Using Cross-Polarized Microscopy To Optimize Wax-Treatment Methods

Abstract

Abstract Paraffin wax deposits are known to be a worldwide problem in the upstream petroleum industry. Considerable resources are expended on inhibiting or removing wax deposits every year. Paraffin wax is not the only type of crude oil based wax. Microcrystalline waxes, composed of naphthene and iso-paraffin molecules, also exist but are not well studied. The predominance of paraffin (also called macrocrystalline) wax and paraffin wax inhibition techniques will likely lead to paraffin wax treatments being applied to naphthene based reservoirs. It is unclear how well a paraffin based treatment will apply to naphthene based oil. The current study investigates the use of Cross-Polarized Microscopy (CPM) to determine if a crude oil contains macrocrystalline or microcrystalline waxes. CPM can determine the size and morphology of wax crystals because crude oil wax crystals are known to be anisotropic. Macrocrystalline waxes are known to present with larger, elongated structures while microcrystalline waxes present with smaller, rounded structures. The type of wax present in the crude oil can be determined using crystal morphology and size. Knowing which type of wax is present in the oil will help to choose which type of wax inhibitor will be used. This study has shown that CPM can be an effective tool to determine the type of wax present in a crude oil. Paraffin wax crystals are shown as being elongated structures with statistically larger sizes than the microcrystals. Microcrystalline waxes are shown to be statistically smaller and rounded structures. CPM is a simple, cost-effective method that can be used in addition to standard tests to determine if a crude oil is paraffin or naphthene based. In the future this test can be used to prevent paraffin wax treatments being applied to naphthene based reservoirs and to help optimize the specific inhibitor chosen. Introduction Wax deposition in production facilities and pipelines is a problem that costs the upstream petroleum industry billions of dollars worldwide every year (Bello et al. 2006). The deposits can plug pipelines or seize equipment leading to costly downtime and expensive remediation techniques. Wax deposition has been extensively studied for paraffin wax but research concerning iso-paraffinic and naphthenic (cycloparaffin) wax is limited (Bello et al. 2006; Hammami and Raines 1999; Dobbs 1999). Currently, no definitive tests exist to determine if a wax is paraffin or naphthene based. This situation coupled with the predominance of paraffin wax treatments will likely lead to paraffin wax treatments being applied to reservoirs that may be naphthene based. Wax Classification A common classification of oil is by base type (Speight 1999). Traditionally oils have been paraffin based if paraffin wax would separate from the oil on cooling and asphalt based if there was no separation of paraffin wax. In more recent years this has been replaced by referring to the wax itself as either being macrocrystalline or microcrystalline (Kumar et al. 2004; Dorset 2000; Elsharkawy et al. 2000). Macrocrystalline wax is the most familiar as it is composed of mostly straight chain paraffins with microcrystalline waxes being composed of high amounts of naphthenic and iso-paraffinic components (Dorset 2000; Elsharkawy et al. 2000). In some publications macrocrystalline waxes are simply referred to as paraffin waxes and microcrystalline waxes are known as amorphous waxes or mal-crystals (Kumar et al. 2004; Elsharkawy et al. 2000; Dorset 2000).