Open Hole Extended Limit Model and Drillstring Rotation Speed Optimization in Horizontal Section of Slim-Hole Horizontal Well
Authors: He Qinyi , Chen Xuyue , Gu Liangju , Yang Qiqi , Jia Zongwen , Zhou Changsuo 
Published: September 2022

ABSTRACT:
Different from conventional drilling, the effect of drillstring rotation on annular pressure loss must be considered when drilling with a slim hole annulus. The rotation factor is introduced to calculate the maximum open hole extension capacity in horizontal section, which could optimize the open hole extended limit model, so that the model could be applied to small hole drilling. Without considering the rotation factor, the drillstring rotation can effectively carry cuttings, which is conducive to hole cleaning, and the maximum elongation increases with the increase of the drillstring rotation speed. Considering the rotation factor, drillstring rotation will increase the annular pressure loss under the condition of small annulus, and the extension limit will first increase and then decrease with the increase of drillstring rotation speed. According to the case analysis calculation, when the drillstring rotation speed is 22-28 RPM, the horizontal open-hole extension capacity reaches the maximum of 4068 m. Sensitivity analysis was carried out for the optimal drillstring rotation speed range, and the effects of drilling fluid density, consistency coefficient and prevalence index on the optimal rotation speed range and the maximum open hole elongation were calculated.

1. INTRODUCTION
Slim-hole drilling technology is cost-effective and can be used to extract oil and gas resources in specific situations. However, different from conventional drilling technology, the annular pressure loss is large due to the small well hole. Many scholars have modeled and calculated the annular pressure loss for the small well hole. Fan and Xie established an annular pressure loss model for slim-hole drilling based on conventional drilling, taking into account the effects of drilling fluid rheology, drillstring eccentricity and rotation, and joint on annular pressure loss (Fan et al., 1998). In the same year, Wang et al. conducted an indoor study on the annular pressure loss in small-hole drilling (Wang et al., 1998). Wang et al. subsequently applied the small-hole annular pressure loss model to a well in Jilin Oilfield and verified the accuracy of the model (Wang et al., 1999). Song et al. proposed the method of annular pressure loss prediction system for small hole, and compiled software to calculate and verify a certain well (Song et al., 2004). Chen optimized the parameters of rotary drilling in real time based on mechanical specific energy mode (Chen et al., 2016; Chen et al., 2018). Wei takes into account the influence of drillstring rotation, drillstring eccentricity and drilling tool joint on annular pressure loss in the process of slim-hole drilling, and further considers the influence of temperature and pressure on annular pressure loss, making the calculation of slim-hole annular pressure loss more accurate (Wei, 2017). Annulus pressure loss plays an important role in calculating the open hole extension limit. At present, there is few calculation models for the maximum open hole extension capacity in the process of slim-hole drilling, and the influence of drillstring rotation on the maximum open hole extension capacity is not considered in the previous calculation model. In this paper, the influence of drillstring rotation on the maximum open hole elongation is considered under the condition of slim hole drilling, and the drillstring rotation speed is optimized under the condition of maximum open hole elongation.