Why do we need to fracture rock to retrieve oil and gas?
The answer lies in where oil and gas is found – sedimentary rocks. Sedimentary rocks are rocks that comprise different
sediments like pieces of other rocks, shell material, and salts. Sandstone, limestone and shale are
sedimentary rocks. The key to
sedimentary rocks is how they formed.
Over time, sediments were deposited in basins and layers of sediments built on top
of each other, resulting in thick accumulations of sediments. Tens of thousands of feet of sediments
accumulated in some basins and the weight caused grains to compact, realign and
break. Compaction, combined with pressure
and fluids moving through sediment deposits worked to cement in between
grains to form sedimentary rock.
Although the rock is compacted and cemented together, the
rock comprises sediments of different shapes that don’t fit together
perfectly. These irregular shapes create
holes or pores in the rocks. These pores
house water, oil and gas. Permeability
is a measure of the rock’s capacity to move fluids through the pores. The more porous a rock is, the easier it is
to move fluid. For example, in the photo below.
On the other hand, the smaller the pieces of the sedimentary
rock, the closer these pieces can be compacted and the smaller the pores and
channels for fluid removal. Shale, for
example, has tiny pores and the connections between the pores are also
tiny. Introducing open fractures,
however, dramatically increases permeability.
Fractures are artificially induced in wells to increase the production
rate and ultimate recovery of gas and oil from shale rock. For example, see the picture of shale rock below.
Shale Rock
What types of sedimentary rocks are geologists looking for?
The two rocks of interest are reservoir rock and source
rock. Reservoir rocks have the ability
to store fluid and transmit the fluid held within pores in economic
volumes. That is, these rocks are porous
and permeable, for example, sandstones and carbonates. Source rocks comprise fine grained sediments
that contain organic material that can become oil and gas. These rocks have tiny pores and low
permeability, for example, shale rock.
America's rich oil shale
deposits likely hold 1.5 trillion barrels of oil,
according to Jack Dyni, a geologist emeritus at the U.S. Geological Survey. According
to the Rocky Mountain Energy Forum,
“that amount is 4 times the proven reserves of Saudi Arabia. And, it’s greater
than all 12 OPEC countries combined, which have proven reserves of about 911
billion barrels of oil.” To put it
another way, “Colorado,
Utah and Wyoming have as much oil as Saudi Arabia, Iran, Iraq, Venezuela,
Nigeria, Kuwait, Libya, Angola, Algeria, Indonesia, Qatar and the United Arab
Emirates combined.”
Oil and gas are less dense than water so as oil and gas
generates within the sedimentary rocks in the basin it moves upward through
fracture and pore systems until something stops or “traps” it. A trap is an impermeable layer of rock that
surrounds a permeable rock, either vertically or laterally. One type of trap is a structural trap. In this mechanism, an impermeable rock seals
the oil and gas trapping it so it can’t escape.
Another type of trap is a fault trap.
In this mechanism, the plates in the earth shift up or down displacing
reservoir rock against impermeable rock so that the oil and gas can’t
escape. Geologists can locate these
traps and drill to access the trapped oil and gas.
How did the oil and gas get there?
Although a common misconception, oil and gas does not originate from dinosaurs. Plant matter and planktonic organisms were
deposited over time in deep oceans, swamps and lakes. These deposits are made of organic carbon molecules
containing carbon and hydrogen or hydrocarbons.
Organic carbon exposed to high temperatures for a long period of time
will yield oil and/or natural gas depending on the temperatures. The type of hydrocarbon produced depends on
the type of organic matter that was deposited.
The role of the geologist, petrophysicist and geophysicist
is a critical component in identifying and validating resource plays. With technology, these experts can identify
areas of potential interest and characterize the reservoirs geometry,
continuity and variability. The data
collected is used to develop drilling plans and select locations. In the next posting we’ll walk you through
the process involved in identifying the next big play. Stand by!
Image credit: www.123rf.com
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