GeoLog

Tectonics and Structural Geology

Geosciences Column: Earthquakes and depleted gas reservoirs; what comes first?

Geosciences Column: Earthquakes and depleted gas reservoirs; what comes first?

An ever growing population means the requirement for resources to fuel our modern lifestyles grows too. Be it in mining, oil/gas extraction or the improvement of renewable technologies, the boundaries of where and how we access resources are constantly being pushed. Previously inaccessible resources become viable prospects as demand increases and our technological know-how advances.

Hand in hand with technological advances, comes an increased awareness of the environment and how it may be affected by the new practices. While the need for more energy is clear, more and more, energy consumers want to understand the impacts of sourcing the energy in the first place. For instance, how seismicity is linked to the extraction of natural resources, namely gas and oil, has become an area of intense research, as well as of media, political and societal focus.

Fracking – the process by which a high pressure mixture of water, sand and chemicals is injected into reservoirs of low porosity and permeability to encourage natural gas trapped within the rock to flow to the surface – makes regular headlines. The debate as to what extent hydraulic fracturing (the formal name for fracking) of rocks, and the subsequent disposal wastewater generated as a by-product, might induce earthquakes is ongoing.

Now, let’s flip the problem, to one which is little studied and even less well understood. What are the risk associated with exploiting conventional oil and gas reservoirs in areas which are earthquake prone? This is exactly the question asked in a recently published paper by Mucciarellie, Dona and Valensise, in the open access journal, Natural Hazards and Earth System Science.

A case study: The Po Plain

In order to explore the problem, the researchers focused on the Po Plain, an alluvial plain which extends for some 45 000 km² (an area roughly half the size of Portugal), over northern Italy. It sits at the foothills of the southern Alps and is bound by the Northern Apennines to the south.

Simplified sketch of northern Italy, centred on the Po Plain and showing the southern Alps and Northern Apennines fold and thrust belts. The location of the largest shocks of the May 2012 Emilia earthquake sequence is shown with red stars. The yellow rectangle outlines the study area (see Fig. 2). Key: SAMF: southern Alps mountain front; SAOA: southern Alps outer arc; GS: Giudicarie system; SVL: Schio-Vicenza line; NAOA: Northern Apennines outer arcs; PTF: pede-Apennines thrust front; MA: Monferrato arc; EA: Emilia arc; FRA: Ferrara-Romagna arc. Modified from Vannoli et al. (2015). Taken from Mucciarelli et al. (2015).

Simplified sketch of northern Italy, centred on the Po Plain and showing the southern Alps and Northern Apennines fold and thrust belts. The location of the largest shocks of the May 2012 Emilia earthquake sequence is shown with red stars. The yellow rectangle outlines the study area Key: SAMF: southern Alps mountain front; SAOA: southern Alps outer arc; GS: Giudicarier system; SVL: Schio-Vicenza line; NAOA: Northern Apennines outer arcs; PTF: pede-Apennines thrust front; MA: Monferrato arc; EA: Emilia arc; FRA: Ferrara-Romagna arc. Modified from Vannoli et al. (2015). Taken from Mucciarelli et al. (2015). Click to enlarge.

Since the 1950s the Po Plain has been systematically exploited for gas and oil. Its structural make-up is similar to many other oil and gas fields world-wide: the reservoir is hosted by growing anticlines (a type of fold which forms an ‘A’ shape) which extend to depths which are seismogenically active. It makes for an ideal case study.

The plain obscures two fold and thrusts belts, – areas of deformed sedimentary rock in which the layers are folded and duplicated by thrust faults – formed due to the proximity to the large orogens. The belts are still contracting, as the European and Adriatic plates continue to collide into one another. The contraction is accommodated by a number of faults in the area which have the potential to generate M 5.5+ earthquakes.

Indeed, the Po Plain was hit by a series of earthquakes and aftershocks in May and June 2012 which ranged in magnitude between 5.9 and 5.1. The costs of the earthquakes were significant, with as many as 100 buildings of historical importance being damaged or destroyed and the tragic loss of 25 lives.

Soon after the earthquakes, speculation start to mount as to whether they might be related to the hydrocarbon exploitation in the area; a notion which came as a surprise to scientists and oil industry professionals alike given that, at the time, studies of induced seismicity in Italy were rare.

Links between hydrocarbon fields and seismicity

Mucciarelli (author of the study) and his co-workers focused on an approximately 150km by 70km section of the Po Plain. They identified a total of 455 drilled wells in the area for the purposes of extraction of hydrocarbons: 190 of which were found to be productive (wells that have been or are producing oil/gas), while 227 were sterile and haven’t been exploited. The geology of the units in the area is generally homogenous and cannot account for the difference in productivity. So, what is the cause?

In a (somewhat simplified) conventional system, oil and gas typically forms in carbon rich shales which act as the source rocks. The hydrocarbons then migrate and accumulate in reservoirs, which are usually formed of permeable and porous rocks such as sandstones. These are capped by a sealing unit of shale or chalk (amongst others), which prevents the hydrocarbons accumulated in porous layers from escaping.

For a reservoir to be productive, the cap rock must be intact and unaffected by fractures or faults which might allow the fluids to escape – something which is not guaranteed in an area prone to earthquakes as is the Po Plain.

Mucciarelli et al. highlight that earthquakes of M 5.5 and above have the potential to cause movement on existing faults leading to new fractures, as well slip on existing faults, thus damaging cap rocks and rendering some reservoirs in the region unproductive as the hydrocarbons would be free to escape. Their argument is strengthened by the finding that a number of the sterile wells they identified cluster around the faults which caused the 2012 earthquakes, while productive wells are found a few kilometres distance away.

What the findings mean for prospective oil and gas fields

Through detailed statistical analysis, the researchers were able to define the characteristics of the productive and sterile areas in greater detail. They found that broader anticlines were less likely to be structurally sound as they were formed by wider, deeper and longer faults which in turn, could be the source of earthquakes. A cluster of unproductive wells would identify such regions during prospecting stages. Conversely, areas of productive wells identify areas unable to generate large earthquakes which would threaten the integrity of the reservoirs. Typically, these would also coincide with smaller anticlines.

The results have implications, not only for the oil and gas industry, but also for underground storage facilities. A CH4 storage facility was being built in an oil depleted reservoir right above the source of one of the May 2012 earthquakes. The research presented in the paper, combined with results from an earlier study by Evans in 2008, show that preference should be given to depleted gas reservoirs over depleted oil and aquifer reservoirs, when designing a gas storage facility in tectonically active areas.

The authors acknowledge that the Po Plain was an ideal case study in which to test their hypothesis. Study of other hydrocarbon producing regions, such as California, North Africa and the Middle East, is now required to fully validate the findings.

References

Evans, D.J.: An appraisal of underground gas storage technologies and incidents, for the development of risk assessment methodology, Prepared by the British Geological Survey for the Health and Safety Executive 2008, RR605 Research Report, 264 ++ tables, figures and appendix, available at: http://www.hse.gov.uk/research/rrpdf/rr605.pdf, 2008.

Mucciarelli, M., Donda, F., and Valensise, G.: Earthquakes and depleted gas reservoirs: which comes first?, Nat. Hazards Earth Syst. Sci., 15, 2201-2208, doi:10.5194/nhess-15-2201-2015, 2015.

Vannoli. P., Burrato, P., and Valensise, G.: The Seismotectonics of the Po Plain (Northern Italy): Tectonic Diversity in a Blind Faulting Domain, Pure Appl. Geophys., 172, 1105-1142, doi:10.1007/s00024-014-0873-0, 2015.

GeoTalk: Anne Pluymakers, Early Career Scientist Representative

GeoTalk: Anne Pluymakers, Early Career Scientist Representative

In addition to the usual GeoTalk interviews, where we highlight the work and achievements of early career researchers, over the next few months we’ll be introducing the Division early career scientist representatives (ECS). They are responsible for ensuring that the voice of EGU ECS membership is heard. From organising short courses during the General Assembly, through to running Division Blogs and attending regular ECS representative meetings, their tasks in this role are varied. Their role is entirely voluntary and they are all active members of their research community, so we’ll also be touching on their scientific work during the interview. Today we are talking to Anne Pluymakers, ECS representative for the Tectonics and Structural Geology Division (TS).

Before we get stuck in, could you introduce yourself and tell us a little more about yourself and your career?

I am Anne Pluymakers, and my background is in Geomechanics and Structural Geology. I defended my PhD last March at the HPT lab at Utrecht University in the Netherlands. During my PhD I performed experiments to determine the effects of CO2 on the frictional and healing behaviour of faults, at pressure and temperature conditions representative for potential CO2 storage sites. Since January I work as a post-doc at Oslo University investigating CO2-shale interactions, which forms part of an international combined CO2 storage/enhanced gas recovery research project. As these project descriptions also demonstrate, it is the combination of science and societal challenges that captivates me.

Although we touch upon it in the introduction of this post: what does your role as ECS representative involve?

As an ECS representative I think it is important to help Early Career Scientists discover what they want achieve, during their PhD but also in their future careers. I feel it is good to start thinking about where you want to go early on, and to try to acquire the necessary skill set that will help you get there. Apart from that, with our TS ECS organising committee we will organise activities that involve people further in EGU and its organisation, as well as try to help people develop such skill sets.

Why did you put yourself forward for the role?

It sounded like an interesting role to take, and a good way to increase my understanding of EGU as an organisation and a community. Besides that, I love organising things and talking to new people and this is a great way to do just that.

Some of the ECS Representatives at the most recent General Assembly in Vienna. From left to right, top to bottom: Matthew Agius (SM), Shaun Harrigan (HS),

Some of the ECS Representatives at the 2014 General Assembly in Vienna. Top, from left to right, (in brackets, the Division they represent) : Matthew Agius (SM), Shaun Harrigan (HS), Wouter Berghuijs (ECS Representative), Roelof Rietbroek (G), Matthias Vanmaercke (SSS), Auguste Gires (NP), Nanna B. Karlsson (CR), Bottom (left to right): Ina Plesa (GD), Lena Noack (PS and Deputy Union Level ECS PC Representative), Sam Illingworth (ECS PC Representative , 2013- 2015), Guilhem Douillet (SSP), Anne Pluymakers (TS), Jone Peter (stand-in for Beate Krøvel Humberset, ST).

What is your vision for the EGU ECS TS community and what do you hope to achieve in the time you hold the position?

I would love to increase the community feeling of all the Early Career Scientists that feel affinity with Tectonics and Structural Geology, and to strengthen the TS network. I think it is important as well to make it easier for the new Early Career Scientists to connect to the existing network.

What can your ECS Division members expect from the TS Division in the 2016 General Assembly?

We have several ideas, amongst which EGU-wide workshops such as a grant writing workshop, and a first-timers workshop. These will be organized in conjunction with the other ECS. Oriented specifically towards our TS community, we will organize a dinner and pub night early on in the conference, so we all can get to know as many people as possible. Later on there will also be a feedback session for all TS-affine ECS to see how we can keep on improving.

How can those wanting to, get involved with the EGU?

If you would like to get in touch with the ECS of the TS Division, and possibly help organise activities, please send an email to me, or to João Duarte.

To find out about all the early career events and activities at the General Assembly and throughout the year be sure to check the dedicated ECS website. There, you’ll also be able to find out who you’re Division ECS representative is, if you’d like to get in touch with them and become involved in the Union. The website also hosts a page full of useful resources for career development as well as a database of undergraduate and postgraduate courses spanning the geosciences across Europe.