Well service companies also use sand and (rarely) walnut shells to prop the rock formations open. Last time I checked, both were environmentally friendly.
In limestone formations, they use HCl, which is found naturally in our stomach's.
The biggest environmental threat lies in their extensive use of dihydrogen monoxide. This stuff is highly corrosive, can kill if inhaled in even small quantities, is a key component of global warming and acid rain, plays a major role in the formation of cancers, and is used to activate most harmful bacterial agents.