Looking Through the Ground to Find the Energy
Alberta’s oil sands make up the second largest recoverable crude-oil holdings
on Earth, after Saudi Arabia. More than 80 percent of these reserves are situated
deep underground and cannot be accessed with surface mining.1
Steam Assisted Gravity Drainage (SAGD) is an in situ (in
place) technology design used to recover crude oil and
bitumen from deep underground. Pairs of wells are drilled
into the ground, and oriented horizontally about 5 metres a
part (Figure 1). Steam is injected into the top well, and the
thermal energy softens the bitumen and causes it to drip
down into the bottom well, where it is pumped out. The
sand is left behind. One way to think of SAGD is “heating
the oil and catching the drips”.2
The technology required to drill and operate these
well pairs is expensive. These processes involve burning
natural gas, and using massive amounts of water from
nearby watersheds.
Figure 1: Steam Assisted Gravity
Drainage (SAGD). Diagram from Petro
Canada: http://www.petrocanada.ca/en/about/666.aspx
Choosing the best location to drill is of obvious importance
to the oil companies. To some extent, it is still very much trial and error. However,
new advances allow geologists and geophysicists use a combination of 3D seismic
data, and core samples to help in this process. Seismic data can suggest where
reserves may be located.
3D seismic data is gathered using explosives, such as dynamite. The areas are
surveyed by placing the explosives in deep holes that are dug in lines running
adjacent to each other on a grid. The holes are dug deep enough to minimize visible
disturbance on the surface.
Geophones are receivers, located on the surface, and situated between explosive
along the lines. The explosives are detonated one at a time and result in elastic
seismic waves that travel outward in all directions. The waves are affected by the
density of the rocks they encounter below, and the distance they must travel. The
speed, and thus, the angle of the waves will vary. They are reflected, refracted,
and, or, transmitted, depending on the density of the surface. The geophones pick
up the seismic waves when they bounce back, like a microphone picks up a voice.
Vibroseis is a second design that is often employed to
create seismic waves in the ground.3 Instead of
digging a hole and detonating dynamite at measured
intervals on the grid, a large truck, or series of trucks,
park at each position. A platform underneath the
truck lowers to the ground and raises the truck. The
Figure 2: Vibroseis truck used by the
weight of the truck is used to allow the platform to
University of Calgary
vibrate at the surface with an appropriate frequency
to generate shock waves. The returning waves are
detected by geophones, positioned on the lines, as before.
The signals from the geophones are transmitted to a seismograph that analyzes
the wavelengths, time, and frequency, and produces a 3D seismogram – a picture of
the world below. Some boundaries reflect and refract more than others, so they
would appear darker in the seismogram.
A 3D time slice shows the rock formations at a specific depth in the seismic data.
It is referred to as a time slice because of the time it took the seismic waves to
reach that particular depth.
Remember that d = vt, where distance is directly related to velocity and time.
Well, v = F, where  is the wavelength and F is the frequency of the seismic wave.
We can relate these equations to the velocity, and see that d = t  F. That is, the
distance travelled by the seismic wave is directly related to the travel time, based
its wavelength and frequency.
The following seismic data was collected from the McMurray Formation in
Northern Alberta, which is located approximately 300 m below the earth’s surface.
The McMurray Formation formed during the Cretaceous Age, which lasted from
about 145 to 65 million years ago. The Cretaceous Age is the youngest period of
the Mesozoic era and defines the boundary between the Mesozoic and Cenozoic
Era. It lasted 80 million years and ended with one of the largest mass extinctions
in Earth’s history, known as the K-T extinction.
The McMurray Formation is commonly subdivided into the lower, middle and upper
formations. The middle McMurray Formation is favoured in the oil and gas industry,
due to the deposits of oil sands. They are mixed in with soft, porous, sandstone,
and hard impermeable siltstone and mudstone deposits. The siltstone and mudstone
act as a barriers to the steam in SAGD production.
The time slices below have revealed an ancient river bed in the middle McMurray
formation in the Athabasca Oil sands project. It is a meandering river bed with a
point bar that was formed from erosion and deposition over time.
The erosion in a meandering river concentrates along the sides of the channel,
rather than the bottom. As water sweeps around a bend, it flows more rapidly
along the outer bank, undercutting and steepening it to form a cut bank.
Meanwhile, along the inner side of each meander, where the water is shallow and
the velocity is low, sediment drops out of the water, as it is unable to carry the
load. It accumulates to form a point bar. Therefore, meandering rivers slowly
change shape over time, and shifts its position as the water erodes material from
one bank and deposits it on another. See Figure 3.4
Figure 3: A cross section of a point bar from a meandering river.
Examine Figure 3 and imagine four horizontal slices through it. They might
resemble the following four 3D seismic time slices that are courtesy of Nexen Inc
(Figures 4-7)5. The arrow in each figure is focusing on an ancient meandering river
channel, showing the deposition of sediment over time on the point bar. As time
increases, and depth increases, notice how the channel shrinks. The time slice at 6
ms represents a higher point in the channel and the time slice at 24 ms represents
a point near the bottom of the channel bed. Remember, the upper McMurray
Formation is approximately 300 m below the surface. In relation to seismic time
slices, it is given a time equal to zero milliseconds (ms). On average, 1 m depth = 1
ms.6
1.6 km
Figure 4: Time = 6 ms
1.6 km
Figure 5: Time = 12 ms
1.6 km
Figure 6: Time = 18 ms
1.6 km
Figure 7: Time = 24 ms
The lines on the point bar in the seismic time slices in Figures 4-7 represent
sandstone and mudstone deposition over time, as also shown in Figure 3. The
deposits on the point bars in the McMurray formation are found to be high in oil
sand concentrations.
When deciding on where to place a well, geologists also use these time slices to
examine the white areas on the point bar. These are areas known as gas caps. Their
presence has been confirmed by extracting core samples and examining them. The
gas caps are traps of gas sealed by siltstone and have been found to decrease
pressure in SAGD production, and thus decrease the efficiency of the well. Using
the seismic images, a geologist with this knowledge can effectively avoid producing
from these gas caps, thereby preventing damage to the oil reservoir.6
3D seismic time slices, and the analysis of core samples from the site, are used to
analyze potential oil sand reserves in ancient river beds. This knowledge helps
geologists and geophysicists, working in collaboration with the oil companies, to
locate the best site for the SAGD oil wells, while minimizing both economic and
environmental cost.
Questions:
1. What does SAGD stand for and how does it work?
2. Describe how geologists use seismology to determine the underground
structures in an area.
3. How does vibroseis compare to the use of dynamite?
4. In relation to seismic waves, what do the dark areas on the 3D seismic times
slices represent? What do the white areas represent? How do they use this
information in their decision to drill a SAGD well pair?
5. Use the program Google Earth to locate a point bar on a meandering river
system. Print or copy and save the picture. Where is the river located? On
your diagram, identify the channel, the cut bank, the point bar, and signs of
erosion and deposition over time.
6. Use the Internet to define an oxbow lake. How does it form? If possible,
identify an oxbow lake on your diagram from Google Earth.
Credits
1. Natalie St-Denis. Alberta Ingenuity Centre for In Situ Energy Pamphlet:
Unlocking Alberta’s Oil Sands Potential. Sundog Printing. 2007.
2. Petro Canada. http://www.petro-canada.ca/en/about/666.aspx
3. http://www.ucalgary.ca/geoscience/January2008
4. S. Hubbard and R. Spencer. Inclined Heterolithic Stratification: Overview
and Reservoir Implications – Insight into the Athabasca Oil Sands Reservoir
from Outcrop Analogue Analysis at Willow Creek, Alberta. Alberta Ingenuity
Centre for In Situ Energy and the University of Calgary. 2007
5. 3D Seismic data courtesy of the Long Lake Project, a joint venture of Nexen
Inc and OPTI Canada Inc.
6. H. Nielson. Sedimentologic Processes and Heterogeneity in Abandoned
Channel Fill Deposits: Reconstructing the Stratigraphic Architecture of the
McMurray Formation, Alberta. Thesis submission to the Faculty of Science,
University of Calgary, Alberta. 2008