Analysis of Drilling Fluid Circulating Pressure Loss in Hole Annulus for Microhole Drilling

Hou Xuejun1, 2, Zheng Huikai3, Gao Longzhu1
1 College of Petrol Engineering, Chongqing University of Science and Technology, Chongqing 401331, China;
2 Harold Vance Department of Petroleum Engineering, Texas A&M University, College Station, TX, 77840, USA
3 Tianjin Boxing Science and Technology Engineering Ltd, Offshore Oil Engineering Ltd. of PetroChina, Tianjin 300451, China

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© Xuejun et al.;

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the College of Petrol Engineering, Chongqing University of Science and Technology, Chongqing 401331, China. Tel: 337-349-4528; E-mails:,


The microhole drilling (MHD) technology is one of cutting-edge drilling technologies with wellbore diameter less than 88.9 mm and taking coiled tubing (CT) as drilling string to deliver the bottomhole assembly to drill ahead. The normal circulation of drilling fluid is affected because of huge annulus drilling fluid circulating pressure loss (ADFCPL) caused by small diameter wellbore, narrow hole annulus and deep well. For three flow regimes of power-law fluid in MHD, the computing models of ADFCPL are built by analyzing the relationships among annulus drilling fluid parameters according to power law fluid flow state equation. The ADFCPL with different flow rate, CT diameter, well depth, eccentricity and so on is calculated for the hole annulus of MHD. The calculation results show that: in 89mm diameter MHD, the ADFCPL is huge. It increases with the increase of outer diameter of CT and average flow rate of drilling fluid, and increases linearly with the increase of well depth, and decreases with the increase of eccentricity. The bigger the hole annulus is, and the lower the flow rate is, the less the impact of eccentricity acts on the ADFCPL is. Thus, the ADFCPL can be reduced by decreasing the outer diameter of CT and the average flow rate of drilling fluid in hole annulus. Only low flow rate can be used to drill deep well. The research results may be useful for MHD parameter selections related to the ADFCPL.

Keywords: Circulating pressure loss, coiled tubing, eccentricity.