Experiment Study of Stress and Pore Pressure in Setting Cement Paste

ABSTRACT: Cement sheath integrity plays an important role in ensuring the wellbore safety. Shear failure, tensile crack or debonding may happen in the cement sheath during the cement sheath setting, later pressure test or production period. The failure condition is up to the effective stress state and strength criterion. However, how to determine effective stress in the cement sheath is difficult. In this paper, a device is designed to test the lateral and pore pressures evolution of a cement column when a constant pressure is applied on the top of the column. Meanwhile, the cement paste volume change could be monitored. It is found that the lateral and pore pressures show a three-stage feature of stable-decrease-stable when the cement paste is hardening. The lateral and pore pressures are nearly equal at the early time and then separate without water charging, both the ultimate pressures are pretty low. The lateral pressure and pore pressure are nearly same too when suppling water to cement. However, the ultimate pressures are slightly higher than the former case. The experiment study could provide an inspect to research cement behavior, which contributes to developing and verifying the cement sheath stress evolution model. 1. INTRODUCTION Cement sheath integrity is important to the wellbore safety. In order to study the cement integrity condition, the stress state is needed to know first. However, it is not easy to determine the cement sheath stress state. Since cement sheath is originally slurry, the cement solidification process changes its material physical and mechanical properties. The stress state of cement sheath may be different from its initial hydraulic pressure. The casing-cement-formation is normally regarded as an entire system, the cement sheath stress state could be obtained based on the thick wall cylinder theory. The methods include theoretical solution (Li et al., 2015; Liu et al., 2017; Thiercelin et al., 1998; Yuan et al., 2013) and numerical simulation (Nabipour et al., 2010; Wong and Yeung, 2005). Both of them could provide an exact cement stress distribution. However, these models usually assume the system is a one-part structure, it means that the stress or displacement boundaries are applied on the model at one time. Actually, such treatment disagrees the fact, the well construction, the hole drilling, casing running and cement pumping are done separately and step by step (Gray et al., 2009). Therefore, the cement stress state should consider the procedure and the cement setting process (Agofack et al., 2019; Amziane and Andriamanantsilavo, 2004; Zuo and Bennett, 2019).