Shale Gas – An Opportunity for Innovation?
The recent exploitation of shale gas has dramatically increased the natural gas reserves in the USA and from a consumer’s point of view has in the short term significantly reduced energy costs. This trend now looks set to be repeated worldwide and in particular in those European countries such as the UK and Poland where similar geological conditions exist.
Like any energy source, Shale Gas exploitation has an environmental impact be that the effect of carbon based energy sources on global warming or localised impact during drilling and well development processes. These issues are not new and lie at the heart of the debate on how best to meet the ever increasing worldwide demand for energy on a more sustainable basis.
What is new about Shale Gas exploitation is that unlike conventional oil or gas well drilling Shale Gas exploitation generates a significant volume of wastewater. Particularly in the short term during the well development stage and to a lesser extent throughout the operational life of the well and in the long term from limited drainage from the fractured strata (produced water).
As the gas is found at great depths beneath the surface in highly compressed low permeability strata it is necessary to artificially increase the permeability of the rock by hydraulically fracturing to allow the gas to flow into the well. Hydraulic fracturing (fracking) was first developed in 1940s and has subsequently been used to enhance the recovery from over a million oil wells. The process is not new, it is well established. The associated risks are known and as far as possible have, based on previous experience, been managed down to an acceptable level.
During the fracking process, significant amounts of fluid are pumped into the formation at high pressure to open existing fissures or develop new ones. Typically this fluid comprises 90% water, around 10% sand (to hold the fissures open whilst oil, gas and formation water flows into the well) and less than 0.5% other additives (to keep the sand in suspension and improve fissure penetration). Once the high pressure fracking pumps are switched off, the sand filled fissures partially close up and a proportion of the fluid flows back to the surface (flowback).
Unlike conventional oil/gas wells the relatively low permeability of the shale means that a significant proportion of the fracking water flows back to the surface for either re-use or disposal. As each well is fractured in a number of short discrete lengths, the returned fluid can, with suitable treatment, be reused to fracture other sections of the well thereby minimising the overall volume of waste water generated. However even with re-use, the development of each well results in the generation of a significant amount of wastewater.
Water also continues to seep out of the formation during the operational life of the well albeit at a much reduced rate. Initially its composition will be similar to the fracking fluid but will eventually revert to that of the formation water.
Historically this water was not reused and was either discharged locally or tankered away to the nearest municipal waste water treatment plant. Options that are no longer economically or environmentally acceptable.
So what are the key issues the water industry is faced with in developing a strategy for treating wastewater from the rapidly developing European Shale Gas industry? Different treatment strategies are required to allow the re-use of flowback water for on-going fracking operations from those used to manage the off-site disposal of both the excess flowback water and the small volumes of production rate generated throughout the operational life of the well.
During fracking on site water treatment systems are required to treat the flowback water to a suitable standard for re-use without detrimental impact on the well. To achieve this, a mobile treatment solution is required to:
* Remove suspended solids through flotation and/or clarification.
* Reduce the concentration of scale forming constituents (such as calcium, magnesium, barium iron and manganese) by chemical precipitation.
* Break down and remove unwanted organic compounds (such as hydrocarbons and organic based additives) by chemical/electrochemical oxidation followed by flotation/clarification.
* Destroy bacteria which could potentially adversely affect the performance of the wells using a combination of chemical oxidation (i.e. by the use of chlorine, ozone, etc.), UV sterilisation or the addition of biocides.
As the characteristics of the water generated from each gas field is different and each drilling contractor using their own blend of additives during the fracking, the on-site treatment technology needs to be not only mobile but sufficiently flexible to cope with this variability. Consequently a one size fits all approach is not always viable and the water treatment system based on a modular approach tailored to site specific chemistry is required to provide the most effective approach in both maximising water re-use and minimising cost. To achieve this, the water treatment industry needs to:
* Work closely with the drilling contractor to fully understand the nature of the contaminants to be removed and treated water requirements.
* Move out of its current comfort zone of working either in the laboratory or a fixed location/permanent water treatment facility and move on to site with the drilling contractor.
* Develop a range of portable water treatment solutions that can be redeployed in a matter of days as the drilling contractor moves from one well location to another.
The requirements for off-site disposal of excess flowback and production waters are dictated by the ultimate need to release this water back into the environment without causing harm. In the USA the most common method of disposal is to pump the water back deep underground via injection wells into suitably isolated geological strata where the natural ground water is similar to that of the wastewater. However such an approach is likely to be less acceptable in Europe due to increased environmental concerns regarding the small but perceived risk of shallow groundwater pollution from leaky injection wells and the possibility of inducing minor earth tremors. Consequently there is increased reliance on the wastewater industry treating this water to a suitable standard for disposal to surface water.
To do this the principal issues that need to be addressed by the waste water Industry are:
* The very high Total Dissolved Solids (TDS – chlorides and to a lesser extent sulphate) concentration found in the water. As these are typically around 100,000mg/l (but can be as high as 300,000mg/l) the use of RO to concentrate the TDS in a reduced volume is not viable and other options such as thermal evaporation are cost prohibitive. As a result, for most applications, the only economically viable option is tankering the wastewater off site and diluting other wastewater streams to concentrations suitable for release into the environment following treatment to remove suspended solids and reduce the BOD/COD.
* The removal of naturally occurring radioactive Minerals (NORMs) to safe level by chemical precipitation.
Shale gas is probably set to become a significant energy source. The challenge for the European Water Industry is to recognise this opportunity and develop appropriate treatment solutions for the wastewater generated.