127
gas. Normally, it is left in the ground until some future date when a pipeline becomes
available, but not always.
Natural gas that occurs without oil is called “non-associated” gas. The methane
in coal seams is an example. When accompanied by petroleum it is known as “associated gas,” which may occur as a free phase in the pore space of a reservoir rock,
trapped above the oil. It can also be present in solution within the petroleum itself,
so that when oil is brought to the surface, the gas comes with it. The dissolved gas
exits from the oil under the reduced pressures at the surface and becomes a
free phase.
Oil produced from the Bakken Shale in the Williston Basin of North Dakota has
natural gas contents as high as one thousand cubic feet (MCF) of gas per barrel of
oil (Nordeng 2010). An MCF is a volume of gas at one atmosphere of pressure and
25 degrees C filling a 10 × 10 ft room with a 10-ft high ceiling; the metric equivalent
is 28.3 m
3
. Pipelines are often used to transport oil, but they are not required. Oil is
a liquid and it cannot be stranded like natural gas. It can go into storage tanks, and
leave the production well in a tanker truck, eventually traveling by tanker train to a
refinery.
One of the downfalls of the shale gas boom is that the supply of natural gas
essentially doubled, while the demand for gas remained more or less constant.
Following the simple laws of economics, prices dropped like a rock. Petroleum, on
the other hand, is a globally-traded commodity, and the balance between supply and
demand is complex and more resilient. It is also worth significantly more money
than natural gas. As of this writing, a barrel of oil is worth about $40, whereas an
MCF of gas sells for less than $1.50. The energy equivalence between oil and gas is
about six MCF of natural gas to a barrel of oil, so the energy cost comparison is
more like $40 to $7.50. No matter how it is calculated, oil is worth more money than
gas. So what happens when a well produces both expensive oil and cheap natural
gas in an area where there are no available pipelines to take the gas? The gas is often
burned off, or flared.
The routine flaring of small amounts of produced gas was common back in the
day when a visible flame called a “flambeau” was maintained on natural gas wells
to prove to investors that gas was actually being produced (refer to Chap. 3). Flaring
has been done on conventional wells offshore in Gulf of Mexico, in the Middle East,
and elsewhere when there was no infrastructure for handling gas. In the United
States, the most notable and prominent flaring has been done on the Bakken Shale
play in North Dakota.
A typical Bakken well arrangement is shown in Fig. 7.3. Five pump jacks on the
right side of the pad are lifting oil to the surface from five separate directional wells.
On the left side of the pad are a set of tanks. The tall, narrow tanks separate the gas
from the liquids in the first stage, and the oil from the produced water in the next.
The shorter, wide tanks are used to store the oil and produced water until each can
be hauled away. The gas is run to a far corner of the pad through a pipeline to keep
it away from the flammable oil, and flared at the top of the stack.
Oil recovery from the Bakken has been remarkable. It has made North Dakota
the second largest oil producing state in the U.S., surpassing other well-known oil
7.2 North Dakota from Space
gas. Normally, it is left in the ground until some future date when a pipeline becomes
available, but not always.
Natural gas that occurs without oil is called “non-associated” gas. The methane
in coal seams is an example. When accompanied by petroleum it is known as “associated gas,” which may occur as a free phase in the pore space of a reservoir rock,
trapped above the oil. It can also be present in solution within the petroleum itself,
so that when oil is brought to the surface, the gas comes with it. The dissolved gas
exits from the oil under the reduced pressures at the surface and becomes a
free phase.
Oil produced from the Bakken Shale in the Williston Basin of North Dakota has
natural gas contents as high as one thousand cubic feet (MCF) of gas per barrel of
oil (Nordeng 2010). An MCF is a volume of gas at one atmosphere of pressure and
25 degrees C filling a 10 × 10 ft room with a 10-ft high ceiling; the metric equivalent
is 28.3 m
3
. Pipelines are often used to transport oil, but they are not required. Oil is
a liquid and it cannot be stranded like natural gas. It can go into storage tanks, and
leave the production well in a tanker truck, eventually traveling by tanker train to a
refinery.
One of the downfalls of the shale gas boom is that the supply of natural gas
essentially doubled, while the demand for gas remained more or less constant.
Following the simple laws of economics, prices dropped like a rock. Petroleum, on
the other hand, is a globally-traded commodity, and the balance between supply and
demand is complex and more resilient. It is also worth significantly more money
than natural gas. As of this writing, a barrel of oil is worth about $40, whereas an
MCF of gas sells for less than $1.50. The energy equivalence between oil and gas is
about six MCF of natural gas to a barrel of oil, so the energy cost comparison is
more like $40 to $7.50. No matter how it is calculated, oil is worth more money than
gas. So what happens when a well produces both expensive oil and cheap natural
gas in an area where there are no available pipelines to take the gas? The gas is often
burned off, or flared.
The routine flaring of small amounts of produced gas was common back in the
day when a visible flame called a “flambeau” was maintained on natural gas wells
to prove to investors that gas was actually being produced (refer to Chap. 3). Flaring
has been done on conventional wells offshore in Gulf of Mexico, in the Middle East,
and elsewhere when there was no infrastructure for handling gas. In the United
States, the most notable and prominent flaring has been done on the Bakken Shale
play in North Dakota.
A typical Bakken well arrangement is shown in Fig. 7.3. Five pump jacks on the
right side of the pad are lifting oil to the surface from five separate directional wells.
On the left side of the pad are a set of tanks. The tall, narrow tanks separate the gas
from the liquids in the first stage, and the oil from the produced water in the next.
The shorter, wide tanks are used to store the oil and produced water until each can
be hauled away. The gas is run to a far corner of the pad through a pipeline to keep
it away from the flammable oil, and flared at the top of the stack.
Oil recovery from the Bakken has been remarkable. It has made North Dakota
the second largest oil producing state in the U.S., surpassing other well-known oil
7.2 North Dakota from Space