Patrick LEDRU
PROBLEMS OF CLIMATIC CHANGE AND RESOURCE DEPLETION:
GEOLOGICAL ANSWERS
Climatic change is being induced by the emission of greenhouse gases produced by
mankind. The rapid increase in the amount of atmospheric CO2 results from the
destocking of fossil carbon through the combustion of hydrocarbons (petroleum and gas)
and coal, the rate of which has steadily increased throughout the industrial era. This is a
phenomenon that affects the outermost layers of our planet (the oceans and the
atmosphere) but which derives its source from the solid layer of the Earth's crust. If the
origin of the problem – without forgetting the intervention (responsibility) of man – is
therefore geological, the solution may also be found in geological engineering.
Two methods are at hand when considering the problem from this angle:
- a reduction of CO2 emission through capture and geological storage
- an avoidance of emission through developing geothermal applications
The paper will discuss these two technologies from the standpoint of site and resource
availability, and from the standpoint of industrial (notably technical-economic) aspects. It
will also consider ongoing research lines and possible future research that could be
envisaged through a forward-looking approach.
1) Geological storage of CO2
The CO2 produced by fixed units burning fossil fuel (coal, petroleum, gas) can be
separated, compressed (to its liquid state), and injected via boreholes into deep
geological layers. All this requires is a presence of permeable geological layers close to
the production site (although transport of the CO2 by pipeline is perfectly feasible using
well-mastered techniques, it would be at a higher cost). The CO2 storage can be in
different types of geological formation, but at a depth of more than 700 m in order to
ensure the pressure necessary for the supercritical state:
- oil fields nearing depletion – this has the advantage of assisting production;
- deep aquifers in sedimentary basins;
- coal beds (with assisted production of methane);
- thick basalt sequences (mineralized trap formations).
The different options will be discussed in terms of their geological, technological and
economic characteristics, based on demonstration operations that have already been
carried out and on projects currently under study. Existing options as regards the
capture of CO2 will also be described and analysed. Needless to say, reference will be
made to the results of the last IPCC report on CO2 capture and storage.
2) Geothermal energy
Geothermal energy – direct use of the energy contained in the Earth's crust – is still little
developed in terms of available resources, which provide many options insofar as
resource type, uses, and operating technologies are concerned. One can notably single
out:
- production of heat to supply urban, industrial and agricultural networks from
deep geological layers in sedimentary basins;
-
production of electricity through releasing the steam contained in active
geological zones in which a magmatic source has enabled the development
of active hydrothermal systems;
- production of heat from shallow resources for heating houses via heat pumps;
- the future exploitation of deep geological formations, commonly granite and
fractured, through stimulating the fractures and developing an exchanger for
the production of thermal energy or electricity. The reference here is the
ongoing European experiment at Soultz sous Forêt.
Discussion could possibly turn around the prospect of such options being developed in
European countries around the Mediterranean, taking natural resources, production
systems, future developments, local technological capacities, research programmes and
required technology transfers into consideration.