Paraffin deposition and its prevention and removal
The crude oil production industry has its own share of complications and challenges. One of the most common issues is the formation of organic deposits in and around the boring regions which can severely impact the flow of oil and thereby, slow down the entire mining process. These organic deposits can be either paraffins or asphaltenes and they can occur in tubing or in the pores of the reservoir rock. So how are paraffin deposits formed and what are the ways to eradicate them or mitigate their impact? Let’s take a closer look at this topic.
Paraffin deposition – the what and the how
Paraffins are high-molecular-weight alkanes that can build up as deposits in the wellbore, in feed lines, etc. These organic deposits can act as chokes within the wellbore, resulting in a gradual decrease in production with time as the deposits increase in thickness. Deposits vary in consistency from soft accumulations to hard, brittle deposits. Usually, the deposits are firmer and harder as the molecular weight of the paraffin deposits increases.
The primary cause of wax or paraffin deposition is simply a loss in solubility in the crude oil. This loss of solubility is usually a result of changes in temperature, pressure, or composition of the crude oil as a result of loss of dissolved gases. paraffins that have the highest melting point and molecular weight are usually the first to separate from solution, with lower-molecular-weight paraffins separating as the temperature decreases further.
How to remove paraffin deposition
Paraffin accumulations are removed by methods that can be broadly placed into three categories:
Mechanical removal of paraffin deposits – Mechanical methods such as scrapers, knives, and other tools are most commonly used to remove paraffin deposits in wellbores. They can be very effective and are relatively inexpensive.
Use of solvents to remove paraffin deposits – The most common solvent used to remove paraffin from tubulars and the near-wellbore region is crude oil. Hot oiling is the least expensive method, where the oil is heated to temperatures of 300°F or more and this heated oil is then injected or gravity fed into the tubing or annulus. The high temperature ensures that the paraffin deposits are soluble in the injected crude, which is then produced back to the surface.
Use of heat to melt and remove the wax – Steam has been used in a number of cases in which severe paraffin problems have resulted in plugged tubulars. The lack of solubility of paraffin in hot water necessitates the use of surfactants with steam or hot water so that melted paraffin can be removed.
Preventing paraffin deposition
While the methods for removing paraffin accumulation are tried and tested, it would be even better if deposits are not allowed to form in the first place. In this regard, several mechanical adjustments can be made in the production string that can minimize the likelihood of paraffin deposition. In general, these steps are designed to minimize the cooling of the crude oil as it is produced to the surface. This can be accomplished by designing pumping wells or tubing sizes and gas lift systems that maximize the flow of oil to the surface and minimize the heat lost to the surrounding formations. The use of more expensive methods such as plastic coatings on tubulars and electrical heaters is severely limited by cost considerations.
Paraffin inhibitors are a class of compounds that consist of crystal modifiers that prevent the deposition of paraffin onto pipe surfaces and pipe fabrication. These surface-active materials retard paraffin deposition by inhibiting the adhesion of paraffin to sites on the tubing walls. Surfactants used in these applications include wetting agents, dispersants, and crystal modifiers. Each of these chemicals needs to be tested for a specific crude oil to evaluate its effectiveness.
The Last Word
In the Oil and Gas industry, continuous and uninhibited production of crude and other energy producing hydrocarbons are necessary to ensure the stability of global demand and supply dynamics. In this regard, paraffin deposits have the potential to impede oil production to a great extent and hence, there has been considerable research into how it can be tackled efficiently so that any adverse impact is minimal. The flip side is the fact that paraffin deposits have other uses in our daily lives, so there may be additional benefits if these deposits are harnessed correctly!