Experimental Study on the Wellbore-Cleaning Efficiency of Microhole-Horizontal-Well Drilling

Microhole-drilling technology is a high-efficiency and low-cost technology that has developed rapidly in recent years. However, during microhole-horizontal-well drilling, cuttings are easy to deposit at the bottom of the wellbore because of gravity and nonrotation of drillpipe. Inadequate drill-cuttings removal can cause costly problems such as excessive drag, and even mechanical pipe sticking. Therefore, many laboratory studies as well as field observations have been directed toward addressing the cuttings-transport problem. In the present study, a full-scale horizontal-cuttings-transport flow loop was set up and a total of 136 experiments were conducted. By analyzing the cuttings volumetric concentration and the dimensionless height of the cuttings bed, the effects of flow rate (0.00058-0.00078 m(3)/s), cuttings diameter (0.0003-0.005 m), rate of penetration (ROP) (0.00211-0.00636 m/s), eccentricity (0-0.8), and wellbore diameter (0.04-0.08 m) on wellbore-cleaning efficiency were obtained. It was found that cuttings-transport efficiency increased first and then decreased as cuttings diameter increased. Flow rate was the main parameter. Higher flow rate, lower ROP, lower eccentricity, and smaller drillpipe/wellbore-diameter ratio all led to higher wellbore-cleaning efficiency in microhole horizontal wells. In addition, a model for estimating the cuttings volumetric concentration and the cuttings-bed height was proposed by dimensional analysis dependent on the thorough understanding of the effects of various variables. The predictions were good when they were compared with the experimental data obtained. Major factors influencing cuttings transport in the field during microhole drilling and conventional rotary drilling were compared, and the reasons for their differences were discussed. The limitations of the proposed model in this study were also discussed. These results could provide a factual basis for improving microhole-drilling hydraulics.