Cementing Steamflood And Fireflood Wells-Slurry Design

Abstract

Steamflood and fireflood wells present special challenges when one designs a cement slurry for such wells. In most cases, the cement slurry is subjected to relatively low temperatures during the cement job and early curing. However, after the cement sets, if must be able to withstand the thermal shock associated with the initiation of steamflooding or fire flooding. Additionally, the cement must be able to preserve adequate compressive strength and low permeability despite the potentially disruptive crystalline changes that occur at high temperatures. Another complicating factor is the weak or incompetent formations often encountered with thermal recovery wells.

This paper discusses the chemical and phase equilibria relationships which prevail when cements are exposed to the high temperatures associated with fire flood and steamflood wells. The CaO-SiO ₂ H ₂O and CaO-Al ₂O ₃-H ₂O systems, which respectively pertain to Portland cement and high-alumina cement, are discussed. In addition, an overview of methods for preparing slurries suitable for high temperatures, yet lightweight enough to prevent damage of weak formations and lost circulation, is present. The use of hollow-glass or ceramic microspheres to extend cement and foamed cement are shown to be particularly advantageous for cementing thermal recovery wells.

Introduction

The application of heat to stimulate oil production has been practiced for over 50 years. Methods such as in-situ combustion (fireflood), downhole heaters, hot fluid injection and steam stimulation have been utilized. In-situ combustion and steam injection are the most popular methods practiced today. These techniques have been the salvation of many oil fields with high viscosity crudes., and essentially involve the trading of BTUs for viscosity reduction ⁽¹⁾.

For reliable long-term performance of the fireflood and steam flood wells, a good cement job is essential. High levels of stress are built up in the pipe and the cement sheath, and the strongest possible pipe/cement and cement formation bonds are necessary. Failure of the bonds could allow interzonal communication and pipe expansion. The ultimate result would be casing failure by buckling or telescoping ⁽²⁾.

In addition, the cement itself must be able to withstand the elevated temperature exposure and thermal cycling associated with steamflood and fireflood wells. The cement must be able to maintain sufficient compressive and bonding strength, low permeability and preferably low thermal conductivity.

Good cementing techniques, such as the use of spacers and washes for adequate mud removal, casing centralization, mud conditioning and pipe movement are extremely important. However, such meticulous preparation and effort are wasted unless the cement is properly designed for long-term stability and adequate performance characteristics. This paper is a discussion of the pertinent parameters one must consider in the design of the cement for a thermal recovery well. These parameters can be divided into two principal categories: (1) chemistry and phase equilibria relationships, and (2) physical characteristics of the well and the formation. A discussion of these parameters is followed by some examples of cement systems which are suitable for various types of thermal recovery wells.