Texas Natural Resources Code - NAT RES § 91.011. Casing

(a) Before drilling into the oil or gas bearing rock, the owner or operator of a well being drilled for oil or gas shall encase the well with good and sufficient wrought iron or steel casing or with any other material that meets standards adopted by the commission, particularly where wells could be subjected to corrosive elements or high pressures and temperatures, in a manner and to a depth that will exclude surface or fresh water from the lower part of the well from penetrating the oil or gas bearing rock, and if the well is drilled through the first into the lower oil or gas bearing rock, the well shall be cased in a manner and to a depth that will exclude fresh water above the last oil or gas bearing rock penetrated.

(b) The commission shall adopt rules regarding the depth of well casings necessary to meet the requirements of this section.