Summary Annular shale creep barriers that can provide long-term well integrity over the entire lifespan of the well can be artificially stimulated by temperature elevation achieved through artificial heating inside the wellbore. Prior work has shown that heating can significantly accelerate barrier formation, but may also potentially damage the shale formation if certain elevated temperatures are applied. In this paper, we report on the optimal thermal conditions for shale barrier formation based on extensive new laboratory and 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, including the Boom Clay in Belgium, the Callovo-Oxfordian (COx) Claystone in France, and the 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. The literature study showed that there is an optimal range for artificial thermal stimulation of shale barriers, with an upper temperature in the range of 200–300°C (392–572°F) that should not be exceeded. At lower temperatures, thermal pore fluid expansion may lead to effective stress reduction and debonding on shale bedding planes. In the optimal temperature range, fluid thermal expansion is effectively negated by thermally induced shale consolidation, and barrier formation is optimally accelerated, as indicated by our laboratory experiments conducted up to a maximum temperature of 150°C (302°F). This is of great practical value for field implementations. Above the optimal temperature range, irreversible dehydration and metamorphosis of the clay constituents of the shale can take place, and the shale can lose its ability to creep, to form a barrier, and to self-heal. This important result shows that heating inside wellbores to improve/accelerate creep of shales needs to be a controlled, engineered process to yield a competent annular 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 provide annular well integrity for very long time periods, including the plugging and abandonment phase. Thermal stimulation will be preferred by operators to accelerate barrier formation because it does not require annular access and can be applied through multiple strings of tubing and casing. 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.
Oort et al. (Sun,) studied this question.