UV Disinfection for Enhanced Oil Recovery and Other Oilfield Applications

One of the biggest challenges facing oil companies today is how to extract more oil from the discoveries made in the last 40 years. Estimates vary, but approximately 66% of the discovered oil gets left in the ground. The International Energy Agency has estimated that if, in the next 20 years, this figure could be reduced to 50%, then this would increase the world's recoverable oil by 1.2 trillion barrels. This would double the world's proven reserves, and extend operating lifetimes for expensive offshore investments. A number of operators can demonstrate progress; BP has increased the recovery factor at the Prudhoe Bay facility on Alaska's North Slope from 40% to 60%, thus extending the field's life by 50 years.   

One of the oldest and most widespread techniques for enhanced oil recovery is well flooding by injecting water downhole. Water is injected into the reservoir to pressurize and displace hydrocarbons towards the producing wells. Injection water is also used in water storage operations in offshore and remote locations.

The quality of the injection water is critical, as the displaced oil needs to flow through porous rock structures. The treatment of injection water is multistage, with filtration to remove solids, and de-aeration or oxygen stripping processes to reduce the oxygen content of the water. Ultraviolet light is used as a penultimate stage to ensure that the water sent downhole is free from microbial contamination without the use of chemicals.

The Problem

The seawater that is used for offshore well injection contains a variety of microorganisms. Some of these species lead to corrosion and scale downhole wells, while others are slime forming. Many species can survive in oxygen rich (aerobic) seawater, or in the oxygen starved (anaerobic) environment below ground.

Aerobic bacteria can convert iron from the ferrous (Fe²⁺ ) to the ferric (Fe³⁺) state, and produce ferric hydroxide (Fe (OH)₃), which is highly insoluble and causes formation damage.  Iron oxidizing bacteria can be slime forming species that form mats of high-density slime that cover surfaces. If allowed into the well, they shield corrosion forming bacteria colonies from chemical bactericides and plug the pores of the matrix holding the oil.  

Anaerobic species such as Sulphate Reducing Bacteria (SRB) can convert sulfate or sulphite that is naturally occurring or be present in a variety of drilling muds into hydrogen sulphite (H₂S).  When combined with iron, iron sulphide is formed which is a scale that is very costly to remove. In addition, SRB species can cause pitting corrosion of steel and elevated H₂S increases the corrosiveness of the water, which increases the possibility of hydrogen blistering, sulfide stress cracking and can lead to costly sweetening of sour oil.

UV systems are usually installed following filtration of the seawater to remove suspended solids. The filters can often be a source of microbial growth, so the UV equipment should be installed post filtration.

UV light has no residual effect, and so periodic conventional disinfection using sodium hypochlorite or other biocides is still required. The advantages of UV being added as a process stage include reduced chemical use, space saving, and improved operator safety.

How does UV work?

UV light is a physical, non intrusive process that has broad industrial and municipal uses. Systems comprise of 316L or Duplex steel chambers that contain high powered UV lamps. Wiping systems keep the quartz sleeves free from fouling. A UV intensity monitor is used to ensure proper disinfection and systems are usually supplied as duty/standby configuration. For offshore use, modular skid mounted packages are usually built and tested before installation.

UV light between 200 nm and 300 nm can pass through the water and is absorbed by the DNA contained in the nucleus of all living organisms. When the UV light is absorbed, the DNA becomes so damaged that the organism is instantaneously rendered non viable. Normal cell function ceases; the organism cannot replicate, respirate, or assimilate food. Once viability ceases, the colony quickly dies.

No organism is capable of surviving UV light.  Many species are now increasingly tolerant to chlorine, and emerging pathogens such as Cryptosporidium and Giardia demonstrate high chlorine tolerance.

UV light has been demonstrated to be very effective against SRB species, and is non selective; any microorganisms present in the seawater will be deactivated.

UV systems are sized based on three main factors: flowrate, water quality, and the challenge organism. Computational Fluid Dynamics (CFD) models are used extensively to design and size UV systems.   The flow profile is produced from the chamber geometry, flowrate and the particular turbulence model selected. The radiation profile is developed from inputs such as water quality, lamp type, and the transmittance and dimensions of the quartz sleeve.

Proprietary CFD software simulates both the flow and radiation profiles. Once the 3D model of the chamber is built, it is populated with a grid or mesh that is comprised of thousands of small cubes. Points of interest, such as at a bend, near a sleeve surface, or close to the wiper mechanism use a higher resolution mesh, whild other areas within the reactor use a coarse mesh. Once the mesh is produced, hundreds of thousands of virtual particles are fired thru the chamber. Each particle has variables of interest associated with it, and the particles are harvested after they exit the reactor. Discrete phase modeling produces delivered dose, headloss, and other chamber specific parameters.

When the modeling phase is complete, selected systems are validated using a third party to provide oversight that determines how close the model is able to predict system performance. Validation uses non pathogenic surrogates such as T1 phase or MS2 to determine the Reduction Equivalent Dose (RED) ability of the reactors. Most reactors are validated to deliver 0.5 log to 6 log reductions of SRB species within an envelope of flow and transmittance. 

The inline style of UV systems are simple to install and they occupy less space, which on an offshore platform is critical.   In addition to injection water for enhanced oil recovery, UV systems are used offshore for the disinfection of drinking water and for the non chemical disinfection of wastewater prior to discharge.

Case Study #1

Eldfisk is an oilfield located near Ekofisk in the North Sea, in sea depth of 200-225 feet. The original Eldfisk development consisted of three facilities: Eldfisk B is a combined drilling, wellhead and process facility, Eldfisk A and FTP are wellhead and process facilities. In 1999 a new water injection facility, Eldfisk E was installed. The facility can also supply injection water to the Ekofisk K. The facility uses horizontal injection wells, injecting into a reservoir between 8000 and 8700 feet (2900 meters).

The UV plant treats 25 MGD and is the largest downhole injection facility in operation. The system comprises of duty and standby units, and was supplied by atg in the UK. It is modular, and has been in operation for more than 10 years.