In various parts of the world, heavy oil projects use high temperatures in order to support the oil production. Most of these projects are still on-going after several years of high temperature well exposure, while others have been suspended due to operational and environmental issues. The well integrity time span should extend itself beyond the field estimated operational date. These wells (observation, productions or abandoned) are highly susceptible to leaks due to cement degradation resulting from thermo-chemical-mechanical loads. Therefore, successful further development of heavy oil reservoirs requires a closer look at the assessment of well integrity and its prediction for extensive periods of time. During the life of the well, the casing-cement-rock system is experiencing, as a secondary problem, a cumulative damage. This could explain the sustained casing pressure that appears after years of good well integrity.
Long time exposure of the well to several loads as well as cyclic load changes may affect the well response due to a rather unexplored degradation of the mechanical, chemical and thermal parameters. In this case, the typical loading mechanisms are thermal, associated with superimposed chemical and mechanical loads. A large number of studies has been conducted to investigate the very complex chemical degradation process of Portland cement systems, which is influenced by temperature, pressure, chemical reactions and their products and, more important, time. A literature review of various investigations on well degradation processes has shown that most of the experiments are time-constrained, and therefore the well integrity is limited to the interval in which results extrapolation is possible. Cement retrogression has also been investigated and research led to recommendations of adding silica flour to the Portland cement. The least investigated aspects of the cements are their cyclic resistance to variable loads.
This paper will critically analyze the various existing well integrity concepts and point out the future need for research to improve the prediction of well integrity. The results of the work show that the major unknowns are long term properties of the downhole environment like casing, cement, and the interaction between casing-cement-formation. Moreover, a simple, yet attention-grabbing experimental investigation has been carried out on several cement samples with different properties by simulating the cement layer associated to the surface casing of a well exposed to temperature variations. Due to the shallow depths, the cementing of the surface casing may yield specific well integrity issues generated by low slurry density requirements, original low temperature environment, high fluid-formation temperature differences, all leading to potential problems related to health, safety and environmental standards worldwide (e.g. aquifers damage, operational risk, etc.).
