By David Watts, CIH | Vice President of Operations
Naturally Occurring Radioactive Materials (NORM) have been considered an occupational hazard for decades, primarily in the mining and oil and gas industry. NORM is commonly found in the natural environment, in man-made materials such as building materials, as well as, precipitates from crude oil and natural gas operations. Worker exposure to NORM may occur from a variety of situations. However, common exposures are related to scales and sludges in water separation systems, condensate collection vessels and associated piping as a result of oil and natural gas recovery activities.
NORM originating in geological oil and gas formations is usually brought to the surface in produced water. As water reaches the surface, temperature changes cause radioactive elements to precipitate. These precipitates are the basis for the sludge and scale formation.
The radioactive sources deep within the reservoir rock are typically composed of parent radionuclides U-238 and Th-232. The progeny of these radionuclides will ultimately be Ra-226 from U-238 and Ra-228 and Ra-224 from Th-232. The collective half-life of the Radium series is about 1600 years, and as it goes through radioactive decay, it gives off alpha, beta and gamma radiation. During this process, the Radium goes through transformations that include Radon Gas and radioactive isotopes of: Polonium, Lead, Bismuth, Mercury, and Thallium. The final result is stable Lead.
A large amount of data has been collected over the years concerning radionuclide concentrations in NORM. Radionuclide activities for Ra-226, Ra-228 and Ra-224 identified in scales and sludges from oil and gas recovery vessels have ranged from 0.1 Bq/g to 15,000 Bq/g. These values represent radioactivity of the isotope as a function of the decay process. Keep in mind that radioactivity does not correspond to human exposure as we may intuitively think. However, knowing the activity provides investigators a basic understanding of source strength and potential deposits with respect to scale accumulation.
NORM monitoring has become somewhat of a craft within the industrial hygiene community due to a number of variables that must be considered during a typical survey. First, the hygienist needs to have a basic understanding of oil and gas processing which drive the scale and/or sludge deposition. Secondly, the hygienist needs to develop a monitoring strategy based on the operation. For instance, monitoring may be identified by task, by routine circumstance or by special occurrence. A task may be as unique as verifying that at an end of shift worker’s clothing does not contain particulates with alpha emitting particles. Routine monitoring may involve conducting monthly surveys which provide feedback regarding equipment shielding effectiveness. Special monitoring could include post-remediation of a vessel for re-entry purposes.
Monitoring is critical due to possible exposures to radiation. The accumulation of contaminated scales and sludges in pipes and vessels may produce significant dose rates inside and outside of these components. While the large majority of radiation in produced water, condensates, and recovered natural gas liquids is composed of alpha and beta emitters, there may also be short-lived the progeny of Ra-226 which may emit gamma radiation capable of penetrating the walls of vessels and piping. Maximum dose rates are usually up to a few microsieverts per hour but in exceptional cases may reach several hundred microsieverts per hour, which is about 1000 times greater than normal background due to cosmic radiation.
Lastly, the industrial hygienist must select the proper monitoring instrumentation from the following three categories: personal radiation detector (PRD), handheld survey meter and radiation isotope identification device (RIID). A PRD is a wearable gamma and/or neutron radiation detector. Most PRDs numerically display the detected radiation intensity (on a scale of 0 to 9) and, thus, can be used to locate a radiation source; however, they typically are not as a sensitive as handheld survey meters and cannot identify the type of radioactive source. The handheld survey meter is a detector which typically measures the amount of radiation present. Most of these devices detect beta and gamma radiation only; however, some models can also detect alpha and neutron radiation. A RIID is a radiation detector with the ability to analyze the energy spectrum of radiation in order to identify the specific radioactive material (radionuclide) that is emitting the radiation. In addition, these devices can be used as survey instruments to locate radioactive material.
Due to the amount of technological advancements in the oil and gas exploration arena in the past decade which have allowed deeper recoveries from previously untapped reservoirs, the presence of NORM will continue to present challenges for the health and safety professional. Staying abreast of the regulatory environment from state to state, instrumentation advancements and process changes that could alter approaches to traditional surveys will be paramount to the health and safety professional.
