When dealing with the production of oil and gas, it is essential to generate a reliable, manageable and profitable flow of fluids from the reservoir to the sales point. Thus, any phenomenon that hinders fluid flow through oil and gas production and transportation systems should be predicted, prevented and remediated.
In the early 1990s, Petrobras coined the term Flow Assurance as the science of providing a low-risk and cost-effective transport of hydrocarbons from the reservoir to surface facilities and so on until the sales point.¹ The concern emerged in the mid 80s with the development of the Albacora field, where Petrobras experienced a wax deposition problem. Since it was the first field in the Campos Basin to be developed over 400 meters of water depth.²
Overall, flow is assured if the transport energy is greater than the resistance imposed on the productive system. Resistance sources are mainly related to fluid characteristics and pipe characteristics. Depending on the fluid composition, its properties (such as number of phases, density, viscosity, etc) will behave differently under a wide range of pressure and temperature. Some environmental conditions may lead to solid formation and deposition, thus jeopardizing production. The so-called big-five deposits are:
1. Paraffin waxes:
Precipitate mostly as a function of temperature drop, as can be seen in Figure 1. The temperature at which the first paraffin particle precipitates is the wax appearance temperature (WAT).
Figure 1 – Representation of a typical wax deposition phase envelope3
2. Gas Hydrates:
Formed when small gas molecules, such as CH4, flowing with water in a pipe under low temperature and high pressure, become trapped in ice-like cages. The presence of gas molecules makes these cage configurations more stable. Therefore these solids form above the freezing point of water itself.
Figure 2 – The natural gas hydrate structures.
3. Asphaltenes:
Amorphous polyaromatic hydrocarbons present in crude oil which contain considerable amounts of heteroatoms and metals. For example, vanadium and nickel¹,³. They precipitate due to changes in pressure, temperature, or composition. So this makes them a possible problem in deepwater operations and in miscible EOR.
Figure 3 – Representation of a typical asphaltene precipitation envelope3
4. Naphthenes:
Deposition can be troublesome in three-phase separators and desalting equipment when producing oils with high acidity, because they form from the naphthenic acids in oil. When fluids are produced and pressure decreases, dissolved CO2 leaves the aqueous phase, thus increasing pH. Then, naphthenic acids in the oil-water interface, dissociate, and become bonded to cations dissolved in water, thus generating the solid naphthenate.¹
5. Inorganic scale:
It can happens because of many chemical and thermodynamic factors. Some important ones are
The concentration of ions in solution exceeding the solubility limit, pressure and temperature (which affect the solubility products of compounds),
Mixing of incompatible fluids (e.g. formation water and seawater can have different dissolved ions which precipitate when mixed together),
The kinetics of precipitation.¹
Flow assurance is not only based on the prevention and remediation of these deposits, though. When particular pipe characteristics, such as its size (diameter and length) and terran, meet multiphase flow, it can lead to hydrodynamic issues. An example of that is severe slugging. It is an unstable phenomenon that leads to large pressure and flow rate fluctuations. This creates potential problems in the platform facilities.
Other than solid deposition and hydrodynamic issues, the field of flow assurance studies corrosion, emulsions, sand (and the erosion caused by its presence) and foaming, since these factors can harm production.
With the increasing exploration of offshore fields and, consequently, the increasing water depths, challenges have increased primarily due to high pressures and low sea temperatures, combined with a peculiar pipeline system design. Therefore, flow assurance became a topic of primary concern so that an economically viable production with minimized issues can occur.
In the next posts, we will introduce the simulation solutions ESSS has in the area of Production and Flow Assurance. We will also discuss how they can be used to tackle these issues. Stay tuned!
References
¹GUDMUNDSSON, J. S. Flow Assurance Solids in Oil and Gas Production. CRC Press. London, UK. 2017.
²CARDOSO, C. B.; ALVES, I. N.; RIBEIRO, G. S. Management of Flow Assurance Constraints. Offshore Technology Conference. 2003.
³LEONTARITIS, K.J. The Asphaltene and Wax Deposition Envelopes. Fuel Science and Technology, vol. 14, fl. 13-39. 1996.
4CARROL, J. Natural Gas Hydrates: A Guide for Engineers. Gulf Professional Publishing. 3rd edition. US. 2003.
5SIAŽIK, J.; MALCHO, M.; LENHARD, R. Proposal of experimental device for the continuous accumulation of primary energy in natural gas hydrates. EPJ Web of Conferences vol. 143, 02106. 2017.
Carolina Barreto
Business Development, ESSS O&G
Carolina Barreto has a Bachelor’s Degree in Chemical Engineering, from the Federal University of Rio de Janeiro, and a Master’s Degree in Petroleum Engineering, from the University of Tulsa. Spanning over 10 years of professional experience in O&G, Carolina initially worked as a process engineer to later focus on petroleum production, in which the use of simulation for engineering applications became indispensable. Carolina has been working at ESSS since 2018 and is one of the responsible for fostering partnerships between Oil & Gas companies, Universities, Research institutions and ESSS, in order to develop R&D projects related to simulation and new software technologies.
