Abstract

Borehole convergence or the change in borehole diameter during or after drilling is one of the primary indicators of borehole instability. Early recognition of borehole instability is critical in achieving successful and timely completion of drilling operations and for borehole exploitation. Commonly used borehole monitoring devices, such as the caliper log and the acoustic borehole televiewer, provide continuous measurement of the borehole diameter when drilling into subsurface formations. However, limited attempts have been made to accurately measure the borehole diameter changes in laboratoryscale studies. In this study, series of laboratory experiments were conducted on Thick-Walled Hollow Cylinder (TWHC) specimens made of synthetic poorly cemented sandstone, where a new laboratory-scale Convergence Measuring Device (CMD) was deployed inside the borehole for monitoring the deformation of the borehole walls. This new CMD was developed based on the concept of Local Deformation Transducer (LDT). Three different cement contents (i.e., 10%, 12%, and 14%) and various lateral confining stress conditions (i.e., 2 MPa, 4 MPa and 6 MPa) were considered to evaluate the performance of the CMD. The specimens with CMD were tested using a modified Hoek cell which enabled visual monitoring of the borehole convergence and borehole wall breakout processes during the tests. The results showed that the CMD was capable of accurately measuring the borehole deformation. Visual monitoring of the borehole allowed precise identification of the locations and shapes of the borehole wall breakouts. Studies showed that the amount of the cement content and the magnitude of the lateral confining stress have a significant effect on the borehole deformation and failure stress. Furthermore, it was observed that the rate of increase in the radial strain of the TWHC specimens prior to the initiation of a borehole wall failure decreases when the cement content was increased.

Keywords:Borehole deformation; Borehole monitoring; Convergence measuring device; Thick-walled hollow cylinder; Laboratory study; Poorly cemented sandstone