Processes Involved
Shale Gas wells are broadly similar to conventional wells. It is
only the modern technologies originally developed for well
stimulation (increasing productivity) of conventional wells that
have opened up shale gas reserves in recent decades.
Well Life Cycle
1) Prospecting
Gas companies employ geologists and engineers to locate areas
where they should attempt drilling. The Department for Energy and
Climate Change (DECC) has reported on shale gas resources in the UK
and their prospectivity [1].
Once an area has been identified, the company may apply to the
DECC for an Petroleum Exploration and Development Licence.
Licencing happens in rounds, and a Strategic Environmental
Assessment must be completed before any licences are offered,
therefore companies will typically apply years in advance of
drilling. At this phase there is no commitment to drill.
2) Exploratory Drilling
Once the company has identified a location within their licence
with favourable prospects, they will purchase or lease the land
from the landowner, apply to the Local Authority for planning
permission and will be inspected by the Health and Safety Executive
(HSE) and the Environment Agency (EA).
The Health
and Safety Executive examine the risk to people - they will be
interested in the safety precautions and the integrity of the well.
They check the operators are following the HSE regulations.
The Environment
Agency (EA) will inspect the site and the operator's
environmental safeguards. They are a statutory consultee. They will
advise the Local Authority on environmental measures such as
bunding around the site and an impermeable membrane on the ground
to contain any leaks. If they find the activity could pose a risk
to drinking water, they must issue a permit in order for activity
to proceed. In the event of a spillage the EA has powers to ensure
measures are put in place to clear up.
At this point the company may face opposition from locals, who
may object to the noise or impact of the drilling rig or the
environmental impact of the drilling.
Once the permissions have been received, the well can be
drilled. It may be drilled horizontally through the shale, to
maximise the area in contact with the gas-rich rock.
3) Completion
This includes installing the final casing and perforating said
casing in the shale strata with a gun, in order to provide a route
from the shale to the surface. This has been achieved in
Cuadrilla's operations in Lancashire [2]
and in 3 Legs Resources' operations in Poland [3].
A properly engineered well completion is critical, as a leak in
the casing could provide a pathway for contaminants from the shale
or fracking fluid to enter the top layers of soil, which sometimes
contain freshwater aquifers.
4) Stimulation
Well stimulation is needed when either the pressure of the gas
reserves or the porosity of the rock is insufficient; in the case
of shale gas, the latter. Fracturing of the rock is needed to
release the gas from the shale; the process involving pumping high
pressure water with sand suspension and chemicals into the well (Hydraulic
Fracturing).
5) Testing
The gas from the well is tested and its chemical composition
explored. The gas may need refinement if its proportions of methane
are not sufficiently high.
6) Production
Production commences when the well is capped - a wellhead (or
Christmas Tree) is fitted and gas is piped off for injection into
the domestic supply or to be burned at a power plant either onsite
or offsite.
If the gas is wet it may need to be refined to remove "gas
condensates" - impurities such as ethane, propane, butane, hydrogen
sulphide etc.
The well site is mostly inobtrusive, however care must be taken
to ensure there are no leaks. The operator should monitor this.
Unconventional well with Christmas Tree at Keele Science
Park
7) Abandonment
When the well is no longer profitable to maintain, it is
abandoned. Fluid may be disposed of into the well bore, or the well
filled with drilling mud and cemented. Well integrity is still
important as fluids from the shale could travel up a faulty well
bore. The well may be re-fractured several times before abandonment
(at 5 year intervals for wells in the Barnett Shale of Texas) [4]
, as fractures can close over time.
Shell's exploratory drilling at Skåne, Sweden, showing from left
to right: Before, during and after drilling. The well was abandoned
as it would not be profitable to complete. Images: www.shell.se
After drilling the well into the shale rock, the rock must be
stimulated. Fracking, or Hydraulic Fracturing, is the most popular
method of well stimulation in shale gas; other methods include
Nitrogen Foam Fracturing and Propellant Stimulation.
1) The well is drilled, with multiple layers of casing and, in
most cases, cement. The area near the top, typically up to the
depth of the lowest freshwater aquifer, has multiple layers.
Schematic of a well in Lancashire. The first casing stage is
drilled to below the lowest freshwater-bearing rock. The
intermediate stage goes up to the shale and the production stage
goes into the shale. All stages will be cemented in place and
pressure tested.
2) The casing in the shale layer is perforated by a small gun
that is lowered into the well.
3) Tens of thousands of litres of "fracking fluid" are pumped
into the well under high pressure. The fluid is mainly water and
sand, with additional chemicals for various purposes
- Biocides - to kill bacteria whose growth might
clog up the well
- Weak acid - to clean away drilling mud at the start off the
fracture (used only in the initial fracture)
- Gelling agent - to hold the proppant
- Liquefier/Breaker - to release the proppant when it is in the
fractures
- Friction Reducer - to reduce the pressure needed
to pump the water in
Composition of fracking fluid. Source:
cosmopolitanreview.com
4) After the fracture is complete, the fluid is allowed to flow
back above ground, where it is collected and taken away for re-use,
treatment, or in some cases stored in pits in the ground. This is
called "flowback water" and ranges from 15-80% of the volume of
fracking fluid used [5]
, meaning that a sizeable proportion is left in the rock.
Of this, some flows back regularly, known as "produced water" -
this occurs even in conventional (i.e. non-fracked) wells. Produced
water contains dissolved chemicals from the rock as well as those
added.
5) More advanced technologies have led to the emergence of
"multi-stage fracking", where only the endmost 300m of the well is
fractured, then a plug added, then steps 2-4 repeated for the next
300m, and so on until the entire horizontal section is complete. Up
to 12 stages may be used in some wells.
Chesapeake Energy explains their method of fracking
Environment
