a body having annular forward and aft portions that selectively telescope with respect to one another between extended and retracted configurations of the body;

a space extending axially through the body;

a helical flight in the space; and

a membrane mounted onto the body and formed into an elongated toroid and that is selectively inverted when the helical flight is rotated, the membrane having a forward section that radially expands into contact with an inner surface of the wellbore when the body is in the retracted configuration, so that when the membrane is inverted contact between the membrane and inner surface of the wellbore generates a resultant force that displaces the tractor assembly within the wellbore.