There is an interesting analog to a quiet revolution that is taking place in the oil field. Anyone who has ever gone bowling might recall feeling a bit squeamish about renting bowling shoes that had been worn by someone else. But a simple process prevents the podiatric contagion the public fears. Commercial bowling alleys treat all returned shoes with ultraviolet (UV) light. That process gets the “bugs” out of the shoes for rent.

It is a well-known fact that bacteria possess cellular DNA, just like humans or animals.When the microorganisms’ DNA absorbs UV light, its genetic structure is damaged, impairing its ability to replicate. Bacteria are everywhere, but their rapid growth is kept in check by UV light absorption. Usually nature provides enough UV light from the sun to control the spread of bacteria; otherwise, the world would be overrun by it. But when technology is used to give nature a helping hand, even the most prolific bacteria can be controlled.

Jumping from footwear to fracturing
The growth of bacteria can be controlled in many ways. The petroleum industry has adapted chemical formulations to kill bacteria or limit its growth. Bactericides are used to treat tank trucks, stock tanks, impoundments, and any place water-based solutions are stored. They also are mixed with fluids before they are pumped downhole. These could include sources of water for injection wells or for hydraulic fracturing treatments. Bacteria are insidious. In nature, even a clean-appearing stream could harbor a biofilm that might be invisible on the surface but that can replicate into a gooey mass when subjected to heat or when combined with certain elements in the formation.

Installed on a single float trailer, the CleanStream unit can be staged at the well site for direct treatment of fluids at a water source such as an impoundment, or where water haulers get their water. (Photos courtesy of Halliburton)

One particularly harmful type of bacteria is sulfate-reducing bacteria. It reacts with downhole sulfates in the formation, turning them into sulfide that, when combined with water, can form sulfuric acid, an aggressive corrosive solution that attacks downhole tubulars and other equipment. Given the right conditions, hydrogen sulfide could form, turning a sweet well into a sour-gas producer. In addition to the risks that sour-gas production poses to humans, the concoction can form iron sulfide scale that can quickly plug oilfield tubular goods. But with the application of chemicals, bacterial growth can be curtailed.

Public perceptions cloud the issue
Despite reassurances to the contrary, the general public is wary of the use of chemicals in hydraulic fracturing treatments. Typically, a slickwater frac contains more than 99% water, with a small trace of biocide-inhibiting chemicals along with chemicals to aid lubricity, inhibit the formation of scale, and forestall undesired reactions with clay minerals. Nevertheless, there is a concern that somehow these chemicals might get into freshwater aquifers and do harm to plants and animals.When gel treatments are called for in the frac design, bacteria can cause gel instability. A small colony of bacteria in a frac tank or impoundment from a previous job can grow exponentially to affect subsequent treatments, especially in summer months when ambient temperatures are above 60°F (15.5°C). Leading pumping service providers, including Halliburton, have moved to a position of openness regarding the chemicals they use, and it is expected that this position ultimately will reassure the public.

Multiple UV light chambers effectively control bacteria at real-time pump rates so treatment can be performed on the fly.

Halliburton’s CleanStream Service, which can be applied to water supplies before the frac treatments are pumped, makes a 90% reduction in the volume of biocide chemicals pumped. Whereas a 5-million gallon slickwater treatment might require 5,000 gal of biocide used to treat aerobic and anaerobic sulfate-reducing bacteria, the same effect now can be achieved with a single UV treatment and only 500 gal of biocide chemicals. If field logistics permit, the UV treatment can be applied on the fly as the treatment is being pumped, reducing the requirement for chemical addition to zero.

Tested in Halliburton’s laboratories, the UV-based CleanStream reactor delivers a 99.9% effective bacteria reduction. UV light intensity is measured continuously by sensors in the processing train to ensure it stays above threshold levels for bacteria elimination. As a backup, periodic samples of treated fluid are taken and subjected to the tests used with chemically treated fluids. The cost of the new service compares favorably with that of chemical additives.

The treatment, while effective in biocide reduction, has no residual effect. That is, it cannot “immunize” the fluid from subsequent contamination by a migrating microorganism. Accordingly, most operators equip their wells with a continuous chemical inhibition service delivered in very small doses through capillary tubes installed with the completion hardware. The continuous chemical inhibition service reduces the formation of waxes, scales, corrosives, and other flow assurance problems in addition to controlling the bacteria population.

Onsite deployment of CleanStream units is straightforward, and up to100 bbl/min can be processed. If job requirements call for higher pump rates, additional units can be deployed.

UV treatment is fast, effective
The CleanStream equipment is modularized onto a single float trailer that is staged between the frac tanks or water impoundment and the pump trucks. One unit can treat up to 100 bbl/min. There are no moving parts, and the treatment is computer-controlled. Alarms tell the operator if a UV light fails, and treatment usually can continue at a reduced pump rate until the bulb can be replaced. Each treatment chamber has multiple bulbs, so a bulb burnout cannot shut the treatment down. Even if there is a catastrophic electrical failure, the unit does not impede the transfer rate of fluid to the frac job since there are no physical impediments to flow through the unit. Cloudy water could diffuse the UV light, reducing its effectiveness, so inline filter units can be deployed as needed to clear suspended solids from the fluid supply.

How can UV light take the place of chemicals?

UV light has four principal wavelengths:

-UV-A;
-UV-B;
-UV-C; and
-UV-V.

Like a prizefighter’s “one-two” punch, the UV-C penetrates the bacteria cell’s membrane, destroying its DNA, while the UV-V destroys the chemicals of the dead cell. These reactions take place at the molecular level and require only microseconds to complete their work. The altered DNA sends incorrect codes to the microorganism, leading to its self-destruction.