Cement plays a vital role in maintaining the long term integrity of oil and gas wells by forming a robust and impermeable barrier and ensuring zonal isolation by restricting the flow of fluids between different geological zones. In-situ thermal stimulation methods such as SAGD place the casing cement under extreme stresses due to the rapid temperature increases that occur during initial steam stimulation. The prevailing mentality in the industry for many years was that compressive strength alone was a satisfactory predicator of a cement's ability to withstand downhole stresses. However, the stresses induced in a SAGD well are often not compressive, but rather radial in nature due to the large thermal expansion of the casing during heat up, making tensile failure of the cement more prevalent. This problem is compounded by the fact that the tensile strength of conventional oil well cements is typically only 8 – 12% of the maximum compressive strength. To best mitigate the risk of cement failure, cement with a higher tensile strength is a better choice.
In this paper a new fiber based tensile strength-enhancing cement additive for thermal oil well applications, such as those used in SAGD, is described. At an optimized loading of 0.5% by weight of blend (BWOB) the fibers gave a 20% improvement in tensile strength under mild temperatures (25–50 °C) and over 35% improvement in tensile strength at 250 °C. This significant enhancement in tensile strength occurs without significant detrimental effects on other mechanical or slurry properties. The stability of the additive is demonstrated by a full mechanical property evaluation (tensile strength, compressive strength, Young's modulus) after long-term high temperature exposure (one month at 250 °C). Extensive field testing with fiber-containing blends demonstrated that the fibers could be readily dry bulk blended on a multi-tonne scale, and successfully pumped with existing conventional dustless mixing systems. The significant enhancement in tensile strength, without sacrificing any other performance parameters, is expected to substantially improve well integrity in the demanding environments encountered in SAGD.