High-temperature mechanical behavior of FKM and its influence on the sealing performance of downhole packers

The packer is a protective tool that is connected to the downhole string and used to seal the annular space between the tubing and casing, or the drill pipe and casing in wells. It enables interlayer isolation, fluid control, and risk reduction by facilitating well control. Fluororubber (FKM) has been widely used as an important material for the rubber cylinder of downhole packers because of its excellent hyperelastic properties. With the gradual expansion of China’s oil and gas exploitation to deep layers and deep sea, the high-temperature and high-pressure environments of downholes pose new challenges for ensuring that FKM packers provide safe and reliable sealing. Therefore, accurate control of the mechanical behavior of FKM in high-temperature environments is important for guaranteeing the effective sealing performance of downhole packers.

A general hyperelastic constitutive model of the rubber was established based on the strain energy density function. Through high-temperature uniaxial tensile and compression tests, the stress–strain curves of FKM samples in the temperature range of 100–200 ℃ were obtained. The experimental results were analyzed using the two-parameter and five-parameter Mooney–Rivlin models as well as the Yeoh model, and the most suitable constitutive model for high-temperature conditions was determined. The evolution of stress relaxation in FKM at high temperature was characterized by the generalized Maxwell model and the Prony series. The evolution of stress relaxation was analyzed using high-temperature tests, and the Prony parameters were determined. An axisymmetric finite element model of the FKM packer was established. The effects of high temperature and stress relaxation on the contact stress of the rubber cylinder were studied through numerical simulation, and the sealing factor was introduced to characterize the overall sealing performance of the downhole packer.

The five-parameter Mooney-Rivlin model was more accurate than the two-parameter Mooney–Rivlin model and the Yeoh model for characterizing the high-temperature mechanical behavior of FKM. For a given setting pressure, a higher temperature led to lower contact stress between the rubber cylinder and the inner wall of the casing, and the sealing performance of the downhole packer gradually decreased with increasing temperature. When the influence of stress relaxation was considered, the sealing performance further declined with increasing setting time. The sealing factor at 100 ℃ decreased by 20.8%, while the decrease reached to 29% at 200 ℃. Finally, a model for predicting the normalized setting pressure of the downhole packer was constructed based on the high-temperature and stress relaxation effects. The model enables accurate calculation of the setting pressure to ensure long-term, stable sealing in high-temperature environments.

The behavior of FKM in the temperature range of 100−200 ℃ was systematically studied, and appropriate high-temperature hyperelasticity and stress relaxation models were constructed to effectively widen the applicable temperature range of FKM. The model for predicting the normalized setting pressure can ensure the sealing reliability of rubber cylinders in high-temperature environments. These data provide theoretical guidance for optimizing the structural design and setting scheme of downhole packers in high-temperature environments.