Tex. Nat. Res. Code § 52.178
The owner of a permit or combination of permits shall have 18 months from the date or average date thereof in which to begin drilling a well for oil and gas on some portion of the land included therein. The drilling on one permit shall be sufficient protection against forfeiture of all the permits included in a combination. Owners of permits or combination of permits included herein shall have three years after the date or average date thereof in which to complete the development of oil and gas thereon, and if oil and gas should not be found in paying quantities and a lease applied for within said time all rights in such permit or combination of permits shall terminate, and the oil and gas in such land shall become subject to the provisions of this law relating to the relinquishment of oil and gas to the owner of the soil.
Tex. Nat. Res. Code § 52.178