October 2012
Kim Roberts, associate editor
SA Instrumentation and Control.
The mechanics of fracking
Shale formations have fine grains with few interconnected pores, resulting in low porosity and permeability. In order for natural gas or oil to be produced economically, individual molecules trapped in pores smaller than the width of a hair must find their way to the well. Hydraulic fracturing creates a network of small fractures in the formation rock, allowing molecules a long distance away to migrate to the fracture and travel quickly to the well.
The production casing of a newly drilled well is lined with steel and cement capable of withstanding high pressures. A tool called a perforating gun is lowered through this and punches small holes in the well casing, cement and rock. Water under high pressure is then pumped down the well and through the perforations at a rate which must be fast enough to maintain the pressure needed to propagate the fractures. (This is known as the propagation pressure.) This initial volume of fluid is termed the pad and typically comprises 20% of the total fluid volume. The actual fracturing generally takes place over several kilometres and below groundwater levels.
Hydraulic fracturing, fracking, is the process of creating fractures in underground rock formations to allow natural gas and oil to flow more freely into wellbores. It is a proven mechanical technique that has been employed in the oil and gas industry since 1947 and in recent years advanced technology has led to a rapid rise in its use for the production of natural gas from dense shale rock.
The mechanics of fracking
Shale formations have fine grains with few interconnected pores, resulting in low porosity and permeability. In order for natural gas or oil to be produced economically, individual molecules trapped in pores smaller than the width of a hair must find their way to the well. Hydraulic fracturing creates a network of small fractures in the formation rock, allowing molecules a long distance away to migrate to the fracture and travel quickly to the well.
The production casing of a newly drilled well is lined with steel and cement capable of withstanding high pressures. A tool called a perforating gun is lowered through this and punches small holes in the well casing, cement and rock. Water under high pressure is then pumped down the well and through the perforations at a rate which must be fast enough to maintain the pressure needed to propagate the fractures. (This is known as the propagation pressure.) This initial volume of fluid is termed the pad and typically comprises 20% of the total fluid volume. The actual fracturing generally takes place over several kilometres and below groundwater levels.
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