Radioactive-Scale Formation
Authors: Smith A.L. 
Published: December 2007

Natural radioactivity in the oil and gas fields has been used extensively worldwide to monitor well performance for more than 30 years. Since 1981, North Sea operators have been finding radioactive scale. The chemistry and physics of this scale are detailed, and a correlation between API gamma ray units, dose rate, and specific activity is explained. Extensive monitoring procedures, operational control measures, personal radiological protection, and environmental discharge controls are necessary.

Introduction

Since the 1970's, polonium-210 has been monitored in onshore terminals of the U.K. southern gas fields. In 1981, radium-226 scale was detected in production tubing from a North Sea oil field. The production systems of several mature fields now contain deposits. Operators have developed extensive procedures to ensure the health and safety of all personnel involved and compliance with all relevant legislation. This paper provides detailed information for those personnel who have provides detailed information for those personnel who have operational responsibility for radiological procedures. The U.K. Offshore Operators Assn. (UKOOA) has produced a set of guidelines and a reference manual to assist operators in gaining a clear understanding of this complex topic.

Radio-Geochemistry of Sedimentary Rocks

Regional zones of mineralization, known as uranium provinces, have been formed within the lithosphere. provinces, have been formed within the lithosphere. Uranium mineral exploration indicates that there is no positive correlation between the occurrence of oil and positive correlation between the occurrence of oil and gas and of uranium. The frequency distribution of uranium in the lithosphere derived from geologic survey coincides with the calculated uranium concentration distribution derived from the radon content of natural gas. This indicates a positive correlation with a phenomenon that is typical of sedimentary rock. phenomenon that is typical of sedimentary rock. Sandstones and limestones from several geologic systems e.g., Devonian, Permian, Jurassic, Cretaceous, and Tertiary have been identified as hydrocarbon source rocks (see Fig. 1). The main radioelements in sedimentary rocks are potassium, uranium, and thorium, and the highest levels are potassium, uranium, and thorium, and the highest levels are in shales. Potassium levels in shale are a result of the clay minerals illite and K-feldspar. Common shales have uranium and thorium levels close to the average for the continental coast. High uranium levels in black shales are probably a result of their increased organic content. Black probably a result of their increased organic content. Black shales can either absorb uranium or reduce the uranyl ion in brines to uranium oxide. Potassium levels in orthoquartzites and arkoses result from K-feldspar, K-mica, and glauconite. Uranium in sandstones is spread mainly through the quartz grains, and the thorium is associated with resistate grains. Zircon and monazite are frequently concentrated as detrital minerals within sedimentary deposits derived from the major igneous rocks. On a microscopic scale, uranium is concentrated in fine-grained accessory minerals at the boundaries of the grains or crystals of the major rock-forming minerals, resulting in a high emanating power. In limestones, uranium ions replace calcium ions within the crystal lattice of calcium carbonate. In phosphatic limestones, which contain uranium in several hundred parts per million, the phosphate ion is replaced. Because thorium per million, the phosphate ion is replaced. Because thorium does not enter the carbonate lattice easily, it is found mainly in the clay and heavy mineral fractions. These also contain the bulk of the potassium in limestones. In sedimentary basins, the evaporites are also a source of radioactivity. Salt deposits contain beds of the potassium chloride minerals sylvine and carnallite. The detrital silicate mineral fraction contains traces of uranium and thorium.

Transportation of Radioelements in North Sea Hydrocarbon Deposits

Southern Gas Fields.

Natural gas deposits in the southern North Sea occur in the Lower Permian Rotliegende sandstone, which unconformably overlies coal measures from the Westphalian stage of the Carboniferous period. The Rotliegende has a porous dune sand facies and is sealed by Zechstein salt deposits. The gas has been formed by the devolatilization of the coal seams, which occur in a normal-coal-measures cyclothemic sequence that, at intervals, includes beds of dark gray to black marine shale. Similar shales occur in the British coal fields and are often radioactive because of uraniferous phosphate nodules containing up to 1,000 ppm uranium. The angular nature of the sub-Rotliegende unconformity causes both coal and marine shale horizons to come into contact with the brines that occupy the pore spaces in the lower part of the Permian sandstones. The brines will dissolve the radium-226 from the parent uranium-238 nodules in the marine shales, and because radium-226 has a half-life of 1,620 years, radon-222 will be carried by the natural gas into the reservoir. Uranium enrichments in outcrops of the coal seams, phosphatic nodules in the Stephanian stage of the Carboniferous, phosphatic nodules in the Stephanian stage of the Carboniferous, and uranium absorbed on the red iron oxides that color the grains of Rotliegende sands may also contribute to the formation of radon-222.