Permeability Characterization of Drilling Fluid Invasion Zone in Ultra-High-Temperature and High-Pressure Gas Wells

In ultra-high-temperature and high-pressure gas well drilling, the use of high-density drilling fluid inevitably invades the reservoir, and perforation operations require penetrating this invasion zone to further connect the reservoir. This paper first conducts core drilling fluid invasion experiments, introducing two parameters—filter cake loss and core slice permeability—to derive the long-term invasion depth pattern of drilling fluid. Then, through three types of perforation target experiments (steel target, cement ring target, and sandstone target), the penetration performance of this type of perforating charge is evaluated. The results indicate that the total volume of drilling fluid invasion depends on the pore permeability characteristics of the core, with a fitted final invasion depth of 7.24 cm after 30 days of actual operation. The edge effect of the perforating gun causes significant differences in penetration depth and hole diameter at different phases, where the near-edge penetration depth and hole diameter are 1.86 times and 1.75 times larger than the far-edge, respectively. When the temperature resistance exceeds 200 °C/100 h, penetration depth and hole diameter decrease significantly with increasing temperature and time, and under 250 °C/170 h conditions, the firing rate markedly decreases. Using sandstone targets with identical strength and pore permeability conditions as a South China Sea ultra-high-temperature and high-pressure well block, the average penetration depth is 197 mm, far exceeding the thickness of the drilling fluid invasion zone. These experimental results provide theoretical support for perforation operations in a South China Sea ultra-high-temperature and high-pressure well block.