The return of drilling fluid in large diameter horizontal directional drilling boreholes

•Calculation equation of drilling mud pressure loss was used to study the drilling mud return for the first time.•The percentages of drilling mud return to the entry and exit points were found.•The theoretical calculation results are quite close to the field observation.•The effect of the elevation difference of entry and exit points on the drilling mud return was discussed.

The return of drilling fluid (drilling mud) in large diameter horizontal directional drilling (HDD) boreholes is a very important issue related to mud pressure, cuttings transport and pull back force, but this problem has not been fully solved in previous research due to the complexity and uncertainty of downhole conditions. The calculation methods of mud pressure loss in the borehole annulus are briefly introduced and the equation using a Power Law model is adopted to study the drilling mud return. A typical reaming hierarchy of a large diameter HDD project has been used to study the return direction of drilling mud and volumes of drilling mud return to the exit and entry points. The research results disclosed that the drilling mud can return to both exit and entry points at the same time, with a large majority of the drilling fluid returning to the exit point if the reamer is closer to the exit point, and vice versa. A parametric study has been conducted to find out the effect of changing parameters in the calculation equation on the drilling fluid return. The drilling mud return for those boreholes with the exit point and entry point located at different elevations has also been discussed. Finally, different types of reamers and other factors such as borehole collapse and borehole shape were taken into the consideration of drilling mud return.

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