Abstract
Annular shale creep barriers which can guarantee long-term well integrity over the entire lifespan of the well can be stimulated by temperature elevation caused by artificial heating inside the wellbore. Prior work has shown that heating can significantly accelerate barrier formation, but may also damage the shale formation if certain temperatures are applied. This paper reports on the optimum thermal conditions for shale barrier formation based on extensive new laboratory as well as literature data.
Thermally accelerated creep behavior was studied for the Lark and the Shetland North Sea shales. Large-scale triaxial equipment was used to study the behavior of shales under downhole stress and pressure conditions while varying temperature. In addition, an extensive literature study investigated the thermal effect of shales used for nuclear containment, such as the Boom Clay in Belgium, Cox Shale in France, and Opalinus Clay in Switzerland. The investigation focused on the impacts of temperature elevation on important shale properties such as creep rate, sealing and self-healing ability, and temperature-induced porosity, permeability, and mineralogical changes.
Both the laboratory investigation and the literature study showed that there is an optimum range for artificial thermal stimulation of shale barriers, with an upper temperature of 200°C – 300°C that should not be exceeded. At lower temperatures, thermal pore fluid expansion may lead to effective stress reduction and shear failure on shale bedding planes. In the optimum range, fluid thermal expansion is effectively negated by thermally-induced shale consolidation, and barrier formation is optimally accelerated, which is of great practical value for field implementations. Above the optimum range, irreversible dehydration and metamorphosis of the clay constituent of the shale happen and the shale loses its ability to creep to form a barrier and self-heal. This important result shows that heating inside wellbores to improve/accelerate creep of shales needs to be a controlled, engineered process in order to yield a competent barrier. This favors the use of a temperature-controlled heater rather than a less-controllable exothermic reaction.
Shale barriers seal annuli much better and more reliably than cement barriers. Moreover, their self-healing ability offers the ability to guarantee annular well integrity for an indefinite period, including the P&A phase. Thermal stimulation is preferred by operators to accelerate barrier formation without requiring annular access. The findings of this paper provide important theoretical and practical guidance on how to optimally stimulate shale barriers and avoid pitfalls associated with thermally-induced shale damage.
