Well cement is placed into the annulus between casing and formation to provide structural support and zonal isolation through the well lifecycle. Nevertheless, operators in the North Sea have been concerned by the ability of the cement sheath to maintain sealing integrity given to the increasing number of reported failures in mature wells. As a result, recent efforts have been undertaken to achieve enhancements in the technology and standardization of tools to assess the status of this barrier. Yet, progress seems slow due to the high complexity of performing the task downhole. Hence, a new laboratory setup is designed to permit a detailed assessment of cement sheath failure mechanisms for realistic wellbore curing and operating conditions.
The laboratory set-up is conceived to allow visualizing the development of possible leak paths throughout the cement sheath, such as de-bonded areas and cracks in the bulk of cement, when exposed to pressure- and temperature-related varying loads. It comprises downscale concentric cylinders of rock, cement and pipe attempting to resemble a cased wellbore section. Temperature and pressure inside the casing can be varied in a controlled manner. In addition, cement pore pressure and rock confining pressure can be controlled independently. Computed tomography (CT) scans of the sample before and along the cyclic loads provide geometric three-dimensional information that aids identifying how and where the cement sheath is damaged.
The first trials of the pressurized cell deal with two samples, one with sandstone and one with shale rock, with centralized pipe and Portland G cement. During the curing process temperature and pressure are kept constant. After the cement sets, pressure is kept constant while casing temperature is varied along several cycles. Initial CT analyses show that cement hydraulic pressure provides a better initial cement job than for previous cemented samples with no pressure. In that sense, it was found better cement bonding to casing and formation, as well as less volume of voids after cement cured. Moreover, the cracking failure mechanisms resulting from thermal cyclic loads were mitigated due to the reduction of tensile stresses, since cement hydrostatic stress and confining pressure around the rock exists.
With this new laboratory set-up it is possible to evaluate the capabilities of any type sealant material to withstand varying temperature and pressure loads by visualization and quantification of de-bonded areas and cracks propagation along the time. The method offers an alternative to study in detail and compare sealant materials used as annular barrier when exposed to realistic wellbore thermal loads.