Page 33 - Fundamentals of Gas Shale Reservoirs

INDUSTRY CHALLENGES 13
1.7.1 Environmental Challenges
TOC (wt%)
As noted by the US Department of Energy (US DOE, 2009),
4.1 4.0
“the key difference between a shale gas well and a conven­
tional gas well is the reservoir stimulation (large‐scale
hydraulic fracturing) approach performed on shale gas wells.”
Also, on a play scale, the major difference between a shale
2.1
play and a conventional gas play is the sheer quantity of
wells required to produce the same quantity of gas.
The environmental concerns regarding shale gas produc­
tion, in particular hydraulic fracturing, are so significant that
certain countries, such as France and Switzerland, have
Marine Lacustrine Terrestrial imposed a ban (SPE, 2012).
FIGURE 1.8 Average TOC by gas shale depositional environment. As a result of these features, the challenges outlined
below are likely to greatly influence the future of the industry.

Average TOC wt(%) by basin 1.7.1.1 Protecting Existing Water Resources In the
United States, the regulatory framework places considerable
2.01 emphasis on protecting groundwater (US DOE, 2009) and
1.87
surface water, due to the potential for shallow fresh ground­
1.44 1.36 1.42 water aquifers (or surface waters) to be contaminated by
deeper saline water, gas, or fracturing fluids during the dril­
ling and hydraulic fracturing process.
Literature (US DOE, 2009) suggests that a substantial
amount of independent research has been carried out to
assess the impact that shale gas operations have on shallow
Passive Rift Intracratonic Foreland Back-arc aquifers and surface water. The US Department of Energy
(US DOE, 2009) have highlighted the importance of the
FIGURE 1.9 Average TOC by gas shale basin type at time of following in managing environmental risks:
deposition.
Drilling, casing, and cementing programs (US DOE,
2009) to isolate water‐bearing zones from gas‐bearing zones.
Figures 1.8 and 1.9 illustrate the relationship among These include consideration of factors such as preventing
TOC, the inferred depositional environment, and basin type drilling mud entering the shallowest aquifers, corrosion of
for each gas shale, respectively. steel casing over time, testing to validate performance, and
designing for redundancy. Studies have suggested that the
current level of redundancy in the systems adopted in the
1.6.5 Clay Content
United States means that “a number of independent events
Qualitative clay content data was available for all non‐US must occur at the same time and go undetected” for fluid
shale gas plays, with the exception of Russia and much of from a pay zone to reach a shallow freshwater aquifer
the Middle East. Values of high, medium, or low were (Michie & Associates, 1988).
extracted. Source documents acknowledge that there is con­ Fracture treatment design (US DOE, 2009) using robust,
siderable uncertainty regarding this assessment, and consid­ yet sophisticated, techniques to produce a controlled
erable use of analogous plays was made when selecting an treatment within a specific target formation, reflecting the in
average value. situ reservoir conditions. This includes the implementation
It is generally accepted that there is a tendency for marine
shales to have a lower clay content (EIA, 2011b) than lacus­ of microseismic‐fracture‐mapping techniques to map the
development of fractures during treatment, and also fracture
trine and terrestrial shales.
design refinement based on the outcome of monitoring.
Fracturing process (US DOE, 2009), including testing/
1.7 INDUSTRY CHALLENGES certification of equipment and wells prior to fracture treatment
to ensure each well is fit for treatment, and the implementation
The fundamental way in which gas is produced within a shale of staged treatments to ensure controlled fracturing of discrete
gas play presents some different technical and environmental intervals. Naturally, the hazard likelihood and hence risk that
challenges in comparison with conventional gas plays. fracture treatments present to an aquifer is a function of the