Environmental Engineering Reference
In-Depth Information
freshwater aquifers which supply water for human use and/or baseflow to
rivers. Any structure that penetrates freshwater aquifers, such as a well, has
the potential to introduce a preferential pathway that could lead to con-
tamination of these water sources if pollutants are allowed to leak or
migrate.
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Drilling requires the use of water and drilling mud to ensure that the drill
bit is lubricated and cooled and so that the drill cuttings can be returned to
the surface. During drilling, the drill string and bit will be in contact with the
geological formation and groundwater. There is, therefore, a risk of
groundwater contamination from the drilling mud and/or mobilisation and
transfer of contaminants.
The risks associated with well drilling have to be carefully considered
during the planning stage and take into account the purposes of the well, the
local surface environment and land use (current and past), the potential
receptors and pollutant/environmental exposure pathways and well design.
Control measures must be put in place to mitigate any identified risks. These
include installation of multiple casings to ensure that different geological
horizons are isolated and to act as a barrier to leakage of fluid inside the
well; blow-out preventers to avoid over-pressuring in deep boreholes and
damage to the casing; and environmental monitoring.
4.2 Hydraulic Fracturing Fluids
To optimise the recovery of shale gas hydrocarbon source rock, the shales are
hydraulically fractured. This involves pumping large volumes of water con-
taining around 5% and and 0.5% of chemicals in to the well at high pres-
sure. The purpose of the sand (proppant) is to hold open the artificially
created fractures and the chemicals to optimise the fracturing process.
The exact composition of the fracturing fluid will depend on the oper-
ational conditions, including the geological formation, depth of the well,
number of fracturing stages, etc. There is no standard recipe and in de-
veloping a mix, different chemicals can be used to provide the same func-
tion. The number of chemical additives is also not prescribed and so this will
vary as well. A representation of the composition of hydraulic fracturing fluid
is shown in Figure 4. This identifies some of the key additives that are often
used.
The viscosity of fresh water tends to be low, which limits its ability to
transport the proppant effectively to achieve a successful fracture stimu-
lation treatment. As a result, some hydraulic fracturing fluids have a gel
additive (gellant) to increase the viscosity. Gellant selection is based on the
hydrocarbon reservoir formation characteristics, such as thickness, porosity,
permeability, temperature and pressure. As temperatures increase, these
gels tend to thin dramatically. In order to prevent the loss of viscosity,
polymer concentration can be increased (polymer loading) or, instead, cross-
linking agents can be added to increase the molecular weight, thus in-
creasing the viscosity of the solution.
