Tapping conventional gas reserves is a matter of drilling a "conventional" vertical well down to the reservoir and allowing the gas to flow out naturally as a result of pressure differences. This will not work for shale gas, however, because it is trapped in sprawling layers of impermeable source rock through which it cannot freely flow. To release the gas, a network of many small cracks, or microfractures, must be created or reopened. These provide paths for the gas to escape and flow toward the well, where it will be recovered.
Shale gas is not contained in a conventional reservoir, but rather disseminated through laterally extensive source rock. Thus, a conventional vertical well would tap only a tiny portion of the gas. The only way to expose more of the source rock pay zone to the well is to drill horizontally. The horizontal drain is kicked off at the base of a vertical hole at a depth of 1,500 to 3,000 meters, and can extend over a distance of several thousand meters - typically between 1,000 and 2,000 meters.
Releasing the Gas: Hydraulic Fracturing
To allow the gas to circulate through the source rock – and flow into the well – the source rock must be made more permeable. This is the purpose of the swarm of microfractures induced by the technique known as hydraulic fracturing or fracking. Fractures are created by injecting a very high-pressure mixture of water, sand and additives into the well via an appropriate casing:
- The pressurized water opens the cracks to create paths for the gas.
- The sand moves into these cracks to hold them open.
- The additives (about 0.5% of the total injection volume) are mainly bactericides to protect the reservoir from contamination by bacteria from the surface, viscosifiers - known as surfactants - to keep the sand suspended in the water and in the cracks,
- and friction reducers.
To optimize hydraulic fracturing and extract the most from the reservoir, around ten fracs are performed per well, a technique known as multifracking. A series of hydraulic fracturing operations is performed, strating from the point farthest from the base of the borehole.
The oil industry discloses the additives used in hydraulic fracturing. The information can be found on dedicated websites such as:
- www.fracfocus.org, the national hydraulic fracturing chemical registry in the United States.
- www.ngsfacts.org, the OGP's hydraulic fracturing fluid and additive component transparency service.
- www.shaleenargentina.com.ar/quimicos.htm, created by Argentina's national oil and gas industry association IAPG.
Fracking in Figures
- Per well with an average of 10 fracs: 20,000 cubic meters of water and up to 2,000 tons of sand.
- Typical dimensions: laterally, about 200 meters (100 meters to either side of the well); vertically, around 70 meters on either side.