Houston-based Sidewinder Drilling, which is controlled by private equity group Avista Capital Partners, said the offer would run until 2 November unless extended.
Launching the planned merger last month, the company said it already had support from the equivalent of 51% of Union Drilling’s common stockholders for the purchase.
The offer requires uptake by 67.2% of Union Drilling’s outstanding shares.
The offer, which is supported by both company boards, is being made by Sidewinder Drilling’s acquisition arm Fastball Acquisition at a 6% premium on Fort Worth-based Union Drilling’s shares last pre-offer close on the Nasdaq of $6.13.
Union Drilling provides contract land drilling services and equipment to US explorers, specialising in unconventional drilling techniques. It owns 53 rigs, including two under construction.
Sidewinder Drilling was only formed in 2011 but benefits from the multi-billion capital warchest of New York- and London-listed Avista.