Oil Drilling in TX.
While it's not all encompassing and work comp is not mandatory in TX, the below might be a good indication of where to place the risk.
Making an assumption that your customer could be classified as SIC 1381 (Oil Drilling and Gas Wells) we can profile the top 20 insurers for this market in TX market as seen at the link belowhttp://compdata.wordpress.com/work-comp-data/tx-oil-drillers/
Looks like your best bet might be Texas Mutual, AIG, Liberty or Ace.
Also, if your doing well at placing these, did you know that there are 307 other oil and gas drilling companies with work comp policies in TX?
(Full Disclosure - Risk Metrics provides this information and I work for Risk Metrics)