The samples were then used to determine radium content, ph was ... using a standard chloride electrode with calibration at the beginning of work and calibration.
Association of Naturally Occurring Radioactive Materials With Mississippi Hydrocarbon Production: A Project Overview Charles T. Swann Mississippi Mineral Resources Institute Rick L. Ericksen Mississippi State Board of Registered Professional Geologists Joel Kuszmaul University of Mississippi, Department of Geology and Geological Engineering John Matthews University of Mississippi, Department of Pharmacology Clayton McKay University of Mississippi, Department of Geology and Geological Engineering
ABSTRACT Naturally occurring radioactive materials (NORM) has been documented from hydrocarbon production in Mississippi. The lack of compositional data regarding NORM concentrations in produced fluids, scales and sludges provided the stimulus to acquire such information in order to aid the hydrocarbon industry and governmental agencies. These scientifically-defensible data are now being utilized to construct elements of best practice and test NORM predictive models. The data base consists of a total of 329 samples, with 275 brine samples from 37 reservoirs and representing all major producing areas in Mississippi. The remaining samples represent samples of scale, soil, sludge, salt, and biological samples. This data base is perhaps the most extensive in the Southeastern U.S. that is available for public use.
INTRODUCTION Naturally occurring radioactive materials (NORM), produced along with oil and gas production, are often concentrated at some point in the hydrocarbon production process. These concentrated NORM deposits could be referred to as technically enhanced naturally occurring radioactive materials or TENORM. In this investigation both NORM and TENORM, are included under NORM acronym, so both will be referred to simply as NORM. The radioactive component is, of course, the major environmental concern that has resulted in significant economic loss to the industry and continues to be an economic concern and legal liability. Exxon and ChevronTexaco, for example, have both been involved in multimillion dollar law suits involving NORM. In the case of Exxon, the dollars were in excess of one billion. NORM - related legal costs to both major oil companies continue as decisions are appealed and other suits are filed. NORM related costs and liability also extends to the smaller independent producers such as those which produce most of Mississippi’s hydrocarbons. To the small independent operators, there is the cost of maintaining NORM storage areas, the increased cost of business in fields which produce NORM as well as the continued legal liability represented by the presence of NORM in storage and NORM present in equipment currently being used. So, there is little doubt of an economic incentive to better understand NORM and its characteristics. The U.S. Geological Survey and the U.S. Department of Energy have contributed significant knowledge of NORM resulting from hydrocarbon production but, their studies, as others, have seldom included the problem of NORM produced in the southeastern U.S., focusing instead on the western portion of the country. Oddly, there is little known of the characteristics, geographic distribution, concentration of radioactive components or how NORM reacts in the humid environments of Mississippi. A review of published literature indicates that Mississippi-specific NORM investigations are essentially nonexistent. As a result of this informational gap the most relevant work to this investigation are the investigations conducted in Texas by Fisher (1),(2). The goal of this set of investigations is to fill this informational gap by providing base line data regarding NORM in Mississippi. It is hoped these data and the conclusions derived from them, will be used by the industry, regulatory agencies, as well as the public. These data have been gathered in an objective, scientifically defensible manner, and the conclusions represent only an interpretation of these data. The data base used in this investigation included a total of 329 samples. The total samples included brine, biological, salt, scale, sludge, and soil samples. The majority, 275 samples, were of brines co-produced with hydrocarbons. Unfortunately, quality control considerations resulted in some the brine samples being rejected for analysis. Some were unuseable due to excess oil and inadequate brine in the sample, for example. Whereas some samples containing nearly fresh water were rejected after it was determined the flow line and stock tanks had recently been steam cleaned. These brine samples represent 37 different reservoirs and were derived from all major producing areas in Mississippi, i.e. the Black Warrior Basin, Mississippi Interior Salt Basin and the Wilcox Trend (Figure 1). Reservoir age varies from Mississippian units in the Black Warrior Basin to Tertiary units in the Wilcox Trend. Reservoir rock types vary widely from well consolidated Paleozoic limestone to poorly consolidated Tertiary sands. Thirty-eight operators have aided the investigation by allowing their wells to be sampled.
METHODOLOGIES
Field Methodology Well sampling is the most basic step in the process of deriving meaningful results in the laboratory as well as the spatial and statistical modeling to follow. With multiple places that can be sampled in the production system, it was necessary to place priorities on potential sampling locations to obtain optimal results in the laboratory. All potential sampling locations are often not assessable at any given well site, so a set of priorities was established for sample location. The sampling priorities are, in order of decreasing priority, 1) from the wellhead, 2) from flow lines, 3) from the heater treater, and 4) from stock tanks. The initial brine samples often consisted of an oil / brine mixture which necessitated an on-site separation by simply allowing the oil and brines to separate in a 19 liter bucket. A valve at the base of the bucket then allowed the brines to be preferentially drained. The excess oil and brine was returned to the system. Each sample consisted of 7.6 liters of brine. Each sample was labeled with a unique sample number that is common to field notes and custody sheets. The field notes recorded date, time, well name, sample number, API number, sampling location, and other pertinent data. The custody sheets included similar information in addition to laboratory handling information. The samples containing solids, such as scale, soil, metal, or biological materials, followed similar labeling and documentation procedures.
Laboratory Methodologies The initial task was to “clean up” the liquid samples by throughly removing suspended or emulsified oil from the brines. The samples were then used to determine radium content, ph was measured and chloride determinations were made. Selected samples were analyzed for additional ions. The methodology used to determine the 226 Ra and 228 Ra content has been reported by Matthews and others (3). This methodology employs the use of EmporeTM Radium Rad Disks to selectively bind and isolate the radium content of a sample. This method allows 226 Ra and 228Ra to be quantified at levels down to 1 Bq/L. The methodology also allows analyses to be made without the need for highly specialized equipment. Anions were analyzed using an ion chromatograph (Dionex DX-120) equipped with a high capacity separation column. Cations were analyzed using an inductively coupled plasma optical emission spectrometer (Perkin Elmer Optima 4300). Chloride measurements were made using a standard chloride electrode with calibration at the beginning of work and calibration checks every 10 samples. Additional chloride measurements were made using standard flame photometry methods. Measurements of pH were also made on each sample using a standard, calibrated pH meter. The mineralogical composition of the scale was determined by standard xray diffraction techniques. Electron microscopy was used to examine the microstructure of the crystalline scales.
Statistical Methodologies The NORM database was used as a tool to help identify relationships between the constituents and to identify evident trends useful in characterizing NORM distribution associated with oil and gas production. We applied statistical tests comparing distribution parameters of subgroups within the data set. The database allowed us to consider samples grouped by basin,
field, producing formation, and spatial coordinates. Analysis for spatial continuity was performed using the major constituents within the database, 226Ra, Cl2, Ba2+, and BaSO42saturation. Barium sulfate saturation was determined using the general saturation index method described by Oddo and Tomson (4). Relatively homogenous populations or groups of data by geologic formation and producing field were identified from the mean of two or more groups and their similarity. These relatively homogeneous groups were considered using experimental variograms of the major chemical constituents within the dataset. These variograms were then used to characterize the degree of spatial continuity from samples taken from particular formations and producing fields.
THE NORM DATA BASE The project has, as illustrated in Figure 1, sampled brines from producing wells throughout Mississippi, including all the major producing areas. As the project was carried to completion, data management became an important aspect of the project. This well sample-derived information was pooled in an electronic spreadsheet format (Excel) that allowed easy retrieval and allowed data searches to be conducted. The data included primary, laboratory-derived analyses such as radium content; secondary information derived from the public domain, such as well completion information; and information derived from the private sector, such as geographic coordinates for well locations. A combination of these data provides a basis for several types of analyses. Perhaps the most important use for the data base was as a source for the spatial and statistical analyses. There is little doubt that other uses could be found for these data. For this reason, it will be made available to the public at the completion of the project
RESULTS A primary goal of the investigation was to determine if radioactive ions were contained in the produced waters and if so, identify the radioactive ions, their concentrations and geographic distribution. Mississippi continues to produce prodigious quantities of brine as a by-product of hydrocarbon production. So, if these brines contained significant amounts of radioactive isotopes, then the environmental problems would be significant. Fortunately, this is not the case. As might be expected, the major radioactive component of the brines were identified as isotopes of radium, i.e. 226Ra and 228Ra. The
226
Ra isotope has a half-life of approximately 1,620 years and 228Ra has a half-life of approximately 5.8 years and both were identified in all the brines we analyzed is small amounts.
The state-wide average radiation values for 226 Ra were 12.6 Bq/L, and 15.1 Bq/L for 228 Ra. With only low amounts of radiation in the brines, we can conclude that the radionuclide content in the produced water represent only a minimal radiation hazard. Since these brines typically contain a significant amount of NaCl, the environmental hazard is well known and rules are in place for proper disposal of saltenriched brines. With low radiation content, if the brines are disposed of in a proper manner for the NaCl content, then the radionuclide content is also taken care of in an environmentally responsible manner. The fact that all the produced waters contain small amounts of radiation, clearly points to the formation of scale as the most important factor in the radiation hazard analysis. If there is little or no tendency for scale formation, such as in Mississippi’s portion of the Black Warrior Basin, then the producer can expect to encounter few problems with NORM-based radiation. The Wilcox Trend also produces less scale than the Interior Salt Basin, although there is some geographic overlap of producing
areas. With scale crystallizing from the brines, the brine chemistry is an important aspect of the brine scale-producing system. The scales sampled during the study were identified as barite (BaSO4). A solidsolution series exists with barite as one end member and celestite (SrSO4) as the other. The boundary separating the two minerals is typically placed at a Ba to Sr ration of 1:1 but, most material falls closer to one or the other end members (5) (6). The scale may, then, vary in the amounts of Ba and Sr present in the crystal as the chemistry of the parent brines vary from which the crystal was formed. Calcium can be substituted in the barite structure for Ba or Sr, but only to a limited degree (6). With Ra and Ca being of approximately the same ionic radius, Ra can also be incorporated into the crystal structure. This Ra derived from the brines is included in the barite crystal lattice as it grows in tubing and flow lines and thus the mineral becomes radioactive. Scale encrusted flow lines with as much as 60 µr/hr have been measured by this investigation from the outside of the steel pipe. There are two important conclusions that should be made regarding scale. The first is that estimates of NORM volumes have assumed that scale is uniformly produced by all hydrocarbon operations in the State (6). This investigation suggests that the assumption of uniform NORM production across Mississippi is incorrect and the amounts of NORM produced varies significantly among the major producing areas. The Black Warrior Basin, for example, produces little scale. The paucity of sulfates in the Black Warrior brines, does not encourage the crystallization of barite, although the Ba ion is present. A similar situation exists in the Wilcox Trend, so scale formation in this area is minimal. The Mississippi Interior Salt Basin brines contain both the Ba ion and the sulfate ion , so radioactive barite scale is common in this basin. This finding also suggests that national inventories of NORM that use similar assumptions of uniform NORM production may require revision. With the sulfate content of a brine apparently controlling barite crystallization, a simple predictive model is suggested to identify wells with potential NORM concerns i.e. brines with high sulfate content is more likely to produce radioactive scale than those with low sulphate content. The second conclusion is that the scale has significantly higher levels of radiation than the brines and require more care in disposal. The disposal of scales have been an important factor in much of the Mississippi litigation. It is suggested that a set of best practices formulated by and adhered to by the industry could be instrumental in reducing the potential for NORM related litigation. A suggested set of elements of best practice is suggested below. Mississippi allows NORM disposal by way of land spreading, which, when rules are properly applied, should result in the release of the property without land-use restrictions. Since the land spreading method of disposal requires the NORM-enriched scales to be in direct contact with the containing soils, it is important to ensure that the barite matrix containing the radium remains insoluble. This idea of barite being insoluble in typical soils is a largely untested assumption, seemingly based on the insoluble characteristics of barite under laboratory conditions. This assumption was tested to determine the validity of the assumption of barite insolubility. Laboratory leaching experiments tested the ability of common solvents (including artificially produced “acid rain”) to leach significant amounts of radium from pulverized barite scale. These experiments supported the assumption that barite is largely insoluble, as little radiation was leached from the samples. These experiments do not, however, imitate the conditions that may be present when scales are contained in a soil matrix. These conditions was imitated by mixing scale and common soils and incubating the mixture. After incubation, the soils were extracted to determine the amount of radioactivity removed from the mixture using artificial rainwater as the solvent. All of the soil incubation conditions resulted in greater extraction of soluble radioactivity than were observed for percolation of artificial rainwater through the isolated NORM samples. The amounts of leached radiation is small. When the abilities of soils to complex soluble radium are considered, however, the indication is that substantial amounts of radium may have been released from the NORM
samples through incubation with moist top soils. This appears be an area where additional research is clearly needed. The majority of brine samples were found to have radium activities greater than 100 pCi/L, while 99% of these samples were also found to have chloride concentrations in excess of 20 g/L. While a strong correlation has been reported between 226Ra and 228Ra by other researchers (1), we found only a moderate correlation (r = 0.74). Mean concentrations of Cl, Ba+2, and SO4-2 vary significantly from one geologic formation to another, while 226Ra appears to show significant variation within each formation compared to variations between formations. Other than Ba2+ and Cl concentration for a few groups, there seems to be little to no other spatial continuity within field of production or producing formation for the variables 226Ra, Cl, Ba2+, and BaSO42--saturation at the sampled scale used for this project.
Elements Of Best Practices For Industry Elements of best practice are suggested below. This set of practices was formulated by reviewing the data generated by this investigation, review of existing laws and regulations, and by interviews with industry regarding how NORM was typically handled and stored. Although these practices are not exhaustive, they are offered as an initial effort to develop a more complete set of best practices. Integral components related to the handling of NORM contaminated equipment includes the unintentional environmental release of NORM into the environment, the reasonable and adequate protection of those workers in the actual handling of NORM contaminated equipment, working in a NORM contaminated environment, and the protection of the general public from accidental or unwarranted exposure and/or ingestion of these NORM-containing materials. As such the following outlines, in a general to specific manner, a cross-section of general methods and procedures by which these pathways may be blocked or minimized to an environmentally reasonable and acceptable extent. In addition to these suggested best practices there are several mandatory and/or suggested regulatory requirements and practices including Mississippi State Oil & Gas Board’s Rules 28, 68, and 69 (8), which contain additional requirements related to NORM. Also note that other regulatory permits and regulations of other agencies of the state as well as federal government may apply such as those of Occupational Safety and Health Administration (OSHA) which relate to workplace safety procedures and training. Many of these other regulatory requirements lie outside the scope of this discussion and as such are not specifically discussed here. The major elements of best practice are listed below. 1) It is suggested that either tarpaulins or plastic sheeting be placed in the area(s) directly surrounding the wellhead and in those areas where the equipment, tubing, sucker rods, etc. may be placed or temporarily stored. The placement of sheeting and/or tarpaulin should be coincident with those areas where scale may become dislodged from the equipment being removed from the well. 2) Suggested best practices as related to the transportation of NORM contaminated equipment include the following components: Prior to the loading or placement of such equipment on/in floats for transport it is suggested that they be lined with one continuous sheet of plastic sheeting which will be able to be completely sealed once the equipment is loaded thereon. Sealing may be generally accomplished the use of any high-quality duct tape. Once the equipment is loaded in the float and properly sealed the materials may then be transported to either of the following: 1) the in field, NORM storage facility or 2) to an approved facility for decontamination and proper, permanent disposal of the NORM containing scale. 3) It is important to note that “closed vessels” may include the following: heater treaters, separators, stock tanks, vacuum trucks, barrels or any other sealed containers containing NORM
materials or NORM contaminated equipment, tools, clothing, gloves, discarded plastic sheeting, etc. Because of the potential for an explosion due to the hydrocarbon components of the produced fluids being collected, it is, of course, absolutely necessary to vent those containers (most commonly tanks) which have held hydrocarbon fractions. 4) It is important to note that closed container(s) which contain NORM may contain radon gas generated from the radium decay. Radon may accumulate to varying levels within the container. It is important that adequate ventilation be provided within the working area to disperse any radon from the containers. 5) Worker(s) should first remove the container lid and allow the contents to come in contact with the exterior air for a minimum of 10 minutes to allow the dispersion of any radon gas which may have been generated and accumulated within the headspace. 6) The worker(s) should always avoid the direct inhalation any gases that may be contained within the container headspace as residual radon gas may be present. 7) Once the inspection is completed the containers lid should be replaced and throughly secured to prevent any leakage of the NORM contaminated materials that may be in a liquid form (e.g. sludges). 8)Contaminated clothing, e.g. contaminated protective outerwear, including gloves and footwear coverings should be disposed of with the scale and/or sludge itself. These items should be placed in barrels along with NORM materials, e.g. scale and sludge, and stored at the in-field storage facility or transported for permanent disposal at a NORM approved disposal facility. 9 )Particularly in dry conditions, workers should wear breathing masks to filter out any NORM contaminates which may have become airborne. This is particularly applicable under conditions which would necessarily create dust, such as in landspreading activities. 10) Maintaining a log of the amount of NORM produced by each well should also be of use to evaluate the economic viability of the well. The NORM costs can be factored into overall operating expenses associated with those particular facilities. 11) It is suggested that any containers with NORM materials of high moisture content or those which are liquid or with significant amounts of free or chemically bound water should have their interiors coated with a rust resistant material. 12) It is suggested that impermeable barrier be included in construction of NORM storage facilities. A barrier, such as a synthetic liner would prevent an liquids containing NORM components from contaminating shallow groundwater systems. 13) Long-term NORM storage facilities should be located as far as physically possible from any dwelling. 14) NORM storage facilities should not be situated adjacent to any surface water (including ephemeral streams or bodies of standing water). Unintended release of NORM materials could occur during flood events or erosion events associated with high precipitation. 15) NORM storage facilities should not be located within the outline of the 100 year flood plain if
practicable. Flood events could result in the loss of isolation of the NORM-enriched materials.
CONCLUSIONS The acquisition of base line data regarding NORM in Mississippi allows the following conclusions to be made. This investigation also provides the base line data needed for responsible promulgation of rules and allows a set of industry best practices to be suggested to better handle NORMenriched scales. 1) Naturally occurring radioactive materials are an integral part of the waters co-produced with hydrocarbons in Mississippi, and are present in small amounts in all basins. 2) The radionuclides of concern are 226Ra and 228Ra. 3) Because of the small amounts of radium contained in the brines we suggest that proper disposal appropriate for the NaCl content also disposes of the radiation hazard. 4) Radioactive scales form from the crystallization of barite from the brines and represent a greater radiation hazard and requires more care in disposal and storage. 5) The sulfate ion appears to control the crystallization of radioactive barite scales. Variation in the sulfate content of brines leads to variation in the potential for a basin to produce scale. This is the reason for little scale production within the Black Warrior and the Wilcox Trend. 6) A predictive model for potential scale formation is suggested by determining a brine’s sulfate content. The greater the sulfate content, the greater the potential for scale formation. 7) All brines contain barium in varying quantities. 8) Barite scales are relatively insoluble when leached in their pure form. When mixed with soil, however, the soil micro-organisms apparently aid to make the scale more soluble and allowing the radium to become bioavailable. 9) A set of best practices to handle NORM-enriched scales is suggested herein as an initial effort to promote a more complete set of accepted industry-wide practices. A complete set of practices would further minimizes the potential liability to the operator of the property, the working interest owners of the property, as well as others who may have had subsidiary activities on or within the confines of the property.
ACKNOWLEDGMENTS This manuscript was prepared with the support of the U.S. Department of Energy, under Award Nos. DE-FC26-02NT15227 and DE-FG26-97BC15035 any findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the U. S. Department Of Energy. The U.S. Department Of Energy contributed $202,962 toward the completion of this project with $50, 881contributed as cost share.
The authors wish to acknowledge the encouragement and support of the Project manager, Mr. John Ford of the U.S. Department of Energy, National Petroleum Technology Office. The authors wish to thank Mr. Charlie Cooper at the U.S. Department of Agriculture, Agricultural Research Service/Sedimentation Research Laboratory in Oxford, MS for assistance with soil selection for the project and for assistance in obtaining the soil samples. Since this work involved brines produced from oil and gas wells, we are, of course, greatly indebted to the independent oil and gas producers in Mississippi for providing access to their production facilities.
REFERENCES CITED 1) 2)
Fisher, S.F., Naturally Occurring Radioactive Materials(NORM) in Produced Water and Scale from Texas Oil, Gas, and Geothermal Wells: Geographic, Geologic, and Geochemical Controls, Austin, Texas, Texas Bureau of Economic Geology, Geological Circular 95-3, (1995). Fisher, S.F., “Geologic and geochemical controls on Naturally Occurring radioactive Materials (NORM) in Produced Water from Oil, Gas, and Geothermal Operations”, Environmental Geosciences, 5, 139-150 (1998).
3)
Matthews, J.C., Bogan, Catrina, Swann, C.T., and Ericksen, R.L., “A Method for Quantitative Determination of 226Ra and 228Ra in Produced Formation Waters (Brines) from Oil and Gas Wells”, Radioactivity & Radiochemistry, 4,55-56 (2000).
2)
Oddo, J.E. and Tomson, M.B., “Why Scale Forms and How to Predict It”, SPE Production & Facilities, February, (1994).
2)
Palache, Charles, Berman, Harry, and Frondel, Clifford, “Dana’s System of Mineralogy”, John Wiley and Sons, 2, 407 - 419, (1951).
2)
Deer, W.A., Howie, R.A., and Zussman, J., “ An Introduction to the Rock-Forming Minerals”, Longman Group Limited, 462 -465, (1983).
2)
American Petroleum Institute, “A Naturally Occurring Radioactive Material (NORM) Disposal Cost Study”, American Petroleum Institute, 39, (1996).
2)
Mississippi State Oil & Gas Board Rules and Regulations, (2003).
Figure 1- Samples for this investigation were derived from all major producing basins. The map locations consist of multiple wells.
