<p>Drilling rigs in the midst of cow pastures are hardly a novelty for Texans. But on a warm May day at a site about 30 miles south of San Antonio, a rig was not trying to reach <a class="meta-classifier" href="http://topics.nytimes.com/top/news/business/energy-environment/oil-petroleum-and-gasoline/index.html?inline=nyt-classifier" title="More articles about oil."><u><font color="#0066cc">oil</font></u></a> or fresh water, but rather something unconventional: a salty aquifer.