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In microhole drilling (MHD) with coiled tubing (CT), the calculation models of drilling fluid circulating frictional
pressure loss (DFCFPL) are studied for different flow regimes of drilling fluid in CT on reel, downhole CT and annulus.
Example analysis of DFCFPL for 89mm diameter microhole is conducted, and the relationships among the total
DFCFPL, local DFCFPL in CT on reel, downhole CT and annulus are obtained. The smaller the CT diameter is, the
higher the local DFCFPL in CT on reel and downhole CT are. The larger the annulus is, the lower the local DFCFPL in
annulus is. So the local DFCFPL in CT is dominant in the circulating system. As for the different diameter CT, the total
DFCFPL decreases linearly with the well depth increasing. In order to use the hydraulic energy rationally, 73.025mm diameter
CT can be used to drill microhole with well depth less than 1000m, 60.325mm diameter CT should be used to drill
microhole well depth of which is more than 1500m. This study can provide references for the CT selection and circulating
hydraulic energy rational utilization design in MHD.