The pros and cons of fracking for natural gas
THE RECENT combination of hydraulic fracturing (commonly called “fracking”) with horizontal drilling has transformed previously unproductive organic-rich shales into the largest natural gas fields in the world. Fracking is a term now commonly heard in Ireland since the oil/gas exploration firm Tamboran Resources recently estimated that the shale bedrock under Co Leitrim harbours up to $55 billion (€41.5b) worth of natural gas, releasable by fracking. However, fracking is opposed on environmental grounds by a protest group called “No Fracking in Ireland”.
Shale gas is the best known of a number of unconventional sources of natural gas. Shale is a common fine-grained sedimentary rock that can harbour gas when organic matter in the shale is broken down at high temperatures to produce “thermogenic” methane. But shale has such a low matrix permeability that commercial quantities of gas can only be released by creating extensive artificial fractures by fracking. Horizontal drilling is frequently used with shale gas wells. First you drill down vertically from the surface, perhaps about 2,000m, to reach the shale layer. Then the drilling direction is changed by up to 90 degrees and proceeds horizontally for another 2,000m. This creates maximum borehole surface area in contact with the shale.
The combination of fracking and horizontal drilling has seen shale gas production in the US grow from 1 per cent of overall gas production to 20 per cent in 2009, with expectations that it will grow to 50 per cent by 2035. According to the International Energy Agency, the new economic extraction of shale gas increases the projected production of natural gas from 125 years to 250 years.
Basic fracking has been used in wells since the 1940s. When the vertical well shaft hits the rock-bearing reservoir, chemically treated water is pumped down under high pressure to stimulate release of the gas/oil. Such fracking typically consumes 20,000 to 80,000 gallons of water per well. However, the new fracking used on shale calls for up to four million gallons of fluid per well. The high pressure fluid cracks the shale and penetrates into the cracks extending them further. To keep the fractures open when the pressure is later relaxed, a solid proppant, commonly sieved sand, is added to the fluid. The propped fractures allow the trapped gas and oil to flow to the well. The injection fluid is also heavily dosed with chemicals to aid its flow – 15,000 to 60,000 gallons of chemicals per well. Tamboran promises not to use chemical additives in any fracking in Ireland.
Up to 75 per cent of the fracking fluid returns up the well bore to the surface when the injection pressure is released. This chemically toxic water is stored in large ponds for later reuse or transport to treatment plants. This initial water return is accompanied by a significant “belch” of methane, a powerful greenhouse gas.
Fracking poses several environmental threats. The water storage ponds must be lined to prevent leakage, but there have been cases of water leaking through torn linings. Also, heavy rainfall can cause ponds to overflow. However, secure containment of pond water is a matter that is surely amenable to simple technical solution.
The “belching” of methane into the atmosphere from worldwide fracking would seriously exacerbate global warming. However, this methane can be trapped with special equipment in a procedure called “green completion”. This could be a regulatory requirement.
Another worry is that underground water blasts may create unexpected pathways for gas or liquid to travel upwards to contaminate surface drinking water. The Environmental Protection Agency (US) claims that one well was contaminated with fracking chemicals and was delivering flammable water.
The oil industry counters that it is almost impossible to conceive of gas or liquid climbing the massive depth of rock between shale and ground water. However one possible route for buoyant gas to travel upwards is through faults in the concrete casing that drillers pour around the steel gas pipe they insert into the bore hole. This casing is designed to prevent the upward passage of gas or liquid outside the pipe that would contaminate ground water if allowed to pass.
Fracking will continue worldwide because conventional reservoirs of oil and gas are running out and because of the hope of independence from Middle Eastern oil and gas. But I don’t think we will see fracking in Ireland for a long time, if ever. Conventionally produced gas from the Corrib field is now 10 years behind schedule in coming ashore.
William Reville is an Emeritus Professor in the Biochemistry Department and Public Awareness of Science Officer at UCC. understandingscience.ucc.ie