A downhole tractor assembly includes tubular main body configured such that a central axis of the main body is parallel with an axis of a tubing string when the tool is disposed in the tubing string. The tractor assembly includes a fluid flow passage within the main body, a turbine disposed in the flow passage, an alternator disposed in the main body and configured to convert the rotation of the turbine into electricity, one or more wheels extending radially at least partially from an exterior surface of the main body, and one or more electric motors configured to, when actuated by electricity from the alternator, drive rotation of the one or more wheels. The tractor assembly is configured to attach to a tubular string disposed in a wellbore, such that fluid flowed through the tubular string flows through the fluid passage and rotates the turbine.