Abstract:
Zhujiadun Structure in Subei Basin is depleted gas reservoir with deep burial, complex strata and low reservoir pressure coefficient, which poses significant challenges to drilling and completion operations in horizontal wells. By analysis of multi-pressure systems in the vertical direction in Zhujiadun strata, the risks in drilling and the mandatory sealing points of casing were determined, and thus the design proposals for a three-stage large-diameter wellbore structure and a multi-target medium-radius three-dimensional wellbore trajectory were finalized. Based on the analysis of the drillability of the strata, the drill bits and screw drills were selected layer by layer, and a drilling speed-up technology for non-standard large-diameter wellbores was developed. By clarifying the geological model building and track adjustment ideas and adopting the integrated logging and steering technology, an efficient wellbore trajectory adjustment was prepared. Through analysis of the physical and chemical characteristics of the selective zones and optimization of the low-density high-performance water base drilling fluid system formula, a drilling fluid system suitable for low-pressure leaking reservoirs was developed. By developing additives and a high-temperature-resistant low-density self-repairing elastic anti-fluid-channeling cement slurry system, a low-pressure-bearing cementing technology for the selective zones was formed. This technology was successfully applied in two horizontal wells. Compared with engineering design, the average drilling cycle was shortened by 14.9%, and the average high-quality rate of the casing sealing section in the production layer reached 84.07%, which meets the gas storage technical requirements of Zhujiadun gas storage facility and provides a reference for the efficient construction of similar low-pressure gas storage wells in China.