Fluid heater and associated methods

A downhole fluid heater 12, 212 heats fluid by agitation. An inner rotor 14, 214 driven by a drive shaft 21 acts on fluid within a heating chamber 18, 218 and turns within a stator 16, 216. Protruding fins 242, 244 on the stator and/or the rotor stir the fluid; fluid friction or viscous fluid shearing imparts thermal heating energy. The fluid may be sealed within the heating chamber 18 so the heated fluid acts to heat the stator 16; external fins 25 transfer heat to wellbore fluid 30. Alternatively, wellbore fluid can be directed into the heating chamber (118, fig 2A) via a port or propeller to directly heat wellbore fluid; in this embodiment (fig 2A, 2B) the heater pumps the wellbore fluid. The fins 242, 244 can be elongate with rectangular planar end tips; grooves between fins are V-shaped interstices 243 defined within the heating chamber. The fins 242, 244 can be curved, undulating or helical to aid fluid movement along the flow path 30. An external housing 223 can define a fluid flow path 230 with a fluid outlet and nozzle (125, fig 2B). A grinder (138, fig 2B) or sand cone crushes trapped particles in the fluid.