Helicopter Geophysical Survey to Detect Brine, Brookhaven Oil Field, Mississippi

Brine produced along with oil at the Brookhaven oil reasons: 1) low altitude flying requires a high degree of maneuverability to avoid numerous populated areas, 2) slow field has contaminated near surface and deeper (at least 100 flight speed provides optimum spatial resolution of m) aquifers. Sources of cultural noise, typical of many oil fields, can interfere with ground and airborne geophysical geophysical signals, 3) mapping of both shallow ( lOO m) subsurface features, and 4) provision of measurements used to map subsurface brine. The first phase of this study, reported here, is to evaluate application of ancillary geophysical measurements uch as VLF (Very Low Frequency) and high resolution magnetic field sensors. airborne electromagnetic (AEM) methods to determine In planning the HEM survey, we used a set of 44 whether any useful AEM data can be collected under Wenner DC resistivity soundings made in and near the oil extremely adverse conditions of heavy cultural noise and a field (Nacht and Barrows, 1985):Interpretation of soundings low resistivity earth. using algorithms of Zohdy and Bisdorf (1975) along a NorthFor this survey a helicopter electromagnetic (HEM) South profile paralleling section C-C’ (Figure 1) are system with a horizontal coil configuration was employed presented as a depth/resistiy 1) 900, resistivity associated with known brine contamination and 7,200, and 52,000 Hz active EM system using horizontal coil suggest areas for additional study. pairs in order map the broad range of resistivities anticipated, 2) high precision total magnetic field sensor, 3) nominal flight BACKGROUND line spacing of 220 meters at an altitude of 60 meters, 4) a The Brookhaven oil field (Figure 1) is one of the high precision UHF navigation system with a backup oldest fields in Mississippi (Kalkhoff, 1986). Since 1943, photographic record of the flight path, and 5) VLF and approximately 54.2 million barrels of brine have been power line monitoring (vertical and horizontal coils) passive pumped to the surface from at least 75 wells. At first, the EM systems. brine was disposed of by pumping it onto the ground or into a nearby stream. Later it was pumped into evaporation pits. TOTAL MAGNETIC FIELD DATA Since 1978 when the above disposal practices were prohibited, The magnetometer system used in this survey consisted brine has been re-injected by Class II wells into the deep oil of a high sensitivity (0.01 nT) airborne sensor. During the producing formations at least 1.5 Km deep. Past and present survey, the magnetic sensor produced a 2-5 nT sinusoidal disposal methods pose a threat to the quality of near surface noise envelope with a wavelength of about 8 seconds along water supplies. Saline waters might seep to the surface the flight path in part caused by motion of the cesium sensor through improperly cased or plugged wells or through within the low inclination (600) local magnetic field. subsurface vertical fractures. The IGRF and a strong east to west regional gradient Gravels of the basal Citronelle Formation and sand were removed from the data in order to enhance local layers in the Hattiesburg Formation are the main aquifers in anomalies. The contour map (Figure 4) shows many short the study area (Figure 2). AI1 of the aquifers in the oil field wave-length anomalies (on the order of 100 m) having have been contaminated by brine to a depth of at least 100 positive amplitudes ranging from 10’s to 100’s of nT. Many m (Kalkhoff, 1986). Contamination outside of the oil field is of these anomalies are associated with oil tanks (100-200 nT unknown. For example, brine can move laterally from the anomalies in the center of the oil field) and steel well Citronelle aquifer to discharge into nearby streams and can casings (several of the smaller anomalies). Frischknecht and move vertically into underlying Hattiesburg aquifers. others (1985) discuss applications of airborne magnetics to locate abandoned cased oil. wells. Certain other cultural DESIGN OF AIRBORNE GEOPHYSICAL SURVEYS features (steel buildings, large machinery, and pipelines) cause Both fixed wing time domain (INPUT) and helicopter some of the magnetic anomalies. frequency domain AEM systems were considered for this Several small circular anomalies are distributed in a survey (Becker and Morrison, 1987). A helicopter way that suggesting a regular drilling pattern (Figure 4) that