Introduction

Asphaltene problems are increasing on a wide scale in the petroleum industry. The deposition of these materials has been increasingly noted in production wells of CO2 floods and in miscible drive floods, after acid stimulations in asphaltic crude oil formations, and as pressures in older fields near the depletion point. pressures in older fields near the depletion point. Many reservoirs produce without evidence of asphaltenes until the oil stability is disturbed. After the initial problems with asphaltenes, even those caused by a single use, catalytic behaving stimulus such as an acid job, many wells continue to exhibit problems long after operations return to normal.

Numerous remedies to remove asphaltic deposits and stabilization methods to control the deposition have been tried with varying amounts of success. This paper presents a review of the mechanisms that contribute to presents a review of the mechanisms that contribute to asphaltene precipitation and laboratory and field results of a class of solvent/stabilizer chemicals that has shown very good performance in removing asphaltic deposits, restoring production, and in preventing some upsets for extended periods of time.

Discussion

Asphaltenes are a complex organic material that are thought to be arranged in stacked, multi-ring structures. They contain nitrogen, oxygen, and sulfur atoms in addition to carbon and hydrogen atoms within the repeating unit. Asphaltenes have a wide variety of potential structures and vary from reservoir to reservoir. potential structures and vary from reservoir to reservoir. The actual structure apparently depends on the source of the oil and the asphaltic material. Asphaltenes are not truly soluble in most crude oils., They exist as 35 to 40 micron platelets and are maintained in suspension by materials called maltenes and resins. These smaller, similar suspending molecules are soluble and act in what has been described as a "micelle-type" arrangement to keep the asphaltic products in suspension. Molecular weight is in the range of 500 to 500,000 and samples may contain many different structures. When stabilizing influences are removed, the asphaltic particles coalesce into larger groups, called flocs, that separate and, with a density of 1.2 g/cc, precipitate from the oil. Asphaltene contents may range from 0 to over 60%. Resin volume-to-asphaltene ratio is on the order of 1:1 to 20:1 in oils that are stable to less than 1:1 in oils that are characterized by rapid precipitation of asphaltenes. Asphaltene content usually increases with decreasing API gravity but instances of asphaltene precipitation in light oil and even wet gas streams are known, though the occurrence is rare.

The treatment of the asphaltene deposition problem should begin with a discussion of the factors involved in inducing precipitation.

Destabilization Forces

The occurrence of an asphaltene deposition in a well may be at any point in the chain of producing events where the equilibrium of the stabilizing influences has been upset. These upsets may include routine production or stimulation operations; even operations that production or stimulation operations; even operations that may have been done before without consequence. The following is a partial list of destabilizing forces that have been identified as factors in asphaltene flocculation. References are cited for some cases and lab and field data are offered in the other cases. The forces may act singly but are often identified as working in combination with others.

  1. CO2 - The destabilization proceeds by lowering pH, interrupting solution equilibrium by changing oilcomposition and by creating turbulence. It is one of the most severe, non-stimulation causes of asphaltene deposition in producing wells. Some level of increased asphaltic deposition has been experienced in nearly every CO2 flood operation. The most noticeable primary location of asphalt materials in the CO2 floods during the early stages of operations is in wellbore and the pumps. The problem increases with the volume increase of CO2 during the maturing of the flood. In many of these cases, asphaltene deposition was never seen before the use of CO2.

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