A gas separation and injection system includes a lower separator that receives and separates a production stream into higher and lower density streams, a turbine-compressor including a turbine that receives the lower density stream to rotate a shaft that drives a compressor and subsequently recombines the lower and higher density streams into a recombined production stream. An upper separator receives the recombined production stream and includes a gas inlet tube that conveys a gas stream to the compressor to produce a compressed gas stream. A shuttle valve assembly axially interposes the upper separator and the turbine-compressor and includes a mandrel assembly received within a body and having the gas inlet tube extending within the mandrel assembly, a valve seat secured to the gas inlet tube, a piston movably arranged within the inner annulus between closed and open positions, and a shuttle valve operatively coupled to the piston.