Abstract
Wellbore collapse frequently happens in the clay shale formation. To maintain wellbore stability, appropriate mud pressure is a vital factor. When clay formation is opened, drilling unloading occurs, modifying rock structure and strength at the wall of borehole, which affects the selection of mud pressure. Currently, mechanism of drilling unloading is still poorly understood which in return will bring a concern to wellbore stability. Therefore, in this study, a combination of triaxial compressive test and ultrasonic wave test has been used to simulate drilling unloading and analyze its mechanism. Results indicate that more void space is created inside the clay shale sample due to unloading. This structure change leads to a decline of strength and acoustic amplitude. Additionally, unloading influence is depended on varying drilling unloading parameters. Small unloading range and fast unloading rate are able to enhance stability. With various degrees of unloading impact, collapse pressure equivalent density has a clear modification, proving that unloading is a non-negligible influencing factor of wellbore stability. Besides, the unloading effect is much stronger in large confining pressure, implying that more attention should be given to unloading when drilling is in extreme deep or high geostress formation. Findings in this paper can offer theoretical guidance for drilling in the clay shale formation.
Highlights
Wellbore instability is a well-known problem in the drilling operation, causing huge economic loss (Chen et al 2003; Rahman et al 2000)
Clay shale has been considered as high-risk formation since numerous wellbore collapses occur (He et al 2016; Wilson and Wilson 2014; Abdulhadi et al 2012)
Under the influence of drilling fluid on rock, mud pressure has to be adjusted with drilling time
Summary
Wellbore instability is a well-known problem in the drilling operation, causing huge economic loss (Chen et al 2003; Rahman et al 2000). If initial mud pressure is too low, wellbore collapse occurs immediately, forming irregular borehole shape and causing irreversible damage to rock around the wellbore (Ding et al 2018). Under this circumstance, adjustment of mud pressure is not so useful, further elevating the challenge of keeping wellbore stability. Adjustment of mud pressure is not so useful, further elevating the challenge of keeping wellbore stability When it comes to initial mud pressure, unloading influence cannot be avoided. Current studies about wellbore stability do not cover the unloading effect, which is not beneficial for drilling in clay shale formation with high risk of wellbore collapsing
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
