Oil and gas companies injected hundreds of millions of gallons of hazardous or carcinogenic chemicals into wells in more than 13 states from 2005 to 2009, according to an investigation by Congressional Democrats. The chemicals were used by companies during a drilling process known as hydraulic fracturing, or hydrofracking, which involves the high-pressure injection of a mixture of water, sand and chemical additives into rock formations deep underground...to tap into large reserves of natural gas...Hydrofracking has attracted increased scrutiny...in part because of fears that the chemicals used during the process can contaminate...drinking water...Some companies are moving toward more disclosure, and the industry will soon start a public database of these chemicals. But the Congressional report said that reporting to this database is strictly voluntary, that disclosure will not include the chemical identity of products labeled as proprietary, and that there is no way to determine if companies are accurately reporting information...