Development and Evaluation of Paraffin Technology: Current Status
Authors: Thomas F. Brent , Bennion D. Brant 
Published: June 2014

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Introduction

Production problems are plentiful in the oil and gas industry. A common one is production of organic solids where a solid phase forms out of the hydrocarbon liquid or gas. These organic solids take many forms (wax, asphaltenes, diamondoids), and their identification and treatment can be crucial to the exploitation of oil and gas fields. Refs. 1 through 4 discusssome of these problems.

Owing to the challenging nature of solids precipitation, techniques are always under development to identify the deposition of these solids more accurately. This article summarizes some of the more recent technologies and identifies the direction for future means of quantitative determination of solids precipitation.

Background

Years ago, problems with solids precipitation were discussed in terms of paraffins or asphaltenes. This dichotomy resists change even today. When organic-solid precipitates are observed in the field, it is not uncommon for the operating engineer to contact the head office and indicate that anasphaltene-precipitation problem exists. When the operating engineer is asked why he or she thinks it is asphaltene, the answer is "because it's dark and it's a solid."

This often is an incorrect means of categorizing solids-precipitation problems. If solids are dark or black and if they cause production problems, they are categorized as asphaltenes. More specific evidence suggests that there is a continuum of organic-solids precipitation. The components that make upmost solids precipitates extend from the lighter components, such as paraffins(which can possess molecular weights of less than 400), up to true multicyclic asphaltene molecules with molecular weights in the tens of thousands. Molecular weight, therefore, can be a means of differentiating between these types of solids. Because of the range of asphaltene types and molecular weights, however, molecular weight alone does not suffice for solids characterization.

Additional insight is available by identifying whether the solids are dependent on temperature or pressure changes. Intuitively, engineers recognize that a system where solids precipitation is dominated by temperature reduction probably would be a paraffin problem, whereas those that depend more on pressure change (compositional change) would be classified as asphaltene dominated. This, however, is not without ambiguity because, in many cases, systems that have some degree of temperature dependence may be predominantly asphaltene but the paraffin molecules may act as nucleation sites where the other more extensive and heavier asphaltenic-type molecules can grow and separate from the liquid phase. To identify onset and type of precipitation conclusively, laboratory testing ultimately must be performed. Some techniques for detection are discussed in the following.

Solids-Precipitation Detection

Many improvements have taken place in the measurement of solids precipitation. Originally, the standard technique for determining cloudpoint temperature (the temperature at which solids begin to form) was to place the sample of oil in a temperature-controlled bath with a thermometer immersed in the subject phase. The temperature would then be decreased, and one would watch for a "clouding" to form in the oil, indicating the point of first deposition of microcrystalline suspended solids from solution. The name cloudpoint is derived from this clouding response. The results may vary, however, depending on the visual acuity of the individual watching the experiment. The technique can be enhanced further by conducting the evaluation under a microscope. On the microscopic level, precipitation initiation may be detected more precisely and the cloudpoint temperature identified more accurately. Nevertheless, the subjectivity associated with a visual measurement is still present.

Recently, visible-spectrum and near-infrared (NIR) -laser techniques have been used. Fig. 1 illustrates an example cloudpoint analysis of an oil with a laser technique. The energy associated with the light transmitted through the sample is on the y axis. The temperature change, as recorded from a thermocouple immersed in the liquid phase, is plotted on the abscissa. Once the cloudpoint is encountered, a precipitous drop occurs in the laser light passing through the sample. The benefit of this laser-based technique is that it is easy to automate and becomes a quantitative measurement of a temperature-dominated solid-phase transition.

A limitation of this technique is that the optical characteristics of the fluid must be such that the light can be transmitted through the fluid; fluidopacity can be a limiting factor for laser systems. NIR laser devices are more effective than visual lasers at transmitting energy through opaque fluids because of their longer wavelength; however, there are still limits to their sensitivity.

Acoustic-resonance techniques also have been developed that respond to abroad range of opacity. The sensitivity of this technique depends on the change in sonic speed with phase transition and is therefore independent of fluid visual characteristics. Fig. 2 shows the results of an acoustic measurement of cloudpoint with a cloudpoint indicated at 55°C. This same point was confirmed with laser and visual techniques. Table 1 presents results obtained from NIR, optical laser, and visual techniques and from acousticresonance. The results are encouraging for the acoustic-resonance approach.