Page 68 - Fundamentals of Gas Shale Reservoirs

48 GEOCHEMICAL ASSESSMENT OF UNCONVENTIONAL SHALE GAS RESOURCE SYSTEMS

it is absolutely necessary to stimulate or fracture a sufficient to be enforced, but are often based on outdated information.
volume of rock to release large volumes of gas. In general, As such, they should be continually evaluated for their
recoveries from these systems range from 10 to about efficacy with regard to present‐day drilling operations and
20% of the gas in place (GIP), although higher numbers updated as necessary.
are often cited. As shale gas development requires hundreds to thousands
Gas flow requires not only large amounts of GIP and of wells, this also puts a strain on infrastructure such as roads
sufficient storage but also a brittle rock fabric as well as exten­ and water use when wells are stimulated with freshwater.
sive knowledge of rock mechanics and stress fields in the Noise pollution is likely a large concern albeit over a short
prospect area to enable stimulation of sufficient volume of period of time. Both can be dealt with by appropriate actions
rock. Even with high‐energy stimulation, the typical shale gas of governments and drilling operations.
well will ultimately produce only a few billion cubic feet (bcf) Seismic activity or small‐scale earthquakes have also
of gas, and as such, requires drilling hundreds to thousands of been associated with shale gas development, although it is
wells to recover large amounts of gas, for example, over actually saltwater disposal wells that have caused such
15,000 wells have been drilled into Barnett Shale gas reser­ seismicity. When disposal wells are drilled near preexisting
voirs to date yielding about 15 tcf of gas. Prospective shale faults, low‐magnitude earthquakes can occur. One of the
gas areas may be located beneath commercial and residential most widely reported seismic events occurred at the Dallas/
areas limiting drilling access. Where such plays are located in Fort Worth (DFW) Airport in 2008–2009, having a maximum
areas where drilling costs are extremely high, the economics moment magnitude scale (M or M) of 3.3 and was likely
w
of drilling a gas well(s) to produce a few bcf of gas may not be due to injection of saltwater into a disposal well (Frolich and
commercially viable at a given price level. Low natural gas Potter, 2013). However, these authors also reported that
prices (ca. $3.69/mcf on average in 2013) in North America more than a decade before this event (1997), a M3.4 natural
have currently resulted in diminished drilling of shale gas earthquake occurred within 100 km (ca. 62 miles) of DFW
wells except in high‐return areas such as the Marcellus Shale Airport. Frolich and Potter (2013) proposed five hypotheses
of the Appalachian Basin, Northeastern United States. for the origin and nature of small intraplate earthquakes as
The search for shale gas outside of North America has might be originated from saltwater injection wells. They cite
identified promising shale gas systems, but to date has only that the consensus of earthquake seismologists agrees that
resulted in marginal commercial success. However, the ear­ small earthquakes (ca. M2.0) are ubiquitous and not indica­
liest wells are typically the most expensive and least pro­ tive of the occurrence or onset of larger earthquakes. Often
ductive, but as companies learn what is necessary to obtain increased seismicity is related to increased recording stations
high gas flow from a given system, commercial development being deployed, so many more low‐level quakes are recorded.
often follows. They also cite that all 50 states in the United States experi­
In addition, there has been much resistance to drilling ence earthquakes, but are small and go undetected. In Texas,
such wells that require high‐energy stimulation that has most M3.5 earthquakes go unrecorded unless near areas of
often cited as concern for the environment, which is a con­ dense population where seismographs are located (Frolich
cern for all including the companies hoping to develop such and Potter, 2013). Avoidance of injection well‐induced
a system. Commercial success requires environmental suc­ earthquakes can be accomplished by understanding regional
cess too, otherwise development would be halted. Operations stress patterns and not locating such wells in areas with
in North America have been safe with over 60,000 shale preexisting faults (Frolich and Potter, 2013).
resource wells drilled and stimulated. While groundwater The societal and political power of developing shale gas
contamination has occurred by ongoing geological processes resources in a friendly and environmentally sound manner is a
and are often reported in shallow water wells prior to drilling tremendous asset in terms of supplying energy needs and also
shale gas wells, these have often been cited as being derived reducing carbon dioxide levels. Natural gas is the most envi­
from the high‐energy stimulation efforts. Shale gas reser­ ronment‐friendly carbon‐based energy source as combustion
voirs are often at depths from 6000 to 15,000 ft or more, emits lower carbon dioxide and other oxide emissions. The
which is one to three miles below the nearest freshwater major source of carbon dioxide emissions in the United States
aquifers. Adequate and effective regulations on cementing is electricity generation, and reduction of CO emissions is
2
and testing of the upper portion of the well bore where it due in part to increased implementation of gas‐fired power
passes through freshwater aquifers should be the focus of plants. In 2012, the United States had its lowest carbon dioxide
tighter regulation. For example, the largest and most tragic emissions in over 20 years (EIA, 2012). Natural gas power is
oil field operation in recent time has been the massive blow certainly not a green solution as there remains a carbon foot­
out in the Gulf of Mexico, where the casing cement job was print; however, it does give us additional time to develop
the cause of various failures and also likely improper well green energy resources. Shale gas is a tremendous resource
testing (National Commission on the BP Deepwater Horizon for our ongoing energy needs resulting in lower emissions
Oil Spill and Offshore Drilling, 2011). Regulations need and lower costs for both consumers and industrial concerns.