Oil and Gas
The majority of sour crude oil and natural gas sources are due to sulfate reducing anaerobic bacteria contaminating the producing formations. In most cases, this was done inadvertently in the past – drilling, fracturing, water flooding, stimulation, etc. with any convenient water source – which wasn’t sterilized. Unfortunately, these non-sterile water sources inoculated the wells’ strata with bacteria and the anaerobic species found a congenial environment – warm with both water and food present.
Today, we’ve learned that any water injected down-hole should be sterilized to prevent souring the formations and plugging with microbiological growth in addition to corrosion products and scale. Initially, non-oxidizing biocides were employed, as they offered a simple “pump and dump” type of application. However, it turns out that this approach has some negatives:
- Non-oxidizing biocides are, in effect, poisons. As such while they may kill the majority of bacteria (a 2-log or 3-log kill), they never kill 100% of them. Some portion of the microbiological population will survive the treatment and still contaminate the formations.
- It’s impossible to determine in the field if an adequate dosage of a non-oxidizing biocide has been fed to obtain the maximum effectiveness. There is no residual, or instantaneous MB test, to determine it. You must apply a dosage and hope it is enough. And that presumes your vendor is not under cost pressure and applies the low end of a generic dosage range to improve profitability.
On the other hand, chlorine dioxide solves these problems. As an oxidizing chemistry, it physically destroys bacterial cells – there is no tolerance or immunity to it. In addition, the reaction chemistry is very rapid, and the process provides a testable residual. Once the chlorine dioxide reacts with all of the compounds present (bacteria, hydrogen sulfide, iron and manganese) there will be an easily testable product residual in the treated water. Thus, within minutes of application you can test for a residual and, if it is present, know the water is sterile.
Most conventional H2S scavengers are some type of amine. In addition to being quite expensive, the reaction with the amines is reversible. This means that if the pH or temperature change significantly after treatment, the H2S can be released and contaminate the environment again.
Conversely, chlorine dioxide permanently destroys H2S by converting to sulfate or in some cases all the way to elemental sulfur. The reaction chemistry is very fast and non-reversible.
Emulsions consisting of water, oil, biomass and corrosion products are ubiquitous in the hydrocarbon processing industries. Very frequently these emulsions consist of oil, water and biomass stabilized by iron sulfide. Chlorine dioxide is extremely effective in breaking up these emulsions for several reasons.
- It dissolves the FeS that is stabilizing it (sulfide to sulfate)
- Rapidly kills and breaks up the biomass
- Allows oil and water to easily separate after elimination of the FeS and biomass
This process is effective in gathering tanks, ponds and well bores. Well stimulation with ClO2 added to a conventional acid stimulation has proven extremely effective in providing long term well-bore cleanup and increased injection or production from the well.
Chlorine Dioxide “Recycle”
ClO2 is so effective on biofilm because of a “recycle” feature – when ClO2 reacts with the bacteria/biomass the majority reverts to chlorite ion. Acid from acid producing anaerobic bacteria species reacts with the chlorite and forms additional ClO2. Thus, a high level of ClO2 is regenerated inside the biomass and results in a rapid and complete kill and dissolution of the biomass.