2 days ago, an already sensationalized study describing the causes of methane gas migration into drinking water wells was published by the National Academy of Sciences (PNAS). What the scientists from Duke suggest has slightly shifted the pace of the fracking debate: that faulty well construction or well casing failure, and not necessarily the fracking treatment itself, may account for most water contamination risks associated with natural gas drilling. After analyzing over one hundred water wells in Pennsylvania, they confirmed that thermogenic methane (and not merely natural surface-level biogenic methane) were found in water samples of wells in close proximity to drill sites. While proper well casing design protects groundwater most of the time, the sheer number of new wells drilled far exceeds the capacity of the few inspectors around to ensure that they’re failproof. Or at least meet baseline regulations.
Anthony Ingraffea has matched the Duke study with other data that show a 6-7% well casing failure rate – or “compromised structural integrity” – of wells drilled within the past three years. Over a 30 year period, even industry studies have reported anywhere between a 2-60% failure rate of oil and gas wells, depending on company, location, and age. Why the variance? And what factors lead to well casing failure?
A giddy operator with the rights to a gas-rich parcel of land can’t just drill willy-nilly. Well design considerations are very complex and attention to detail must span the construction, testing phase, and decommissioning of the well post-production. Moreover, drilling wells are often constructed uniquely with regard to the geology and geography of the specific location. For instance, because much of the shale formation in Pennsylvania lies beneath a shallower gas formation, it is easier for the shallower gas to escape
during the initial drilling process. This in turn has made it difficult for drillers to design failproof wells that can be sealed off from the younger deposits completely.
How gas wells are made
After a 16″ hole is drilled below the water table, the actual well casing is then inserted and the gap between this pipe and the earth is cemented. More casing is then run down in a smaller hole, cemented, and repeated until proper depth is achieved. A good image for this is a telescope – the drilling well narrows with greater depth. The well itself is made of several layers of steel – or multiple concentric pipes – surrounded by cement; these sections of steel pipe are screwed together end to end and encased with cement to make a chain. Isolating and cushioning the wellbore with this chain of steel as well as cement – if correctly designed – should prevent leaks. Primarily, the purpose of cement casing is to prevent fluid loss in the production zones or aquifers, and hence prevent contamination of fresh water sources.
How gas wells leak
The well’s susceptibility to functional failure relates to the experience level, standards, regulations and oversight used to design, build, operate and plug the well. For this discussion, active life and plugging are essential to the wells performance over time…The caveat to ensuring a long-term, environmentally safe well after abandonment is dependent on the experience level of the contractors, adherence to best management practices, sophistication of the regulatory agencies and degree of oversight. These vary state-by-state. (Barry Stevens)
Though the risks for future failure in the short-term are unlikely, the steel and cement casing in the wellbore are subject to greater risk of mechanical or chemical failure over time. While 6-7% may not seem very high a failure rate, if over 100,000 wells have been drilled over the Marcellus reserves alone, the rate takes on new – and frightening – significance. Creating a buffer between the water table, the earth, and the wellbore using cement is tricky. If the cement doesn’t bond properly with the walls of the well, contaminants or methane can more easily leak into water supplies. For this reason, faulty well design has proven problematic for on-shore and off-shore drilling projects. And it’s not the only factor.
Loss of well integrity is related to these factors:
- Pressure: high casing pressure from fracking operations & lack of a pressure relief system
- Age: as wells age, the steel casing is subject to corrosion; likewise, cement linings may shrink, debond, or crack.
- Construction or design flaws
- Damage during handling or fracking
- Rapid development of gas field
- Disturbance of young cement due to other drilling occurring on the same pad
- Deviated wells
- Drilling in shallower high-pressure gas horizons
Risks to well failure occur predominantly in the vertical wells. Generally, fracking is employed thousands of feet below this area in the directional drilling holes and is thus less directly associated to water contamination risks. However, researchers from the Society of Petroleum Engineers sought to assess risks for failure in horizontal well stability and found that the risk increases when the fracking fluid temperature is lower, or where the casing and cement system has to undergo too much potential variation in temperature and pressure during a fracking treatment. More research has yet to be carried out to assess well failure in the deeper directional drilling zones.
90% of casing failures occur at the connecting points that link the steep pipe together and so improper cementing of the well at these connections is the leading risk factor for water contamination. Where have these cases been reported? What are states and companies doing to mitigate the risks? This discussion continues in the next post, “Well casing failure violations”.