Self-aligning open-hole tractor

A self-aligning open-hole tractor assembly adapted for use in a subterranean open-hole wellbore comprising:

• an elongated body;
• a first centralizing portion extending from said elongated body;
• a second centralizing portion extending from said elongated body;
• and
• a first tractor portion disposed intermediate said first centralizing portion and said second centralizing portion, the first tractor portion independently rotatable with respect to each of the centralizing portions, and the centralizing portions each independently rotatable with respect to the elongated body and the first tractor portion,
• the first tractor portion configured to adjustably engage the wall of the wellbore and drive the assembly along the wellbore and comprising no more than one pair of radially extending circumferentially spaced arms, disposed about 180 degrees apart around said elongated body, the arms having respective traction elements at their outer ends, said arms being urged radially outwardly from the body to urge the traction elements into engagement with the wall of the borehole, the arms adapted to align themselves with the largest diameter of the well to permit the tractor assembly to self-align within the open-hole wellbore and maintain optimized driving friction within the open-hole well;
• and
• a second tractor portion disposed adjacent said second centralizing portion, wherein said second centralizing portion is between said first tractor portion and said second tractor portion, said second tractor portion extending from said elongated body for independently rotating about the longitudinal axis relative to said first centralizing portion, said second centralizing portion, and said first tractor portion.