Let's first define a few things. "Shale gas" is natural gas (methane) and natural gas liquids (larger molecules in a liquid state found with methane: ethane, propane, and butanes) found locked up in impermeable shale rock (sedimentary rock made from compacted mud, clay and silt). Because of how common the conditions are for shale gas to be formed (350-500 million years ago, algae died and settled on the bottom of sea and river beds, covered by silt and clay, and cooked under pressure and temperature), there's a lot of it. Shale gas has been rendered an economically viable source of natural gas by "hydraulic fracturing," which is a process of drilling down into gas-containing shale, injecting high pressure water, sand, and some chemicals to break open the impermeable rock and increase porosity, allowing the gas to flow out and be collected. "Proppants" like sand hold open the fissures, allowing the pressure to be relaxed, the water removed, and gas to flow out. The water that comes back (not all of it does; most stays injected underground) is called "flowback water."
The water, sand, and chemicals are pretty key components to the hydraulic fracturing process. One well will use between 4 and 6 million gallons of water in its productive lifetime, which is about 1-2 years. To put this into context, it's about 10 olympic swimming pools, or the amount of water that 4 average homes will use in a year . That's a lot of water for one well. In terms of what's also injected, water makes up ~92% of the fracking fluid, sand ~5%, and the remaining 3% is chemicals comprised of (mostly) acids and pH buffers to prevent scale build up (more on that), biocides to kill bacteria that would return topside with the flowback water, and detergents and oils to change fluid properties. The blend and chemicals change from location to location and are optimized for a given geological area.  As I mentioned, most of the water (and chemicals) stay injected in the well: 65-85% of the injected water stays underground and 90% of the chemicals are either consumed in process (like acids removing scale) or also stay underground.
The stuff that comes out is salty. Super salty. Like 10x saltier than ocean water. Stuff that's literally 25% salt. And there's 1.5 million gallons of it to deal with. That's 100 tanker trucks. Of super salty water. That you can't simply pour into the nearest river. In almost all cases it's trucked to special EPA-controled injection wells that basically bury the salty brine in similar geological structures as it came . The reason the fresh water that goes in comes out super salty is because the water that's already down there has been sitting in minerals and rock formations for hundreds of millions of years; the fresh water mixes with it and flushes some of it up to the surface.
Of course after the water comes out, the natural gas that we went through all this trouble to get finally comes out. A well will produce about 250,000-400,000 thousand cubic feet over its lifetime (enough to heat 6500 homes for a year ), and at $4/MMBTU (million BTUs) and 1 thousand cubic feet per MMBTU, is valued at $1-1.5 million. But natural gas isn't the only product that comes out; there's also natural gas liquids (NGLs). These are higher-valued commodities that can replace oil as petrochemical feedstocks, and as such are priced similar (but at a slight discount) to oil: something like $70/barrel (42 gal). In current gas plays, there's about 4-12 gallons of NGLs per thousand cubic feet of natural gas. This means that the value over the course of the well is something north of $3 million for NGLs. Most of the value is in these commodity chemicals. This weight toward NGLs basically means that natural gas is viewed as a byproduct and suggests that low prices will continue for some time.
I've summed up the list of grievances against shale gas and hydraulic fracturing as the following:
- Exorbitant use of fresh water
- Current inability to treat or dispose of the produced water from wells
- Accidental spills and leaks of chemicals and methane into surface and groundwater
- "Fugitive methane emissions:" methane released accidentally or purposefully into the atmosphere
- Drudging up of natural radioactive sources in flowback water
- Discounting of renewable energy technologies
In a second post, I'm going to address 1 and 4 directly with personal analysis, as they were my primary concerns when I started learning about shale gas, and I'll discuss what I've learned about 2, 3, and 5 from research. I'll write about 6 in a final post as I think it merits a greater discussion.
Soooo...yeah. There's a lot going on with shale gas. My 3 little (alright, maybe not so little) posts are certainly not going to be the final say on the matter, but I hope it at least puts things into perspective and adds some understanding.
---------------------------------------------------------------------------------------------------------------------------------- Rao, Vikram, "Shale Gas: The Promise and the Peril." RTI International: 2012