Technical parameter optimization of downhole packer-while-drilling for deep-sea drilling

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.