Page 49 - Introduction to Petroleum Engineering

OIL AND GAS RESOURCES 33
Economic production of hydrocarbons from shale or tight sand became possible
with the development of directional drilling and hydraulic fracturing technology.
Directional drilling is the ability to drill wells at angles that are not vertically down-
ward. Hydraulic fracturing is the creation of fractures in rock by injecting a water‐
based mixture into a formation at a pressure that exceeds the fracture pressure of the
formation. The orientation and length of the induced fracture depends on formation
characteristics such as thickness and stress. Once fractures have been created in the
formation, a proppant such as manmade pellets or coarse grain sand is injected into
the fracture to prevent it from closing, or healing, when injection pressure is removed.
The proppant keeps the fractures open enough to provide a higher permeability flow
path for fluid to flow to the production well.
Shales are typically rich in organic materials and often serve as source rock for
conventional oil and gas fields. Production of oil and gas from shale is considered
unconventional because shale functions as both the source rock and the reservoir.
Shale deposits can be found throughout the world. Figure 2.8 shows shale plays in
the contiguous United States. Shale gas deposits include the Barnett Shale in North
Texas and the Marcellus Shale in Pennsylvania. Shale oil deposits include the Bakken
Shale in North Dakota and the Eagle Ford Shale in South Texas.
Holditch (2013) and McGlade et al. (2013) provided estimates of the global
volume of unconventional gas resources. Table 2.2 summarizes their estimates of
technically recoverable reserves for coal gas, tight gas, and shale gas. The differences
in tight gas and shale gas estimates illustrate the range of uncertainty.

Example 2.2 The Darcy Unit

Express the following permeabilities in darcies: 1 md, 1 µd (microdarcy), and
1 nd (nanodarcy).

Answer

 1D 
1md = 1md×   =× − 3
110 D
 1000md 
−
1µd = 1µd×( 10 md/d) ×   1D   =× − 6
3
µ
110 D
 1000md 
6
−
1nd = 1nd×( 10 md/nd) ×   1D   =× − 9
110 D
 1000md 
2.3.4 Tar Sands
Sand grains that are cemented together by tar or asphalt are called tar sands. Tar and
asphalt are highly viscous plastic or solid hydrocarbons. Extensive tar sand deposits
are found throughout the Rocky Mountain region of North America, as well as in