Energy Technologies: How They Work and Their Induced Seismicity Potential - Induced Seismicity Potential in Energy Technologies - page 59

waterflooding in the United States is about 108,000; in Texas alone, current data from the
Railroad Commission of Texas indicate that more than 36,000 wells are currently permitted
to use saltwater injection for the purposes of secondary recovery. 4
Injection pressures and volumes in waterflooding projects are generally controlled to
avoid increasing the pore pressure in the reservoir above the initial reservoir pore pressure.
Nonetheless, reservoir pore pressure can increase as a result of waterflooding, and felt induced
seismic events at 27 sites globally (18 of which have been in the United States) have been
caused by or likely related to waterflooding (Chapter 1, Box 1.1; Appendix C). Waterflood-
ing at the Rangely Field in Colorado induced seismic events with magnitudes up to M 3.4
(Chapter 2, Box 2.4). Near Snyder, Texas, seismic events with magnitudes as large as M 4.6
occurred in 1978 after the initiation of a large (25 million barrel per year [10.2 trillion gallons
per year]) waterflooding project in Cogdell Field (Davis and Pennington, 1989; Nicholson
and Wesson, 1990; see also Appendix C).
Tertiary Oil and Gas Recovery (EOR)
Tertiary recovery is the process of recovering greater amounts (often greater than
50 percent) of the original oil and gas contained in a reservoir (DOE, 2011) and is gener-
ally, though not exclusively, initiated after the use of secondary recovery operations. 5 In
addition to maintaining reservoir pore pressure, EOR methods help displace the hydro-
carbons toward the production well. These methods can be broadly grouped into three
main categories: thermal, miscible displacement, and chemical injection (polymer flooding)
(Shepherd, 2009). Chemical injection methods are primarily used in California but are not
commonly used elsewhere in the United States and are not discussed further. Note also that
“other” methods in Figure 3.8 include microbial, acoustic, and electromagnetic methods;
these are not frequently used and are not discussed further.
Thermal techniques change the viscosity of oil in the reservoir by heating it through
the injection of steam or air (Shepherd, 2009). Heating lowers the viscosity of the fluid
and allows hydrocarbons to flow more easily through a reservoir toward a production well.
Over 40 percent of EOR operations in the United States use this method; it is most com-
monly employed in fields with high-viscosity oils (DOE, 2011). Miscible displacement is
generally used for lower-viscosity oils and involves injecting gases such as nitrogen or CO 2
that can reduce the viscosity of the oil and physically displace it toward production wells
(Figure 3.10). Nearly 60 percent of EOR projects in the United States use this gas injection
technique (DOE, 2011). In the United States, over 600 million tons of CO 2 (11 trillion
standard cubic feet; ~540 million metric tonnes) have been injected in ~13,000 wells for
4 See www.rrc.state.tx.us/data/wells/fluids.php.
5 See www.glossary.oilfield.slb.com/Display.cfm?Term=enhanced%20oil%20recovery.