Operators involved in the recovery of hydrocarbons from heavy-oil reservoirs often face the problem of maintaining well integrity in steam-injection wells. A significant portion of these wells suffer various forms of leaks and in the most severe case complete steam breakthrough to surface. Throughout the life of heavy-oil wells, cement material degradation and stresses in the cement sheath induced by extreme temperature cycling result in severe mechanical damage and ultimate failure of the cement sheath. These problems motivate different operators to explore new cementing technologies that are capable of achieving reliable longterm zonal isolation in these extreme conditions.The main challenge for operators is to design thermally stable cement with mechanical properties sufficient to withstand stresses induced by the large temperature changes. This paper describes the development of a new cement system, which is stable, strong, impermeable and flexible up to a temperature of at least 350°C(650°F), corresponding to the maximum steam injection temperature. Depending on the curing temperature this new cement system provides low Young's modulus of 1,800 to 4,000 MPa while maintaining excellent compressive and tensile strengths compared to cements currently used in the oilfield industry. Aging the cement system for 6 months at steam temperatures demonstrates the stability of the set material properties, including maintaining a low permeability.Field trials in North America show that this new cement system can be easily implemented into standard cementing operations using conventional equipment. Cement evaluation logs after cement operations confirm that excellent zonal isolation and wellbore integrity are readily achieved.By keeping adequate strength and flexibility, this new cement system reduces the risk of cement sheath failure and steam migration throughout the life of these steam-injection wells. It provides a long-term well integrity solution for any well exposed to very large temperature increase after the cement initial set, such as in fields exposed to steam temperatures.
Proper well construction involves long-term integrity; thus, accurate characterization of the physical properties of set cement systems is mandatory. Chemical stability, compressive strength, and permeability are commonly the main parameters determined for oilfield cement systems. The knowledge of these properties is most often enough to estimate if a cement system will maintain well integrity. However, some hydrocarbon recovery processes are highly aggressive toward the cement sheath. Under thermal processes for recovery of heavy oil, such as steam-assisted gravity drainage (SAGD) or cyclic steam stimulation (CSS), the well temperature can reach up to 350°C. The casing expands during the heating-up phase, and this outward expansion increases the stress on the annular cement sheath. Under such conditions, simulations show that a key intrinsic thermal property, the linear coefficient of thermal expansion (LCTE), is equally as important as the other physical properties to maintain well integrity. To determine the LCTE of set cement, one needs to understand the thermal-expansion theory of solid materials. An experimental setup for measuring this parameter for set cement has been developed, and essential precautions for performing accurate measurements have been formulated. The LCTE of cement is a critical parameter in the development of new cement technologies for high-temperature (HT) well applications; the chemical composition of the cement system and the curing conditions can affect the LCTE of oilfield cement systems. Improved understanding of this critical parameter has allowed significant improvement of the reliability of cement systems used in these hostile environments and provides better solutions in the form of thermally responsive cement systems.
There are more than 1200 heavy oil deposits worldwide containing approximately 13,000 billion bbls of oil. Various steam injection methods are used to mobilize the viscous fluid to flow so that it can be produced to surface. During the production process, the cement sheath maintaining wellbore zonal isolation is subjected to large mechanical strains imposed by temperature variations. Under these extreme conditions, mechanical and thermal cement properties are key factors to maintain wellbore integrity and to achieve the desired reservoir productivity. Significant heavy oil reserves are found in unconsolidated sandstone formations and carbonate formations. To avoid fracturing of these formations during cement placement, systems with densities below 1,800 kg/m 3 are required. Further, during exposure to steam temperatures, formation type may affect cement structure and, hence, its material properties. A new steam resistant cement system was designed to have tailored mechanical and thermal properties for slurry densities in the range of 1,400-1,700 kg/m 3 . To simulate the influence of formation composition on the material properties, cement samples were aged in autoclaves containing either water or water-saturated sand. Aging was continued up to six months at temperatures of 250-344 degC. A two-fold improvement of the material properties in the conditions simulating contact with the formation (water-saturated sand) is observed compared to standard curing conditions. Cement sheath integrity is simulated for the new and conventional steam resistant cements for typical conditions of heavy oil wells, including the effect of the formation. The higher durability of the new cement material compared to conventional systems is confirmed. Operators can count on cement sheath integrity as they apply thermal processes to produce heavy oils.
Most heavy oil resources are found in shallow reservoirs. The challenging conditions of steam injection process to recover heavy oil from these wells impose additional requirements on zonal isolation material properties. At minimum, the ideal cement system must be designed with mechanical strength and flexibility sufficient to withstand the stresses induced by significant thermal loads. However, this ideal system has to acquire these properties at low placement temperatures and keep them at the minimum required level until abandonment and above. Conventional systems traditionally used for steam injection application don't satisfy these minimum properties requirements, which lead to various forms of leaks or even to complete steam breakthrough to the surface.New specialized cement was developed to reliably maintain zonal isolation in shallow wells under steam injection conditions. This system, designed at the density range of 1,400-1,700 kg/cm3 (11.7-14.2 lbm/gal) is able to develop an excellent combination of mechanical strength and flexibility at the placement temperatures of 15-30 °C. Cement obtained after the initial low temperature curing was then aged at 350°C for 6 months. Every month, a temperature cycle (350°C to ambient temperature to 350°C) was simulated. Mechanical properties (compressive and tensile strengths and Young's modulus) were measured after 1, 3, and 9 months. Numerical simulations of sealant integrity performed with these properties in an annular geometry under shallow well conditions confirm long-term durability of the new system.The cement's ability to withstand stresses encountered during steam injection was validated in a specific large-scale laboratory setup. A piece of cement sheath was subjected to two consequent temperature cycles from ~ 20 to 180°C with the a ramp-up of 0.6°C/min, equivalent to the worst case scenario for the proposed injector wells in the validation location. New cement system was capable of passing these conditions whereas conventional systems failed to maintain integrity of the well.Thus, this study confirmed the capability of the new specialized cement system to ensure zonal isolation in shallow wells.
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