U.S. Well Services filed a patent infringement lawsuit against rival Halliburton Co. and shale oil producer Cimarex Energy Co. on April 15, alleging they infringed its electric fracking patents.

The lawsuit, which seeks unspecified damages and costs, alleges Halliburton’s all-electric fracturing fleet violates several of U.S. Well Services’ patents for electric pumps and pumping systems and monitoring and control systems.

Halliburton said it had just received the lawsuit and was reviewing it. Cimarex Energy did not immediately reply to a request for comment.

The action comes as electric fracking, or e-frac as it is often called, has won greater interest among oil and gas producers seeking to reduce carbon emissions. The technology uses natural gas or the grid to power hydraulic fracturing equipment.

There are a handful of oilfield service companies that own and operate e-frac equipment, including U.S. Well Services and rival Evolution Well Services. E-frac fleets can be costly to build—running up to $60 million apiece compared with as little as $30 million for diesel-powered systems.

Halliburton CEO Jeff Miller had said the company would be “slow” to move on the technology because of its cost. In January, however, Halliburton said it had deployed the industry’s first grid-powered fracturing system for Cimarex Energy in the Permian Basin, the top U.S. shale field.

There are fewer than 20 electric frac spreads in the U.S., according to consultancy Primary Vision, representing a fraction of the 206 active frac fleets. U.S. Well Services is running five e-frac spreads, while Evolution Well Services runs seven, according to Primary Vision.

Other hydraulic pumping firms, such as NexTier Oilfield Solutions Inc. and ProPetro Holding Corp., are also exploring the technology, they have said.

One industry observer said e-frac providers are also in greater demand as their technology has been shown to be effective. “They’re showing the efficiencies that were promised with e-frac—and they’re finally manifesting” cost and environmental benefits, said George O’Leary, managing director at Tudor, Pickering, Holt & Co.

The case is U.S. Well Services versus Halliburton, Cimarex Energy, U.S. District Court for Western District of Texas, No. 21-cv-00367.