Using Airborne Geophysics to Identify Salinization in West Texas

RI0257. Using Airborne Geophysics to Identify Salinization in West Texas, by J. G. Paine, A. R. Dutton, and M. U. Blum. 69 p., 59 figs., 2 tables, 3 appendices, 1999.  Print.

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ABSTRACT
Salinization of soil and water is a chronic environmental and agricultural problem in arid regions. In this study of a 91-km² area in Runnels County, Texas, we integrated high-resolution airborne and ground-based geophysical surveys and chemical analyses of soil and water to identify near-surface salinization and determine its origin. Possible causes of salinization are migration of brine alongnatural conduits (faults, fractures, joints, and permeable stratigraphic units), infiltration from brine-disposal pits and leaking oil and gas wells, and evaporative concentration of shallow ground water.

An airborne geophysical survey of the Hatchel area, where more than 700 oil and gas wells have been drilled since the 1920's, measured magnetic-field intensity and ground conductivity at three electromagnetic frequencies to identify (1) conductivity anomalies caused by salinization and (2) magnetic-field anomalies caused by well casings and other ferrous objects. Water samples were analyzed to verify airborne data and distinguish salinity types. We combined airborne geophysical data with oil- and gas-well locations to identify 107 conductivity anomalies consistent with oil-field salinization.

Ground-based geophysical measurements, aerial-photograph interpretations, and record inspections of 54 anomalous sites revealed that at least 42 had oil-field salinization and that 22 might be wells that are leaking or have leaked in the past.

We created a geophysical "profile" that captured 20 of the 22 potentially leaking wells identified during field investigations: a site that (1) has a magnetic anomaly or a known well location and (2) has anomalously high conductivity as measured by the high- and intermediate-frequency (56,000- and 7,200-Hz) airborne coils. These results suggest that airborne geophysics can be combined with well locations for identifying most potentially leaking wells without requiring ground investigations at every anomaly.

Used alone, airborne methods distinguish natural salinization from oil-field salinization but have difficulty discriminating among oil-field sources (pits, spills, and leaking wells). Used alone, ground-based surveys can map salinization extent and determine whether wells might be leaking, but unknown salinization is missed. In small areas where well locations are known, ground-based surveys can determine which wells might be leaking, and they are an inexpensive alternative to airborne surveys. Airborne methods are most effective in typical oilfield areas of tens to hundreds of square kilometers, where well locations are uncertain or multiple salinity sources are expected. Airborne data can be used to determine the extent and intensity of salinization, locate source areas, focus ground investigations, and estimate chloride mass in the ground.

Keywords: airborne geophysics, electromagnetic induction, oil-field pollution, salinization, water quality

CONTENTS
ABSTRACT 
INTRODUCTION

      Geology and Soils

      Hydrogeology

      Causes of Salinization

      Oil and Gas Activity

GEOPHYSICAL METHODS

     Airborne Geophysics

     Ground-Based Geophysics

          Single- and Multiple-Frequency Conductivity Profiling

          Time-Domain EM Soundings

WATER AND SOIL SAMPLING

RESULTS

     Airborne Geophysical Survey

          Magnetic Field Data

          EM Data

               56,000-Hz Vertical Dipole Data

               7,200-Hz Vertical Dipole Data

               900-Hz Vertical Dipole Data

     Regional Conductivity Patterns and Local Anomalies

          Site Selection

          Regional Conductivity Patterns

               Northwest Low Conductivity Zone

               Central Conductive Zone

               East-Southeast Low-Conductivity Zone

          Known Wells Having Conductivity and Magnetic Anomalies

               Site 76

               Site 34

               Site 43

               Site 51

         Conductivity and Magnetic Anomalies

               Site 17

               Site 71

          Known Wells with Conductivity Anomalies

               Site 12

               Site 16

               Site 73

          Other Local Anomalies

              Sites 65 and 67

     Chemical Composition of Water Samples

          Water Salinity

          Chemical Composition

          Comparison with Regional Trends

          Comparison of Soil and Water Salinity with Measured Conductivity

DISCUSSION

          Effectiveness of Airborne Geophysics

          Geophysical Profile of a Leaking Well

          Utility of Airborne and Ground-Based Geophysics

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

APPENDIX A. Hatchel area geophysical sites

APPENDIX B. Chemical composition of ground-water, surface-water, and oil-field-water samples

APPENDIX C. Chloride content and electrical conductivity of soils collected in this study in Runnels County

Figures

1. Map of the Hatchel, Texas, quadrangle

2. Generalized geologic map of the Hatchel quadrangle

3. Soil map of the Hatchel quadrangle

4. Total dissolved solids in brine from Guadalupian Series oil fields

5. Potentiometric surface of brine in Guadalupian Series based on equivalent fresh-water hydraulic head

6. Conceptual model of West Texas salinity sources

7. Oil- and gas-well locations in the Hatchel quadrangle

8. Photograph of helicopter lifting DIGHEM magnetometer and EM birds in preparation for airborne geophysical survey

9. Photograph of RRC worker using a metal detector to locate an abandoned well

10. Photograph of Bureau worker using Geonics EM34-3 ground-conductivity meter to perform reconnaissance EM survey

11. Exploration depth of various coil separations and orientations of the EM34-3

12. Protem 47/S transmitter input and receiver response

13. Instrument configuration of Protem l LEM sounding

14. Photograph of LCRA and CRMWD staff sampling a water well

15. Location of water and soil samples collected by CRMWD, LCRA, and the Bureau

16. Map of enhanced total magnetic field strength

17. Map of shallow ground conductivity at 56,000 Hz, vertical dipole coil orientation

18. Map of moderately deep ground conductivity at 7,200 Hz, vertical dipole coil orientation

19. Map of deep ground conductivity at 900 Hz, vertical dipole coil orientation

20. Changes in estimated exploration depth with ground conductivity for 900-, 7,200-, and 56,000- Hz airborne EM coils

21. Transient decay and resistivity models for TDEM soundings at site 76

22. Transient decay and resistivity models for TDEM sounding at site 82

23. Transient decay and resistivity models for TDEM sounding at site 81

24. Transient decay and resistivity models for TDEM sounding at site 83

25. Transient decay and resistivity models for TDEM sounding at site 84

26. Transient decay and resistivity models for TDEM sounding at site 85

27. Transient decay and resistivity models for TDEM sounding at site 86

28. Transient decay and resistivity models at TDEM soundings at site 17A

29. Transient decay and resistivity models for TDEM sounding at site 87

30. Sketch map of site 76

31. Apparent ground conductivity at site 76, measured using multiple coil separations and horizontal and vertical dipole orientations

32. Two-layer conductivity models that fit multiple-coil-separation data for east-west line 76B and north-south line 76C at site 76

33. Sketch map of site 34

34. Apparent ground conductivity at site 34, measured using 20-m coil separation and horizontal and vertical dipole orientations

35. Sketch map of site 43

36. Apparent ground conductivity at site 43, measured using 20-m coil separation and horizontal and vertical dipole orientations

37. Sketch map of site 51

38. Apparent ground conductivity at site 51, measured using 20-m coil separation and horizontal and vertical dipole orientations

39. Photograph of vegetation-kill area adjacent to abandoned brine-disposal pit, site 17

40. Sketch map of site 17

41. Apparent ground conductivity at site 17, measured using multiple coil separations and horizontal and vertical dipole orientations

42. Sketch map of site 71

43. Apparent ground conductivity at site 71, measured using 20-m coil separation and horizontal and vertical dipole orientations

44. Sketch map of site 12

45. Apparent ground conductivity at site 12, measured using 20-m coil separation and horizontal and vertical dipole orientations

46. Sketch map of site 16

47. Apparent ground conductivity at site 16, measured using 20-m coil separation and horizontal and vertical dipole orientations

48. Sketch map of site 73

49. Apparent ground conductivity at site 73, measured using 20-m coil separation and horizontal and vertical dipole orientations

50. Photograph of abandoned Early A No. 2 well leaking saltwater at site 67 49

51. Sketch map of sites 65 and 67

52. Apparent ground conductivity at site 65, measured using 20-m coil separation and horizontal and vertical dipole orientations

53. Histogram of TDS in ground-, surface-, and oil-field-water samples in the Hatchel area

54. Relation between electrical conductivity and TDS and Cl content of water and soil samples

55. Chemical composition of hydrochemical facies in ground-, surface-, and oil-field-water samples in the Hatchel area

56. Relationship between Br :Cl ratio and Cl concentration in water samples collected during this study, compared with regional data presented by Richter and others (1990)

57. Relationship between Cl:S04 ratios and Na:Ca ratios in water samples collected during this study, compared with regional data presented by Richter and others

58. Comparison of TDS of ground-water samples and ground conductivity measured by 7,200- and 56,000-Hz surveys

59. Comparison of measurement of ground conductivity by airborne geophysical survey and measurement of Cl content and electrical conductivity of soil samples

Tables

1. Generalized stratigraphy of the Colorado River watershed

2. Summary statistics for airborne geophysical anomalies


Citation
Paine, J. G., Dutton, A. R., and Blum, M. U., 1999, Using Airborne Geophysics to Identify Salinization in West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 257, 69 p.