Characteristics of wastes produced by shale gas exploration
In shale gas exploration wastes are produced at well drilling, reservoir stimulation (hydraulic fracturing) and well abandonment/site reclamation stages.
Wastes produced during well drilling operations include:
- chemicals used for preparation and adjustment of drilling mud,
- oil derivatives,
- corrosion inhibitors,
- products of mud component decomposition,
- completion and hydrocarbon flow stimulation chemicals,
- reservoir fluids in the form of brine water and crude oil.
Detailed composition and characteristics of wastes primarily depend on the type of mud used (water- or oil-based mud, inhibitors added, etc.).
In addition, well drilling, casing, cementation and reservoir stimulation jobs produce relatively small amounts of other other wastes, such as:
- cement slurry leftovers,
- post-reaction fluids after stimulation jobs.
At the stage of reservoir stimulation, flowback water recovered from the well is the main waste.
Chemical composition of flowback water may vary and is primarily contingent on:
- the properties of fracturing fluids,
- geological conditions of the well,
- chemical reactions between the rock and fluids used,
- total time of fluid presence in the rock mass.
In addition to substances used for fracturing fluid preparation, flowback water may contain components that have been leached out from the fractured rock, including:
- heavy metals,
- radioactive elements (e.g. radium, thorium, uranium),
- hydrocarbons (e.g. benzene).
So far, 21 hydraulic fracturing operations have been made in Poland, of which only 8 in horizontal legs. Information on chemical properties of flowback fluids has not been made available with the exception of the Łebień LE-2H well. Polish Geological Institute – NRI made in-depth tests of the fluid in 2011 for the report titled "Studies on environmental aspects of hydraulic fracturing made in Łebień LE-2H well".
Flowback fluid from the Łebień LE-2H well had variable chemical composition (depending on the time of its recovery from the well) and the values of selected parameters in fluid samples tested ranged in very wide intervals:
- pH value: 5.73 – 6.35
- chlorides (Cl): 4 100 – 48 000 mg/l
- sulfates (SO4): >5 – 52 mg/l
- calcium (Ca): 318 – 7 568 mg/l
- ammonia nitrogen (NH4): 9 – 159 mg/l
- boron (B): 2,5 – 40,1 mg/l
- barium (Ba): 5,3 – 217,9 mg/l
- potassium (K): 82 – 536 mg/l
- sodium (Na): 2 118 – 22 596 mg/l
In overall, flowback fluid portions that were first recovered from the well displayed lower concentrations of particular components. The longer the fluid remained in the well, the higher was the content of chemical components, even several dozen times higher than the initial concentration.
Flowback fluid may be recovered over a period of several weeks and the flow rates tend to decrease with time. A majority of the returning flowback fluid (up to 95%) is recovered in the first days following the job.
autor: Anita Starzycka