As a geoscientist with nearly three decades of experience in the oil industry, I have been watching the debate on fracking (or as we called it, "fracing") with great interest. After a recent discussion with someone who stated that there had never been a blowout related to fracking like there was on BP's Macondo well, I thought that it was time to help explain exactly what fracking is and where the potential problems lie from a geoscientific viewpoint.
Production of hydrocarbons from a reservoir requires both porosity (the holes in the rock) and permeability (the measure of how connected those pores are to each other). When I'm explaining the use of hydraulic fracturing to laypeople, I always use the analogy of a sponge. Hydrocarbons are found in the pores of the sponge but, unless those pores are connected to each other, the hydrocarbons will not flow to the surface. In the case of shale, the porosity is very high, often in the order of 30 percent or more compared to half that in many sandstones and limestones, however, the pores are not interconnected meaning that the permeability is very low. In those cases, hydraulic fracturing techniques are used to connect the pores to each other, allowing the oil or natural gas to flow to the surface.
Fracking is a production enhancement technique that has been in use for decades in low permeability reservoirs. Twenty years ago (or even less), fracking involved pumping what we considered to be large volumes of sand or gel (the material that would help connect the pores) in a semi-fluid state under high pressures through holes that were perforated in the production casing. This "cracked" the rock and, after the fluids that contained the sand flowed back out of the hole, the sand (the proppant) was left behind in the formation, enhancing the permeability. Over the past decade, fracking has changed a great deal. Rather than a single frac, oil companies are now using multi-stage fracs that require massive volumes of water and chemicals to enhance permeability in formations that were once considered to be non-productive.
Many of us have heard of the unfortunate souls that live in areas where fracking is now commonplace (i.e. Pennsylvania, parts of southern Alberta etcetera) that find themselves with tap water that is contaminated with natural gas as shown here:
The oft-heard mantra is that the source of the methane is biological (i.e. it is sourced from decaying plant life close to the surface of the earth), however, that is not necessarily the case. Let me explain why.
When the oil industry drills a borehole to depth, as you can imagine, the sides of the borehole are very unstable and rock continually sloughs into the hole, causing all manner of problems. To alleviate these problems, a long string of hollow steel production casing of varying diameters is run into the hole to the total depth of the well (or somewhere below or at the depth of the producing formation depending on the type of well). The diameter of this casing is somewhat smaller than the diameter of the borehole; the space between the two is known as the annulus. Once the casing is in place, cement is pumped down the casing and flows back up the well between the casing and the sides of the borehole through the annulus. The cement is allowed to harden and tools are run to ensure that the "cement job" is sound. The purpose of the cement is three-fold; it holds the casing in place, it prevents the fluids used in the well completion operations from flowing to the surface and it prevents fluids from inside the borehole from flowing into the surrounding formations once the well is completed and on production. For example, if there is a water-bearing formation above the productive zone, the production casing and cement will seal off that formation, preventing the formation water from flowing into the well bore.
Here is a cross sectional diagram showing a completed coal bed methane gas well and its components:
So, what could possibly go wrong? Sometimes, the cement fails to displace the drilling mud in the annulus and completely fill the void between the casing and the formation and other times it fails after a period of time. This situation allows the formation fluids, which are under pressure because of the weight of the rock that lies on top of them, to flow through the annulus to the surface where the air pressure is far lower. It is this exact situation that can result in contamination of near surface aquifers even when hydrocarbon-producing formations are many thousands of feet below the source of groundwater.
How prevalent is the issue of failed well bore integrity? Before you continue reading the accompanying data, please keep in mind that this data is oil industry-sourced; it is not sourced from any anti-fracking lobby with an axe to grind. Let's start with a graph that shows the rate of sustained casing pressure (i.e. failed well bore integrity) in wells in parts of the Gulf of Mexico from the Autumn 2003 edition of the Oilfield Review to us some idea of how common the problem is:
The well bore integrity in five percent of these wells can be expected to fail immediately ramping up quickly to 40 percent in eight years. From fourteen years and onward, half or more wells are expected to lose integrity.
In case that isn't enough proof for you, here is a slide from a 2011 presentation by Archer, a global oilfield service company, showing the percentage of wells with integrity failures in the Gulf of Mexico and the North Sea in both Norway and the United Kingdom:
I find it stunning that 45 percent of wells in the Gulf of Mexico had integrity failures. Archer also states that 20 percent of wells with integrity issues are cement integrity and an additional 41 percent are related to failures of the equipment in the hole (tubulars).
Horizontal drilling and hydraulic fracturing – coming soon to a shale basin near you!
The red blobs on the first map outline the world's major shale basins that have been assessed as containing economic volumes of shale gas. In total, 32 nations have a projected 5760 trillion cubic feet (TCF) of technically recoverable natural gas on top of the 862 trillion cubic feet in the United States. With the U.S. consuming 25.5 TCF in 2012, that's a 33 year domestic supply. As well, the U.S. Energy Information Administration (EIA) projects that 14 percent of global natural gas supplies will be sourced from shale by the year 2030 and the U.S. Department of State notes that this will provide "…the reserve base necessary for expanded consumption in a business as usual scenario." I guess there is no need to conserve after all! Thank goodness fracking will be there to bail us out!
Lastly, according to the Society of Petroleum Engineers, over the next decade, the oil industry will drill more wells than they have in the last 100 years and that of the world's current inventory of 1.8 million wells, roughly 35 percent have integrity problems.
I find those observations to be most disturbing, particularly given that so much of our energy future is now hinging on the ramping up of hydraulic fracturing. We're betting our future on a very risky energy pathway, one that depends heavily on improvements to well bore integrity.
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