Experimental study on wellbore stability during volcanic gas well test: A case study on Well Yongtan 1 in the Permian of the Sichuan Basin

During the completion test of Well Yongtan-1 (volcanic gas well)in the Permian of the Sichuan Basin, the formation collapsedand a large amount of volcanic debris returned, which blocked the wellbore and surface flow and finally impacted the well completion test seriously. In order to optimize the following completion mode and develop similar gas reservoirs efficiently, this paper analyzes reservoir rock mineral compositions, micro pore structures and rock mechanical parameters. Then, the wellbore stability in the process of completion test and production is researched based on Von-Mises shear failure theory. And the following research results are obtained. First, the lower volcanic reservoir in the work area has high clay mineral content and developed fractures and micro fractures. After the action of working fluid, clay mineral expansion, particle dispersion and migration occur easily, resulting in wellbore instability. Second, different working fluids influence rock mechanical parameters, and then impact the wellbore stability. In the reservoir treated by the modified polysulfonate well test working fluid, there is a wellbore instability point on the wall of the vertical well where the root mean square of maximum rock shear stress is greater than that of rock shear strength. Third, the critical gas production for the production of volcanic rock debris in the lower volcanic reservoir is 1.13×104 m3. Fourth, when the daily production of simulated gas well is between 1.42×104 m3 and 28.35 ×104 m3, the converted annual debris production is in the range of 270.3-2 183.2 kg, and the converted shaft sand setting height is 12.7-102.5 m, which can impact the production easily. In conclusion, the experimental results lay a foundation for the efficient development of similar volcanic gas reservoirs. In the completion test and production process of volcanic gas wells, wellbore stability shall be maintained by optimizing the working fluid formula, controlling the production pressure difference and optimizing the completion method, so as to give full play to the productivity of gas wells. © 2022, Natural Gas Industry Journal Agency. All right reserved.