Geoscience Reference
In-Depth Information
BOX 3.5
Hydraulic Fracturing
A hydraulic fracture is a controlled, high-pressure injection of fluid and proppant into a well to fracture the
target formation (see Figure). "Proppant" refers to sand or manmade ceramics used to keep the fractures open
after fluid injection stops. The injected fluid is usually a combination of water and small amounts of chemical
additives that reduce pipe flow friction, minimize rock formation damage, and help carry proppant into the
fractures (see also Box 2.3; DOE, 2009; King, 2012). Horizontal wells are hydraulically fractured in multiple
pumping “stages,” starting at the far end of the horizontal well and progressing toward the wellhead. Each
fracture stage is isolated within the horizontal well with packers or mechanical sleeves that open and close each
zone. After the entire hydraulic fracture procedure is completed, the injected fluid is allowed to flow back into the
well, leaving the proppant in the newly created fractures. The amount of fracturing fluid used in one horizontal
well fracturing stage varies, depending in large part on the geologic formation, and is on the order of millions
of gallons per well. Generally, water volumes are estimated from 2 to 5.6 million gallons per well (DOE, 2009;
King, 2012; Nicot and Scanlon, 2012; Soeder and Kappel, 2009). Horizontal wells can be hydraulically frac-
tured in one to more than 30 stages depending on the length of the horizontal well.
The distance and direction of the manmade fractures propagating from the well vary depending on the
type of hydraulic fracture treatment and the geologic properties near the well, including the rock toughness and
stress state in the formation. In general, the fractures are observed from geophysical surveys such as micro-
seismic (Appendix I) and tiltmeters (Cipolla and Wright, 2002) to propagate perpendicular to the direction of
the minimum in situ stress. The induced fractures can form a complex fracture network in areas of low horizontal
stress differences or simple fracture geometry in higher differential stress areas. Although the extent and direction
of the fractures are not known precisely, hydraulic fractures may extend on the order of one hundred to over a
thousand feet from the well. The upward growth of the hydraulic fracture tends to be limited by the horizontal
layering (bedding) of the shale formations and by the vertical stress exerted by overlying rock and rarely extends
up more than a few hundred feet (less than 100 m) from the wellbore (Fisher, 2010; Fisher and Warpinski,
2011). The geometry of hydraulic fractures can be estimated using a special seismic monitoring technique
termed microseismic mapping (see Appendix I), although this geophysical procedure is completed on only a
small percentage of hydraulically fractured wells, largely due to the cost.
BOX 3.6
Felt Earthquakes Near Blackpool, England, Related to Hydraulic Fracturing
Hydraulic fracturing of the Preese Hall-1 well in the Blackpool area of England caused seismicity in April
(
M
2.3) and May (
M
1.5) 2011. The April earthquake was felt in northern England and was widely reported
in the press. The well was drilled and hydraulically fractured by Cuadrilla Resources to explore the gas potential
of the Bowland Shale Formation.
The Preese Hall-1 exploration well was stimulated vertically to 9,004 feet measured depth with five hydraulic
fracture stages. The April
M
2.3 event occurred during stage 2, and the May
M
1.5 occurred during stage 4; in
Search WWH ::
Custom Search