Lead Deposits in Dutch Natural Gas Systems
- F.A. Hartog (Shell Research and Technology Center) | G. Jonkers (Shell Research and Technology Center) | A.P. Schmidt (Inst. of Earth Sciences, Utrecht U.) | R.D. Schuiling (Inst. of Earth Sciences, Utrecht U.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- May 2002
- Document Type
- Journal Paper
- 122 - 128
- 2002. Society of Petroleum Engineers
- 4.3.3 Aspaltenes, 4.3.4 Scale, 6.5.4 Naturally Occurring Radioactive Materials, 4.1.5 Processing Equipment, 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating, 4.6 Natural Gas, 5.2 Reservoir Fluid Dynamics, 5.1.1 Exploration, Development, Structural Geology, 6.5.2 Water use, produced water discharge and disposal, 2.2.2 Perforating, 4.5 Offshore Facilities and Subsea Systems, 5.1.2 Faults and Fracture Characterisation
- 0 in the last 30 days
- 252 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 5.00|
|SPE Non-Member Price:||USD 35.00|
During the production of natural gas from Dutch Rotliegend and Kupferschiefer sediments that are next to "conventional" carbonate and sulphate scales, lead deposits sometimes also form. These deposits, which vary in composition from galena (lead sulphide) to elemental (metallic) to lead and complex lead (hydr)oxides, may form a serious threat to operations because they tend to block production equipment or injection pumps and tubing. In some cases, radioactive lead (lead-210 with a half-life of 22.3 years) is incorporated into these compounds at levels greater than regulatory concern, which necessitates additional safety measures during operations and maintenance.
In sour gas wells, the deposition of galena can be explained by local supersaturation (caused by a sudden temperature or pressure drop) where bisulphide anions react with lead cations in the produced water, in which concentrations of up to 150 mg lead per liter brine have been detected. In the absence of hydrogen sulphide and even in "dry" gas wells, the deposition of metallic lead has been observed locally at rates of 10 g/D or more. An electrochemical mechanism is held responsible for lead deposition on carbon steel tubing: iron from the tube wall is oxidized by lead ions in solution, leading to corrosion of the wall and deposition of lead scales in thicknesses of 10 mm or more. Under unfavourable conditions or during maintenance operations, metallic lead is easily converted into secondary lead minerals.
Detailed knowledge on the geological origin, the transport pathways to surface facilities, and the mechanisms of lead scale formation might enable the future development of techniques that can prevent lead deposition.
Naturally occurring radionuclides (NORs) are found throughout the Earth's crust. Their presence is the major cause for the (largely) unavoidable natural background radiation to which all humans are exposed. Many human activities, such as mining and milling of ores, extraction of petroleum and natural gas resources, use of groundwater for domestic purposes, and living in dwellings, may alter the natural background radiation environment, either by moving NORs from inaccessible locations to places where humans are present or by concentrating them. Exposures resulting from displaced or concentrated NORs are termed technologically enhanced natural radiation doses, while materials in which the NORs appear are termed technologically enhanced, naturally occurring, radioactive materials (TENORM). Generic reviews on the sources and origins of TENORM have been published in recent years,1-3 whereas the distribution of TENORM in various streams from the exploration and production (E&P) industry has been reviewed in 1997.4,5 In view of the health, safety, and environmental aspects6 and because of the costs associated with the presence of TENORM in E&P product and waste streams,7,8 which are estimated to amount to several billions of dollars, fundamental knowledge of TENORM's origin, transport, and deposition mechanisms is of paramount importance for the development of prevention/ inhibition techniques.
The origin of TENORM in product and waste streams from the gas and oil industry is fairly well understood and documented,4,9,10 the best known example being radium that contains low specific activity (LSA) scales (see the Appendix for definitions of terms, abbreviations, and units). Radium is found only at extremely low chemical concentrations (in a typical scale at the parts per billion, ppb, level), incorporated in common conventional (calcium, barium, and/or strontium sulphate and/or carbonate) scales. Both the mechanism for the transport of radium from the reservoir to topside facilities with produced brine and the formation of radium-containing [226Ra from the uranium-238 (238U)-decay-series and 228Ra from the thorium-232 (232Th)-decay-series; see Figs. 1 and 2] scales is fairly well described and documented.4,11,12 Next to 226Ra and 228Ra, the radioactive isotope lead-210 (210Pb) can also (sometimes) be detected in these scales. This lead isotope originates almost exclusively from in-situ decay of 226Ra in the scale. Because the emanation factor for radon (222Rn) in these scales is very low, the scale's age can be calculated from its 210Pb/226Ra ratio.
The deposition of 210Pb in natural gas liquids fractionating units, such as depropanizers and debutanizers, in which a concentration of the noble gas radon can take place, is also well known. After several years of operation, very thin (nearly invisible to the naked eye) 210Pb deposits are found in some plants with very high contamination levels.4,13 These deposits originate from the decay of 222Rn, which has a fairly long residence time in the installations. The same phenomenon has been observed for gas transport14 and distribution systems, in which the residence time of the gas is also in the order of days.
The origins as well as the transport and deposition mechanisms of 210Pb are fairly well documented and understood. A short overview of these (based on Ref. 4) is given in Figs. 1 and 2. In the last few years, attention has been focused15 on LSA scales containing "unsupported" 210Pb, radioactive lead in the absence of its radiogenic ancestor 226Ra.
In these scales, which are almost exclusively encountered in gas-producing facilities, no or too little 226Ra is present to account for the observed 210Pb activity.16 In marked contrast to the deposits found in natural gas transport, storage, and fractionating units, 210Pb here is always accompanied by stable (i.e., nonradioactive) lead, which acts as the carrier for the radioactive isotope. These deposits can be found both distributed throughout entire production/ treatment systems as scales with thicknesses of a few millimeters or in equipment very locally as massive lumps (weighing a few kilograms, approximately the size of a man's fist), which may clog the entire tubing and block water-injection pumps. The origin of this lead and its transport and deposition mechanism are poorly understood. In 1995, a research project in these areas - sponsored by the N.V. Nederlandse Aardolie Maatschappij (NAM) - was initiated and carried out at Utrecht U.17 and at the Shell Research and Technology Centre in Amsterdam in close cooperation with NAM. The results from these studies are summarized in this paper.
|File Size||827 KB||Number of Pages||7|