The fracking is the targeted disruption of low permeability geologic formations by means of the injection of a high-pressure fluid to crack a rock. This fracturing can be performed near the surface or deep (more than 1 km, or more than 4 km in the case of shale gas) and from vertical wells, horizontal or inclined.
This relatively old technology (1947) was invented for conventional oil deposits and was renewed due to its association with horizontal drilling (developed from 1980). This is the restraint of the economic viability of the association for non-conventional sources, which led to the recent development in the use of these: it is made ??accessible once inaccessible resources or which have been exploitable at exorbitant costs and slow.
It is carried out by fracturing the rock with a mechanical stress using a fluid injected at high pressure from a borehole surface, to increase the slightest macro porosity and micro-porosity. The fluid may be water, a slurry, or a technical fluid whose viscosity was adjusted.
When the pressure of the fluid injected to the desired depth is beyond the point of application created by the weight of rocks located above, one or more fractures are introduced exactly when the pressure exceeds that of the pore water in the rock. Fractures can be widened with continuous injection of fluid possibly up to several hundred meters as fluid intake is maintained. The direction of the fracture is, of course, a subject of preliminary studies, but is far from being fully controllable.
To prevent the fracture network from closing on itself when the pressure drop, the fluid is enriched (about 10%) in proppants: powders of hard materials, mainly sifted sand or ceramic beads. They will fill the fractures and, once in place and re-compressed by the weight of rocks, constitute a sufficiently porous medium to allow the subsequent movement of products outside. The injected fluid also contains a complex mixture of products of the chemical industry (typically 0.5% in total), drawn from a list of more than 750 trade references. These include additives suitable for fracturing in place, and often biocides. These are intended to prevent the development of any bacteria that complicate the extraction process. (These bacteria feed on chemicals in the basement, including hydrogen sulfide, dissolved iron…)
Typically, an individual fracturing operation is carried out within several hours, exceptionally – days, and numerous fractures are spaced along a same single horizontal drilling. Finally, during the extraction phase, the areas of regularly spaced artificial cracks will permit drainage volumes of rocks relatively remote from the axis of the well. But no more: extractable areas are confined to the vicinity of cracks and created the impermeability of the rock taking up quickly beyond. Therefore, the productivity of a fractured well fall quite rapidly with time: a quarter of the volume is recovered in the first year, he productivity is reduced to 10 % after five years.
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