Drill string torque and drag is one of the major limitations of horizontal wells. Torque and drag modeling is regarded as an invaluable process to assist in well planning and to predict and prevent drilling problems. Soft string model are widely used in the industry, which has good matching result with field data when bit off bottom, but has low matching accuracy in horizontal wells when bit on bottom. In horizontal section the string is in compression when drilling. The compression force is not always in the same direction with well bore axis. The compression force has a component perpendicular to well bore and this force should be considered when modeling the drilling torque in horizontal wells. Off bottom torque and on bottom torque should use different models as the string in horizontal section has different status.

Horizontal section drill string is separately discussed in the paper. By study the off bottom torque, on bottom torque and bit torque, the WOB effect on torque was captured. String status when bit on bottom was analyzed. New model was presented considering the WOB affect along drill string for horizontal section of wells. Model validity was checked by field case at Changbei project.

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