An Experimental Study on the Influence of Cyclic Loading and Eccentric Casing Geometry on Cement Sheath Integrity

Abstract

ABSTRACT: Zonal isolation is essential for the longevity and productivity of any oil or gas well. Losing zonal isolation can lead to environmental problems such as contamination of freshwater tables, sustained annular pressure buildup (SAP), and extensive and expensive remedial work. In this study, we studied the fatigue failure of cement through cyclic loading experiments on thick-wall-cylinder samples. To simulate the actual wellbore conditions, we designed a two-layered sample, incorporating casing and cement. A dedicated tri-axial facility was modified to introduce cyclic internal casing pressure. Low and high-impact cyclic loading tests were conducted for both concentric and eccentric thick-wall-cylinder samples. The fatigue behavior of Class H cement and CNTs-reinforced Class H cement was studied. Besides the study of fatigue behavior, the influence of casing eccentricity was studied by analyzing the first-cycle failure. The Mohr-Coulomb failure criterion was employed to analyze the experimental data, supporting the validity of the analytical eccentric casing model established in prior research. The experimental results demonstrate the fatigue properties of the cement sheath and validate the impact of an eccentric casing geometry. This outcome is significant for enhancing cement system design and optimizing wellbore operational strategies, including applications like hydraulic fracturing and enhanced oil recovery processes. 1. INTRODUCTION The main goal of cementing is to provide reliable zonal isolation throughout the life of a well. The integral of cement protects the casing and prevents the formation fluid from migration. Cement quality is influential to wellbore integrity and zonal isolation. Zonal isolation is essential for the longevity and productivity of any oil or gas well. Nowadays, most wells around the world are extending deeper and deeper. Downhole conditions become increasingly complicated. Changes in downhole conditions, in terms of temperature and pressure, expose cement sheath to deductive stress that impacts the integrity (Goodwin & Crook, 1990). There are many reports about cement sheath failure in deep wells where high pressure and temperature exist. Cement failure is manifested by interzonal annular fluid movement and abnormally high annular pressure at some point behind the casing up to and at the surface (Nelson, 2006). Subsurface geo-mechanical changes can also cause severe defects in oil and gas wells' cement barriers. Damages in cement sheath, such as voids, cracks, and channels or debonding between cement and casing, caused by either a poor cementing operation or due to the thermal/pressure fluctuations in a well, can threaten the integrity of cemented sheath and compromise zonal isolation.