Updated electromagnetic profiling
techniques hold promise for evaluating shallow oil and gas plays
along basin margins.
An electromagnetic (EM) exploration system has been
commercialized by Montason Exploration Inc. using recently developed
theory combined with computer and measurement instrumentation.
The basic geophysical properties of subsurface reservoirs
indicate the range of resistivity variation is much larger than
the range of P-wave seismic velocities (figure
1). By measuring subsurface conductivity, a "virtual" resistivity
log can be derived for geological mapping, the company says.
Since resistivities of hydrocarbon-filled and wet
reservoirs vary significantly, EM resistivity data may define reservoir
fluid content without drilling.
Real World Field Tests
Data acquisition consists of an electrical transmitter
(Tx) with magnetometer receivers (Rx) positioned up to 1.5 miles
away. Data is sampled between the transmitter and receiver (figure
2).
The system records the magnetic induction caused
by electrical signals put through a transmitter on the ground. The
input signal and the earth's magnetic response are both monitored.
Computer processing outputs a conductivity log.
Accuracy depends on the thickness-depth ratio, conductivity
contrast and background noise.
Some of the scientific theory has been published
in the United States and Russia. Successful lab and field tests
have been independently conducted.
Testing was done in the shallow Cretaceous Niobrara
chalks in the D-J Basin in eastern Colorado, where:
- Niobrara gas pay varies from 25 to 70 feet
thick.
- Porosity is 30-40 percent.
- Formation permeability is below a millidarcy.
- Productive wells need fracture stimulation.
Niobrara gas fields are structurally trapped accumulations.
Typical gas wells produce under 500 MCFPD and have reserves averaging
from 100-700 MMCF up to 2 BCF per well.
Formation resistivity is 1-2 ohm-m in wet wells and
4-25 ohm-m in producers.
Beecher Island Field covers 27 square miles; production
is at 1,400 feet, with 200 feet of structural closure.
One of the EM tests was done on the field's crest.
The results (figure 3) show that when
the Niobrara is gas saturated, EM can measure the depth and resistivity
of the pay zone.
A survey also was conducted over the structural lead
shown in red (figure 4) at an 1,800-foot
depth. The survey resulted in 29 "virtual" logs being added to existing
subsurface data to produce the EM-derived structure map.
The lead was verified to the northwest, but proved
to be substantially smaller than expected. EM verified a structure
that was too small to drill because of remote pipeline access.
In this case, EM saved thousands of acreage, drilling,
completion and other exploration dollars that otherwise would have
been spent on an uneconomic venture.
Applications
Reconnaissance work can be done as well as prospect
evaluation, because the method is fast, adaptable and relatively
inexpensive.
- Since lateral facies boundaries can cause
resistivity changes, the system can be used for stratigraphic
exploration.
- Reservoir work, such as gas storage projects,
can use EM for field boundary delineation. Reservoirs like the
Eagle Sand in Montana, shallow gas in California and the Trenton,
Clinton and other shallow formations in the eastern United States
can be mapped.
- EM can map shallow coal beds because they
have strong resistivity contrast with surrounding rocks.
- Local conductivity variations in coal beds
also can be mapped.
- Other areas that can benefit are minerals
and groundwater exploration, archeological and environmental work.
The current EM system's depth limit is about 2,500
feet, but signal penetration is area dependent and some areas allow
deeper penetration. Advanced system designs will soon permit recording
well below 5,000 feet.
A conductivity contrast is necessary for the tool
to work. In the examples presented, productive Niobrara generally
has a 100 percent or greater contrast, but EM detects much lower
contrasts. Mineralized zones are identifiable since they generally
show very high contrasts.
Effective analysis of EM profiles requires calibration
to known subsurface conditions. Cultural problems affecting use
are electric transmission lines, pumps, pipelines with cathodic
protection and high traffic areas.
EM surveys are highly efficient — analysis is completed
in a few days, allowing for great acquisition versatility. Because
of this, the crew can be redirected to sample an anomaly on a tighter
grid before moving.
EM also is easy on the environment, lowering permitting
costs due to negligible surface disturbance.
The Bottom Line
The main advantage of an EM survey is its low cost
compared to 3-D seismic designed for high frequency at shallow depths.
This is especially true when the cost of three-component data necessary
for subsurface fluid detection is added to the basic 3-D cost.