Abstract
The primary objective of wellbore cement is to provide zonal isolation as a hydraulic barrier and ensure controlled fluid flow from the reservoir to the production facilities. During the lifetime of a well, cement has to cope with variations in stress, temperature and pressure. Frequent and dramatic oscillations of stress, temperature and pressure can cause damage within cement and its interfaces as well, which may leads to damage of cement sheath integrity, resulting in loss of zonal isolation and sustained casing pressure. This study investigated the effect of the mechanical compression and cyclic temperature variations on the mechanical properties of the cement, such as hardness and Young’s modulus; furthermore, this study included evaluation of cement slurries typically used in the deep drilling in the Gulf of Mexico US for cement compression.
Micro indentation tests were conducted on cement samples to evaluate the changes in mechanical properties with various cement designs. Analysis of indentation results on the cement samples showed: (a) a decrease in hardness and Young’s modulus right after compression while an overall increase one month after compression; (b) cyclic temperature variations significantly decreased neat cement hardness and Young’s modulus, but this effect varied when cement had other chemical additives; (c) hardness and Young’s modulus decreased after increasing water to cement ratio. Based on these results we are further investigating how to optimize cement slurries for field application that are characterized by thermal oscillation and under confined compression of cement sheath.
1. INTRODUCTION
1.1 Wellbore Cement
The main engineered hydraulic barrier in wellbores, cement sheath, can be subjected to many types of failures during the well life. The primary functions of wellbore cements are to provide zonal isolation, mechanical support of the metal pipe and prevent corrosion of metal component of wellbore system. Failures in cement sheaths can lead to the contamination of fresh water aquifer, migration of reservoir fluids from high pressure sands to low pressure sands, and sustained casing pressure as a result of fluid migration from the reservoir to the surface [1, 2]. The integrity of the wellbore cement sheath is a function of the mechanical properties of the hydrated cement, the geometry of the cased well and the properties of the surrounding formation [3]. During cementing operations, insufficient mud removal and improper cement placement might lead to weak bonding on both interfaces of the cement sheath. [4, 5]. Debonding may be caused by casing movement, pressure and/or temperature changes, and cement shrinkage. Pressure and temperature oscillations caused by completion and production operations contribute to the development of fractures and microannuli within the cement matrix [6]. Cement fractures created due to internal pressurization of the casing generally cause loss of annular zonal isolation in the lower quarter to third of the well, while large temperature changes cause cement sheath fracturing in the upper third to half of the well [7] . Both tensile and shear failures of the cement sheath have been shown to have a strong relationship with wellbore pressure and temperature [8].
