Mining & Mined Caverns - Parsons Brinckerhoff

B. Other factors – Logistical, economic, environmental and
political considerations (listed below but not further
discussed)
• Proximity to refineries, production or consumption centers
and transportation networks of pipelines, roads,
railways and waterways
• Availability, zoning status and cost of land, compatibility
with activities conducted on neighboring tracts
• Availability of utilities
• Permitting requirements
• Security of site
• Construction costs (including union versus non-union
labor)
Storage caverns have been mined in all general
rock types, which are classified into three broad
categories - sedimentary, igneous and metamorphic. In
the U.S., shale caverns are most numerous, followed
by those in limestone and igneous rocks, and lesser
numbers in other rock types. Nearly impermeable rock,
with adequate strength, is the ideal and some shale
formations readily qualify in this regard. Of greatest
concern is the possibility of encountering major ground
water inflows unexpectedly during construction which
could cause abandonment or very expensive remedial
action. Highly fractured rock of any type and cavernous
carbonate rocks (some dolomites and limestones) are
examples of high risk rock.
Subsurface geologic and hydrologic conditions
of importance for storage cavern development include
the interrelated physical and chemical engineering properties
and ground water conditions of rock formations
being considered as potential hosts for desired cavern
construction. For a prospective rock formation to be considered
suitable for cavern construction, it must meet the
following geotechnical requirements:
1. Adequate structural strength to allow economical mining
of reasonably large openings which will remain stable
for decades, with a minimum of artificial support.
2. Low permeability which will prevent major ground water
inflow into the cavern, and potential leakage of
stored product.
3. Presence of favorable and stable ground water conditions
which will remain dependable throughout the
planned lifetime of the cavern to assure containment
of the stored product.
4. Physical and chemical inertness among the stored
product, the cavern host rock and ground water.
Adequate Structural Strength
Compressive strength of a rock can be measured on
core samples in the laboratory, but more important is
Network
the strength of the rock mass which cannot be directly
measured. Following are the principal interrelated conditions
that affect the rock’s overall strength and how it will
behave when intersected by excavated cavern openings:
• Compressive strength
• Type, spacing, orientation, cohesion, width, filling material
and surface character of structural discontinuities
including faults, fractures, joints, shear zones,
bedding and foliation planes, contacts, veins, dikes
and open cavities.
• In situ state of stress.
Low Permeability
Permeability of a rock is a measure of its ability to transmit
fluid under a pressure gradient. Two types of permeability
must be considered:
• Primary permeability – The permeability allowing fluid
flow between mineral grains. Primary permeability in
directions parallel and normal to bedding in sedimentary
rocks is often quite different, with horizontal values
generally higher. Significant primary permeability
is considered to be very negative with respect to suitability
for cavern construction because it cannot be
remedied by grouting.
• Secondary (or fracture) permeability – permeability due
to fractures or dissolved openings. Moderate fracture
permeability can often be significantly reduced by cement
grouting during cavern construction. Cavernous
solution permeability in dolomite or limestone could
be catastrophic to cavern mining.
Favorable and Stable Ground Water Conditions
Mined underground storage caverns have historically
been developed under a basic hydrologic containment
principle. Simply stated, the cavern must be placed at
sufficient depth below the natural water table (or below
the piezometric level in the case of confined water-bearing
rocks) to ensure that the ground water pressure exerted
at the level of the cavern is greater than the vapor
pressure of the product stored within the cavern (at the
prevailing temperature). In the case of a permeable host
rock, this will allow potential ground water seepage into
the cavern, while preventing product leakage upwards or
outwards from the cavern. One vertical foot of ground
water exerts a pressure of 0.433 psi per foot at the base
of the column. The depth of a cavern must be greater
than the theoretical minimum depth needed, with respect
to ground water pressure, to provide a margin of safety
against the potential danger of overfilling or overpressuring
the cavern, lowering of the water table and even some
human error during storage operations.
Mined Caverns
APRIL 2012 http://www.pbworld.com/news/publications.aspx
47