmental regulations. Concerns over the concentrations of benzene, ethyl benzene, toluene and xylene (BETX) in aromatic solvents have been related to a perceived need to steer away from an EPA "hit list" of toxic chemicals.

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