Location of Abandoned Wells by Magnetic Surveys: acquisition and interpretation of aeromagnetic ...

65 Pages·2010·0.93 MB·English

Location of Abandoned Wells by Magnetic Surveys: acquisition and
interpretation of aeromagnetic data for five test areas
by
F. C. Frischknecht1, R. Grette2, P. V. Raab3, and J. Meredith^
Open-File Report 85-614A
1985
Prepared in cooperation with the Environmental Protection Agency
This report is preliminary and has not been reviewed for .conformity with U.S.
Geological Survey editorial standards. Any use of trade names is for
descriptive purposes only and does not imply endorsement by the USGS.
J-U.S. Geological Survey, Golden, CO
o
^Edmonton, Alberta, Canada
3Edcon, Denver, CO
^Mass. Inst. Tech., Cambridge, MA
I. INTRODUCTION
The Underground Injection Control Regulations (UIC), issued by the
Environmental Protection Agency (EPA), regulate injection wells for the
protection of actual or potential underground sources of drinking water as
required by the Safe Drinking Water Act. One provision of the UIC regulations
establishes a radius of review around proposed new injection wells, based on
the hydrogeologic properties of the subsurface, within which a search must be
made for possible conduits such as abandoned wells from the injection stratum
to overlying aquifers containing potable water. The problem presented by
abandoned or unknown wells is especially acute in petroleum producing regions
where the total number of wells may reach densities as high as 2,000 per
square mile. Particularly in the early days of petroleum production the
locations of wells were not always recorded. Some recorded locations are
erroneous, or described only in broad terms and many old records are not
readily available. Hence there is a need for other means of locating
abandoned wells.
In an earlier phase of this study Frischknecht and others (1983)
evaluated the use of geophysical methods, particularly magnetic and electrical
techniques, for locating abandoned wells containing steel casing. The first
step in the study of magnetic methods for locating wells was to characterize
the magnetic fields of representative well casings. Ground magnetic
measurements were made around a number of known wells located near Denver,
CO. It was assumed that the field of the casing could be represented as the
field of a number of magnetic pole pairs. A non-linear least square curve
fitting or inversion program was used to determine the strength and location
of a set of pole pairs that produce a field which is a close approximation to
the observed field. The parameters determined by this process were then used
to calculate the expected fields at various heights above the surface. It was
concluded that it should be possible to detect steel casings by means of low-
level aeromagnetic surveys. The results of the modeling were used to design
aeromagnetic surveys which were carried out in four small areas in Oklahoma
and one in Colorado. The primary purpose of this report and of USGS Open File
Report 85-614B is to present and to evaluate the results of these airborne
surveys. Persons interested in the detailed results for all of the test areas
will need to refer to Open-File Report 85-614B as well as to this report. A
summary of some of the results has been given by Frischknecht and Raab (1984a,
1984b, 1984c); the latter report, which has been released also by the EPA,
contains suggestions for non-specialists regarding use of geophysical
methods. All of the work described in this report was sponsored by the
Environmental Protection Agency.
Aeromagnetic Surveys
Survey Design
The principal factors which govern the design of an airborne survey for
location of steel casings are (1) the variations in the magnitude and shape of
the magnetic anomaly with distance from the casing, (2) the instrumental noise
of the system, and (3) magnetic anomalies or noise from all sources other than
the casing. Time variation in the earth's field could also be a factor but it
can essentially be eliminated by not flying during periods of abnormal
magnetic activity and by use of a well placed ground monitor.
Extrapolation of ground magnetometer anomalies near casings to predict
aeromagnetic results over casings should be fairly reliable. However, except
for the results of Frischknecht and others (1983) little published data are
available which can be used for prediction of airborne anomalies. Their
results indicated that most of the wells they studied could be located by
aeromagnetic surveys using reasonable flight heights and line spacings.
Generally the wells studied contained at least 200 feet or more of 8 5/8 inch
diameter casing. There are, no doubt, abandoned wells, in which only a small
length of casing was used or in which the near-surface casing has been
removed, that are impossible or economically impractical to locate and such
wells were not considered in the survey design.
Instrumental noise is fairly easy to determine although it does depend
somewhat on field conditions such as turbulence at the time of the
survey. After computer corrections for maneuvers, the instrumental noise in
the airborne system used was estimated to be 0.2 gamma (nanotesla).
Noise from cultural features other than casings and from geologic sources
is difficult to predict. Injection wells and water wells not related to
petroleum production are likely to be found in industrial areas where there
are many other sources of cultural noise. Steel objects such as pipelines,
tanks, steel buildings and large machinery are associated with petroleum
fields and are possible sources of anomalies. Also, many oil fields are found
in industrial and moderately populated areas where there are other steel
structures. In general, sedimentary rocks in which petroleum is found are
very weakly magnetic and the geologic sources of anomalies over petroleum
fields are usually the metamorphic and igneous rocks beneath the sediments.
The anomalies due to these deep sources have broad wavelengths and gentle
gradients. However, locally some sedimentary rocks contain enough magnetite
to cause substantial short wavelength magnetic anomalies. In fact, there is
evidence that some oil fields are marked by the existence of magnetite or
pyrrhotite formed in the near surface rocks by reaction between the rocks and
gases or fluids migrating upward from the petroleum deposit (Donovan and
others, 1979, Henderson and others, 1984).
Lacking adequate quantitative information on cultural and geologic noise
levels it was assumed that a total field anomaly from a casing should have an
amplitude of at least one gamma to be recognizable. Based on the group
of Colorado wells studied, a flight elevation of 61 meters (200 ft) and a line
spacing of 100 meters (328 ft) was chosen. In choosing this line spacing it
was assumed that errors in actual spacing would not exceed 24 meters (80 ft).
In selecting the sampling speed of a magnetometer the tradeoff between
sensitivity and sampling speed (see table I) must be considered.
Original plans were to operate the available magnetometer (Table 1) at a
sample rate of 0.73 s., which provides a sensitivity of 0.1 gamma or better.
However, this relatively low sample rate tends to introduce errors due to
aliasing and does not permit adequate definition of anomaly shape. Therefore
a sample rate of 0.40 s., which gives a sensitivity of 0.2 gammas, was chosen
for the magnetometer. Other data were sampled at an interval of 0.2S.
Possible use of gradiometers was considered by Frischknecht and others
(1983). Instrumental noise levels of about 0.023 gamma/m (0.007 gamma/ft) for
the horizontal gradient and 0.0098 gamma/m (.003 gamma/ft) were assumed. It
was also assumed that because of other noise the smallest recognizable anomaly
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gradiometer measurements would have to be made on a closer line spacing than
total field measurements to locate small casing anomalies. However, it should
be noted that gradiometer systems measure total fields as well as gradients so
that, in any case, a gradiometer system would provide more information than a
total field system, although at a greater cost. Also, gradient measurements
would be useful in discriminating against long wavelength anomalies produced
by geologic sources.
Airborne system used
The aircraft used was a Fairchild Porter. This single turbine engine
STOL aircraft can be operated at speeds of 100 knots or less. Its high rate
of climb, low stalling speeds and reliability of the turbine engine make it
more suitable than most fixed wing aircraft for extremely low level surveys.
Geophysical and auxiliary navigation and equipment are listed in table I.
Rotation of the sensor and magnetic fields of the aircraft represent
sources of noise and error in the airborne measurements. Ferromagnetic parts
of the aircraft are sources of both induced and permanent magnetic fields.
Electrical circuits in the aircraft constitute current loops which are sources
of both alternating and DC fields. Rotation of the skin and frame of the
aircraft relative to the earth's magnetic field induces transient eddy
currents in those structures which are sources of transient magnetic fields.
The magnetometer sensor is placed at the end of a tail boom or stinger in
order to remove it as far as possible from the fields of the aircraft and yet
have it rigidly coupled to the aircraft so that its motions are known. The
following measures have reduced the magnetic fields of the aircraft at the
sensor: (1) some of the ferromagnetic parts of the aircraft have been
replaced with non-magnetic stainless steel or other non-magnetic parts; (2)
other parts are regularly demagnetized using a degaussing coil; (3) two wire
circuits rather than a single wire with airframe return are used in all long
circuits carrying appreciable DC current.
Most of the remaining field of the aircraft at the sensor is
compensated. DC currents are driven through a three axis coil placed about 37
inches ahead of the sensor in the tail boom to cancel the permanent field.
Small permalloy strips are placed near the sensor to cancel the effects of
induced magnetization; the fields from these strips vary in the same way as
the field from the aircraft, as the attitude of the aircraft changes. The
proper currents in the compensation coils and the placement and length of the
permalloy strips are determined empirically through flight tests. Closed wire
loops can be placed near total intensity fluxgate sensors to compensate for
eddy current fields but this is not practical for proton magnetometers due to
the degradation of the signal-to-noise ratio from electrical losses in the
compensation loops.
Two different measures of the residual field of the aircraft are commonly
used, heading effect and maneuver noise. Heading effect is the variation of
observed field with slow changes in heading or azimuth of the aircraft when it
is in level flight. Maneuver noise is the variation of the magnetometer
readings in a constant earth's field when the attitude of the aircraft is
changed from the horizontal (pitch or roll maneuvers) or the heading is
rapidly changed (yaw maneuver). Heading effect is independent of eddy
currents but rapid maneuver noise is dependent on both eddy currents and
uncompensated fields of permanent and induced origin. Rotation of the sensor
causes small changes in the signal of a proton magnetometer and constitutes an
additional source of noise.
Before correction, heading error was roughly 1/2 gamma for the work
described here. Noise due to slow maneuvers was about the same. The chief
source of high frequency noise is from eddy currents induced by turbulence and
aircraft maneuvers and, in some cases, it was higher than one gamma. To
provide information for identification and removal of maneuver noise, roll and
pitch signals from a vertical gyro and yaw signals from a fluxgate compass are
recorded.
In locating well casings, accurate positioning of the aircraft along the
flight path and accurate recording of the position are necessary. In general,
positioning the aircraft visually or by Dopplar radar are not adequate, and
recovery of the flight path by a tracking camera or by recording the Doppler
results is generally not adequate, although in some areas there may be enough
landmarks for visual navigation and for recovery of the flight path using a
tracking camera.
To provide adequate information for pilot guidance and flight path
recovery, a microwave navigation system which determines position by
trilateration was used. To use this system, transponders are placed at either
end of a known baseline. The range to each of these transponders is measured
by a transceiver in the aircraft and the two range numbers are recorded
directly in digital form by the data acquisition system. Using the known
transponder locations and measured ranges, an on-board data processor
determines the position of the aircraft in the local coordinate system. The
position of the aircraft relative to pre-programmed flight lines is also
determined and displayed on indicators in the cockpit for pilot guidance. If
the transmission path is interrupted by an obstacle or another factor prevents
determination of valid range information a warning light comes on in the
cockpit and a flag message is recorded. To achieve uninterrupted transmission
paths the transponders were placed at the top of 10 m (30 ft) masts on top of
hills or high points and the receiving antenna was placed on a long rod which
was extended about 3 feet beneath the aircraft while flying.
The accuracy of actual locations depends on the accuracy of the
transponder locations, the accuracy of the range information, and the geometry
of the triangle. Information is most accurate when the angle of intersection
of the range lines is 90°. For this angle, the maximum error is 1.4 times the
range error. To avoid large errors, the angle of intersection was kept
between 30° and 150°, in which case the maximum error is four times the range
error in one direction and much less in the other direction. The accuracy of
horizontal locations also depends on knowledge of the aircraft height relative
to the transponders. However, the terrain in the test areas was flat enough
that this factor was not a significant source of error. Transponder locations
were surveyed relative to each other and to prominent landmarks using a laser
ranging theodolite.
Height of the aircraft above ground is measured with a radar altimeter.
The output is displayed for pilot guidance and recorded as well. Also
recorded, were outputs from a barometric or pressure altimeter and an
indicated airspeed transducer. This information is often useful in processing
conventional aeromagnetic data but was not important for this project.
A 35 mm tracking camera was used to take overlapping frames of the flight
path. For this project the photographic information was used primarily to
check the accuracy of the microwave navigation system at selected points and
to tie photographically determined well locations to the flight path.
The primary data acquisition system records data on 1/2 inch tape; the
recorder has a read head which permits verification of the data recorded on
tape. Information from the magnetometer, the microwave navigation system, and
the system clock is entered directly in digital form. Analog data are entered
through a multiplexer and A-D converter. A strip chart recorder is used to
monitor the output of the magnetometer and some of the microwave navigation
data were printed on the terminal used to control this system. Fiducial marks
on the strip chart and frame numbers on the tracking camera film are
controlled by the digital acquisition system.
Field Procedures
The aeromagnetic test area near Denver (figure 1) is part of the area in
which ground magnetometer measurements were made (Frischknecht, et. al.,
1983). Four test areas near Oklahoma City (figure 2) were selected by the
University of Oklahoma (Fairchild and others, 1983, Fairchild, 1984). The
Oklahoma areas selected had 14 or more wells in each section, as determined
from records of the Oklahoma Corporation Commission, and are
predominantly rural in nature. Information on the size and amount of casing
in the holes and the names and addresses of landowners was provided. The four
areas were inspected to determine the locations of dwellings and livestock
which should be avoided by the aircraft and the locations of towers and other
features which might be a hazard to the aircraft. Large parts of three of the
original four areas (Horseshoe Lake, Moore, Oklahoma City) were excluded from
the survey area, primarily because of the presence of dwellings.
Tentative sites for the microwave transponders were selected from a study
of USGS topographic maps. The sites were selected to be on tops of hills to
provide line-of-sight transmission paths to the aircraft and they were located
so that the angle of intersection of the range lines to the aircraft was
between 30° and 150°. Next, the landowners of the tentative sites and
landowners who control access to the sites were contacted in person or by
telephone to obtain permission to temporarily place a transponder on their
property. After permission had been obtained, the actual sites were inspected
to make certain that there were no unusual problems such as nearby powerlines
or livestock which might damage the masts or guy ropes. Tentative sites
within the survey area for the base station magnetometer were selected during
these inspection trips. In general, landowners were cooperative and willing
to allow temporary placement of the transponders and magnetometer. In
one instance it was necessary to pay a landowner a nominal sum for use of his
land. In another instance it was necessary to place the site in a pasture
where the equipment had to be protected from cattle. The greatest problem in
this process was contacting the landowners and, in many cases, the person
leasing the land.
104°45' 104°30'
BENNETT
e
O>
CO
WATKINS
Piney Creek
Test Site
o
4-
CO
e
o>
CO
MILES
5 10 15 20
10 15 20 25 30
KILOMETERS
Figure 1. Location of Piney Creek, CO test site.
con1 97°15'
97°30
Arcadia Test Site
ARCADIA
LUTHER
Horseshoe Lake
Test Site
V///////A
JONESr^
o
CO
o
in
CO
HARRAH
METRO/AREA
Oklahoma City
Test Site
Moore Test Site
MILES
5 10 15 20
10 15 20 25 30
KILOMETERS
Figure 2. Location of Oklahoma test sites.
9