Oil billionaires Dan and Farris Wilks have taken around a 10% stake in hydraulic fracking firm U.S. Well Services, according to a regulatory filing, bringing their total investment to $47.5 million in the past month.

The Wilks family office purchased $25 million of notes convertible into common shares in late June, worth some 25.7 million shares, according to a filing with the U.S. Securities and Exchange Commission.

Around that same time, the Wilks brothers’ pressure pumping firm, ProFrac, separately invested around $22.5 million in U.S. Well Services as part of a licensing agreement that allows them to build and operate electric frac fleets.

ProFrac converted those notes into three licenses, valued at $7.5 million each, following the transaction close. It has the option to purchase seven additional licenses at $7.5 million each, and 10 more at $9 million each.

“We believe these investments not only demonstrate the value of our intellectual property, but also offer further evidence that our industry is rapidly transitioning towards next-generation fracturing technology, led by electric fleets,” said a spokesperson for U.S. Well Services in a statement.

Dan Wilks has been scooping up shares in oilfield services firms hard hit by the downturn in drilling and fracking with stakes in ProPetro Holding Corp. and NexTier Oilfield Solutions.

The Wilks brothers also have been engaged in a lengthy court battles seeking to take over Canadian pressure pumper Calfrac Well Services Ltd.