Abstract

One of the main objectives of a primary cement job is to prevent formation fluids from migrating into the annulus. To achieve this objective, the cement sheath should withstand the stresses induced by the various well operations and maintain integrity during the life of the well. However, the majority of the cement design programs in the industry today consider only the slurry properties and do not assess the effect of the mechanical properties of the cement sheath on the final well design.

A design procedure has been developed to estimate the risk of cement failure as a function of cement sheath and formation characteristics and well loading. A few examples of well loading are pressure testing, well completion operations, hydraulic fracturing, and hydrocarbon production. The design procedure is based on a finite element analysis and simulates the sequence of events from drilling through cement hydration, well completion, and production operations. The cement failure modes simulated are debonding, cracking, and plastic deformation.

The cement is assumed to behave linearly as long as its tensile strength or compressive shear strength are not exceeded. The material modeling adopted for the undamaged cement is a Hookean model bounded by smear cracking in tension and Mohr- Coulomb in the compressive shear. Shrinkage and expansion of the cement are included in the material model.

The need to design a fit-for-purpose cement sheath is accentuated by the sustained casing pressure observed on a number of wells after they were put on production and on some HPHT wells after the displacement fluid was changed over to a well completion fluid. The pressure in the annulus side sometimes results in an inability to continue further operation. Applications are discussed and examples are provided. From the processes reviewed in this paper, one can estimate the risk of failure of various cement systems and select a fit-for-purpose system that will minimize the overall cost. This process should improve the economics of constructing and producing oil and gas wells (cost effective life cycle design) and also improve safety because zonal isolation failures may be reduced.

Introduction

The main purpose of a primary cement job is to provide effective zonal isolation for the life of the well so that oil and gas can be produced safely and economically. To achieve this objective, the drilling fluid should be removed from both the wide and narrow annulus and the entire annulus should be filled with competent cement. The cement should meet both the short-term and long-term requirements imposed by the operational regime of the well. Typical short- and long-term properties that are required from the cement are listed in Table 1.

Traditionally, the industry has concentrated on the short term properties that are applicable when the cement is still in slurry form. This is necessary and important for effective cement slurry mixing and placement. However, the long-term integrity of cement depends on the material/mechanical properties of the cement sheath such as Young's modulus, tensile strength, resistance to downhole chemical attack, etc. The need to consider properties of the cement sheath for long-term integrity is critical if the well is subjected to "large" changes in stress levels.

After placing cement in the annulus, if there is not any immediate migration of fluid to the surface, it is likely that short-term properties such as density, rate-of-strength development, and fluid loss of the cement have been designed satisfactorily.

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