Experimental study of shale drillability with different bedding inclinations under varying wellbore pressure conditions

Shuai Chen,Jie Wu,Heng Bao,Xiangchao Shi,Leiyu Gao

doi:10.1007/s13202-021-01140-2

Abstract

In the practice of shale gas development engineering, it is important to understand the physical and mechanical properties of shale. The bedding inclinations of shale are known to significantly influence its physical and mechanical properties. This study mainly examined the influence of bedding inclinations on drillability under different wellbore pressures. The bedding inclinations used in this study varied from 0° to 90°, with a gradient of 15°. The wellbore pressure values used varied from 0 to 25 MPa, with a gradient of 5 MPa. The results show that the drillability index of shale increases exponentially with increasing wellbore pressure at different bedding inclinations. The proposed exponential empirical model can describe the relationship between the drillability index and wellbore pressure. When the wellbore pressure is less than 15 MPa, bedding inclinations significantly influence the drillability index, and the drillability index of shale shows a “W”-type variation trend as the bedding inclinations increase in the range of 0° to 90°. The influence of bedding inclinations on drillability decreases gradually with increasing wellbore pressure. When the wellbore pressure increases to 25 MPa, the impact of bedding inclinations on drillability is virtually undetectable. The results of this study can provide reasonably insight into the effect of bedding inclinations on shale deformation under the drill bit, and useful prediction for the drillability index under in situ conditions.

Highlights

Rock drillability was first proposed in 1927 by Tillson, who suggested that drillability reflects the degree of difficulty involved in breaking rock (White 1969)
Shear through the bedding plane occurred on shale samples with bedding inclinations of 75° and 90°
We found that the drillability index increases exponentially with increasing wellbore pressure for shales without different bedding surfaces

Summary

Introduction

Rock drillability was first proposed in 1927 by Tillson, who suggested that drillability reflects the degree of difficulty involved in breaking rock (White 1969). Microdrilling experiments are generally adopted as an effective method to evaluate rock drillability The simulated bottom-hole conditions of a microdrilling experiment can be used to objectively evaluate rock drillability under real drilling conditions. The results of field drilling show that bottom-hole pressure conditions significantly influence rock drillability (Carpenter and Chris 2015; Nas et al 2010), and laboratory experiments have shown that there are significant differences in the same rock under different bottom-hole pressures (Mao et al 2018). Strong dipping strata usually result in severe borehole inclination (Da et al 2015), which is an important technical problem in drilling engineering. Few studies have examined how bedding inclination causes borehole inclination (Teng et al 2017), especially in simulated bottom-hole conditions. Controlling the wellbore along a predetermined trajectory is critical for subsequent oil and gas production

Methods

Results

Conclusion