Radiological survey of the covered and uncovered drilling mud depository

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 C_U-238 12 (10-15) Bq/kg, C_Ra-226 31 (18-40) Bq/kg, C_Th-232 35 (33-39) Bq/kg and C_K-40 502 (356-673) Bq/kg. The same radionuclide concentrations in the clay were C_U-238 31 (29-34) Bq/kg, C_Ra-226 45 (40-51) Bq/kg, C_Th-232 58 (55-60) Bq/kg and C_K-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.