Gas invasion in deep-sea shallow gas drilling may cause serious accidents, such as blowouts, if not properly managed. Therefore, the implementation of effective well control has attracted increasing attention. The downhole packer-while-drilling (PWD) is an effective emergency tool to address such issues. It can prevent wellhead blowout by plugging the drill pipe/casing annulus in a timely manner when a gas invasion is detected, which is of great significance for well control during deep-sea shallow drilling. A rubber cylinder is the core sealing element, and its structure and mechanical properties determine the setting performance of the PWD. Therefore, an optimization analysis of the rubber cylinder based on on-site conditions is expected to provide optimal technical parameters for engineering applications.

Considering that some factors affecting the setting performance of PWD are uncertain or difficult to quantify accurately, orthogonal optimization experiments and fuzzy comprehensive evaluation (FCE) were combined to construct a technical parameter optimization system for the PWD. The optimization parameters included the size and mechanical parameters of the rubber cylinder, the thickness of the spacer ring, and the wall friction coefficient. Based on fuzzy theory, a three-layer fuzzy evaluation system was constructed for the technical parameter optimization of the PWD, in which the target layer was the sealing factor of the PWD, the primary layer contained the main factors affecting the setting performance, and the secondary layer included specific technical parameters, such as the structural and mechanical parameters of the rubber cylinder and other relevant parameters. Using the orthogonal optimization design method, a structural parameter sensitivity analysis of the rubber cylinder was conducted, and the primary and secondary relationships of the parameters affecting the PWD setting performance were determined. Based on the range and fuzzy membership functions of each parameter obtained from the orthogonal experiments, 18 optimization schemes were determined for the final FCE.

According to the orthogonal experimental design and sensitivity analysis, the order of influence of each structural parameter on the setting performance was as follows: pore radius, total length, thickness, and inclination angle. Through the FCE of the 18 sets of technical parameters, the optimal values were: pore radius of 6 mm, thickness of 40 mm, total length of 343 mm, inclination angle of 50°, elastic constants C₁₀ = 1.77 and C₀₁ = 0.88, spacer ring thickness of 12 mm, and wall friction coefficient of 0.2.

A finite element simulation was used to verify the PWD setting performance before and after technical parameter optimization. The maximum von Mises stress of the optimized rubber cylinder increased by 1.13 MPa owing to the addition of a pore, which had a little impact on the strength of the rubber cylinder. However, the setting performance of the rubber cylinder increased by 10.28% after parameter optimization. The analysis results indicated that orthogonal experimental design can achieve optimization using only a small dataset, and the PWD technical parameter optimization based on orthogonal experiments and FCE is effective.

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.