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FYI
To Frack or Not To Frack -
some information on the pros and cons of the action.
What is Fracking? It is a general term used for the fracturing of any given source rock intersected at depth, after drilling a borehole, to release the fluids or gases one wishes to extract, collect and put to use.
Fracking has been used for a very long time in water boreholes drilled into a water aquifer where the host rock is not porous enough locally to allow the water to flow freely into the borehole and be pumped out for domestic, agricultural or industrial use. Fracking in water boreholes usually only consists of exploding some dynamite, for example, at specific depths where sandstone, dolomite, limestone, chalk or other sedimentary rock units are known to host large aquifers of water. This water is either fossil water or where the aquifer recharges over time from surface water seeping down through porous rocks or fractures to be contained above a uniform non-porous rock unit such as metamorphosed rocks (for example shales) or a volcanic unit. No chemicals or other fluids are used in the process and the fracking is normally only completed once.
The various forms of gas deposits hosted in the Earth's near surface rock units are illustrated in the figure above.
This note describes the basic processes and some of the pros and cons in hydraulic fracking used for the release of petroleum related gases from coal beds or thick, discrete, continuous shale layers.
Those gas deposit types pertinent to the proposed work at the old Tilmanstone colliery site are coalbed methane (CBM) and gas-rich shale. The rock strata in which shale gas is trapped are almost impermeable to gas flow (Figure 2).
Access to the gas in both these deposit types is normally achieved by drilling a large diameter hole down to the host rock material, then drilling horizontally or at a shallow angle within the host rock so that a number of gas release points can be created. Thousands of tons of water mixed with sand and various chemicals are then injected as illustrated for shale gas extraction in Figure 3.
The access borehole for both coalbed methane (CBM) and gas-rich shale has to be lined with steel casing, and must include completing cement grouting between the casing and the rocks at the edge of the borehole. Through any porous units such as chalk or sandstone, the upper portion of the borehole should include up to three layers of casing and cement grouting as has been submitted by Coastal Oil & Gas. This is to ensure that there is no passage either into or out of the borehole for any fluids or gases.
This is a key part of the process involved and it is necessary to prove that the cement grouting has been properly completed. A geophysical wireline survey of the hole, at least though the porous rocks such as the chalk, should be completed with at least a Full Wave Sonic tool and an Acoustic Televiewer.
I have completed this kind of work for a CBM project in southern Africa, where a company went bankrupt as they could not extract the gas consistently. The geophysical results clearly showed that the cement grouting was poorly completed and fine grained sedimentary material was falling down between the borehole wall and the casing and blocking the exit holes through the casing.
Water Consumption
Fracking for these gas deposits consists of two initial phases: first several access points to the gas-rich rock are opened by exploding a number of charges with material that can pierce the casing and fracture the host rocks behind the casing; then a fluid with propping agents, specifically sized to the fractures and size of the fractures, is injected at high pressure down the borehole and through the fracking points created by the explosives.
The fluid used can be up to 99% water, which is normally collected locally. The amount of water that may be used will generally be within the following ranges:
