Radiological survey of the covered and uncovered drilling mud depository

https://doi.org/10.1016/j.jenvrad.2017.10.020Get rights and content

Highlights

  • Remediated Hungarian drilling mud depository was investigated from radiological aspect.

  • The drilling mud depository has no radiological risk from the all radiological aspects.

  • Long term monitoring activity is not necessary from radiological point of view.

Abstract

In petroleum engineering, the produced drilling mud sometimes contains elevated amounts of natural radioactivity. In this study, a remediated Hungarian drilling mud depository was investigated from a radiological perspective. The depository was monitored before and after a clay layer was applied as covering.

In this study, the ambient dose equivalent rate H*(10) of the depository has been measured by a Scintillator Probe (6150AD-b Dose Rate Meter). Outdoor radon concentration, radon concentration in soil gas, and in situ field radon exhalation measurements were carried out using a pulse-type ionization chamber (AlphaGUARD radon monitor). Soil gas permeability (k) measurements were carried out using the permeameter (RADON-JOK) in situ device. Geogenic radon potentials were calculated. The radionuclide content of the drilling mud and cover layer sample has been determined with an HPGe gamma-spectrometer. The gamma dose rate was estimated from the measured radionuclide concentrations and the results were compared with the measured ambient dose equivalent rate.

Based on the measured results before and after covering, the ambient dose equivalent rates were 76 (67–85) nSv/h before and 86 (83–89) nSv/h after covering, radon exhalation was 9 (6–12) mBq/m2s before and 14 (5–28) mBq/m2s after covering, the outdoor radon concentrations were 11 (9–16) before and 13 (10–22) Bq/m3after covering and the soil gas radon concentrations were 6 (3–8) before and 24 (14–40) kBq/m3 after covering. Soil gas permeability measurements were 1E-11 (7E-12-1E-11) and 1E-12 (5E-13-1E-12) m2 and the calculated geogenic radon potential values were 6 (3–8) and 12 (6–21) before and after the covering. The main radionuclide concentrations of the drilling mud were CU-238 12 (10–15) Bq/kg, CRa-226 31 (18–40) Bq/kg, CTh-232 35 (33–39) Bq/kg and CK-40 502 (356–673) Bq/kg. The same radionuclide concentrations in the clay were CU-238 31 (29–34) Bq/kg, CRa-226 45 (40–51) Bq/kg, CTh-232 58 (55–60) Bq/kg and CK-40 651 (620–671) Bq/kg.

According to our results, the drilling mud depository exhibits no radiological risk from any radiological aspects (radon, radon exhalation, gamma dose, etc.); therefore, long term monitoring activity is not necessary from the radiological point of view.

Introduction

Oil is the main energy source of our civilization. In 2013, it contributed 31% of the total energy consumption. Oil and gas not only play a role in power and transportation, but are also the raw materials for the petrol chemistry industry (Alessandro, 2013, Lior, 2010, IEA, 2015). In 2014, 4200 million tons of crude oil was mined in the world and 3524 billion m3 of gas was produced. During the production of oil, various types of waste are produced, such as sludge, mud and water. These waste products of the oil industry (mud, sludge, sand, oil shale, ash, etc.) can contain elevated concentrations of radionuclides from the uranium and thorium series, and because of the elevated radioisotope concentrations they often fall into the NORM (Naturally Occurring Radioactive Material) category (EU BSS, 2013, IAEA-TECDOC-474, 2013, Unscear, 2008 B). An oil well can produce up to 1000 tons of NORM solid waste annually (AL Nabhani et al., 2015). The radionuclide concentrations in the waste of the oil industry can have variations of several orders of magnitude (0–1000 Bq/kg) depending on which part of the technology produced the mud or scale (Bakr, 2010, Gazineu et al., 2005, Shawky et al., 2001, Al-Saleh and Al-Harshan, 2008, Xhixha et al., 2015, Hilal et al., 2014). This can cause a significant increase in the radiation dose experienced by the workers in the oil industry, leading to detrimental health effects (Darby et al., 2005). This is the reason why the waste products from the oil industry have to be characterized radiologically, and if the characterization reveals that it is necessary, constant monitoring is also required.

In this study, drilling mud deposits were surveyed from a radiological point of view. The deposits originated from the digging of Hungarian oil and gas wells. The drilling mud provides cooling and adequate lubrication to the drilling head during operation, regulates the pressure in the drilling hole, seals off porous geologic formations, and carries geologic drill cuttings from the bottom of the well up to the surface (Penn et al., 2014, Oreshkin et al., 2015, Ukeles and Grinbaum, 2004). Typically, water-based drilling mud (WBM) is composed of colloidal clays (bentonite), potassium-chloride, sodium-hydroxide, lignite, barium-sulphate, mica, ground nutshells, polymers, and a number of other additives, depending on the needs of the particular well (Whitaker et al., 2016). There are multiple opportunities for the valorization of drilling mud that has been used several times and that cannot be reused again. Possible uses for drilling mud include underground injection, application to agricultural fields, and usage in the building industry as a raw material. There are multiple choices for the handling of material that cannot be utilized, either because of its properties or for economic reasons, such deposition in pits or mud boxes, use in landscaping, dispersal over the ground, discharge to the sea or ocean, slurry injection, disposal in salt caverns, on-site burial (reserve pits) or storage in hazardous waste landfills. In the case of NORM disposal, the routes of exposure include external radiation, inhalation of dust (including radon and thoron progenies) and intake through the food chain (consumption of leafy vegetables, drinking water, fish and meat) (Kontol et al., 2015). These waste products, if placed in nature after utilization, can contribute to the radiation dose of the general population through the aforementioned routes; thus, in addition to the standard chemical characterization of the materials, it is wise to consider the radiological aspects as well. The harmful effects can be mitigated by the insulation and covering of the depositories (Várhegyi et al., 2013, Jonas et al., 2017).

The main aim of this survey was to determine whether the application of any long-term radiological monitoring system is necessary for the covered or uncovered depositories.

Drilling mud depositories in Zalatárnok, Hungary have been surveyed from a radiological point of view both before and after they were covered with clay. The ambient dose equivalent rates on the field have been measured and the gamma dose rates calculated from the concentrations of the terrestrial radionuclides. The migration through air has been checked by the measurement of radon in the soil gas, in situ radon exhalation (exhalation) and the radon concentration in air. There are no active faults or tectonic movement that could cause radon anomalies (Iovine et al., 2017). By measuring the U-238 and Ra-226 concentrations and their ratios in the water of monitoring wells, the radioactive contamination of ground water and deeper water layers has been investigated. In the case of oil and gas fields, the ratio of U-238/Ra-226 in the reservoir rock and the new NORM material is different. The U-238 series, up to Th-230, is not or is hardly able to be mobilized from the reservoir rock, while Ra-226, as is true of other elements in the II group of the periodic table, is water soluble (IAEA-SAFETY REPORTS-34). This is the reason why NORM materials from the oil and gas industry can have this disequilibrium, and in cases in which increased amounts of radionuclides get into the environment (if there is no other disturbing effect) this disequilibrium (which can be greater than an order of magnitude) can be observed (Hilal et al., 2014).

Section snippets

Sample collection

In the current study, we have carried out a radiological survey of the waste depository of the MOL Group at Zalatárnok. There are three depositories in the area (Fig. 1.) Two of them (Depository C, 90 × 150 m, and Depository B, 90 × 150 m) were already covered before the start of this study (and consequently only the covered state could be surveyed), but the third (Depository A 70 × 30 m) was investigated both in a covered and in an uncovered state.

During recultivation, a 1.5 m clay covering

Ambient dose equivalent rate H*(10), and gamma dose rate Dγr

The Ambient dose equivalent rate, and the calculated gamma dose rate results are shown in Table 1 and Table 2.

It can be seen in Table 1 that the range of the ambient dose equivalent rate measurements was 76 (67–85) when uncovered, which increased to 86 (83–89) nSv/h after the cover layer was applied. Depositories B and C were measured only in their covered state, because they had already been covered at the date of the measurement campaign. These values are shown in Table 2. The ambient dose

Conclusion

This survey of uncovered and covered drilling mud deposits from oil drilling has been carried out from a radiological perspective. In the case of the uncovered depository, the ambient dose equivalent rate at a height of 1 m from the surface of the drilling mud was 76 (67–85) nSv/h, which is in line with the average in Hungary (70 nSv/h) and the world average (57 nSv/h) (UNSCEAR, 2000). After the same depository had been covered, the ambient dose equivalent rate marginally increased to 86

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