Drilling Fluid Technology for Ultra-High Temperature Fractured Bedrock Reservoirs

The Kunteyi gas field is located in the #1 buried structure of the Kunteyi sag on the northern margin of the Qaidam Basin, the bedrock reservoirs of which are developed with fractures and have a weathering crust, and the formations of which have weak segments and natural channels for mud losses, together with complex geological conditions such as ultra-high temperatures (approximately 200 ℃) and abnormal high pressures (pressure coefficient up to 1.63). To address the drilling fluid challenges confronted in drilling ultra-high temperature bedrock reservoir formations, such as thick and loose mud cakes, low pressure-bearing capacity and ease of lost circulation etc., an ultra-high temperature drilling fluid with high plugging capacity for the prevention of lost circulation was developed through synergistic design of core additives. In the drilling fluid “SC-200+Redu240” are used to construct a high-temperature colloidally stable framework, and “nano-silica + white asphalt NFA-25” used to achieve plugging of the multiscale fractures and finally an integrated function of “filtration control + fracture plugging + wellbore stability” is realized. Experimental results confirm that this drilling fluid functions normally at temperatures up to 200 ℃. After aging, the rates of change in the apparent viscosity and plastic viscosity are both less than 3%, the high-temperature high-pressure filtration rate is less than 12 mL, and the drilling fluid can resist contamination by 15%NaCl. Additionally, the sand-bed filtration rate of the drilling fluid after aging is only 4.2 mL, and the filtration rate for fracture plugging is 8.6 mL, indicating that the drilling fluid has excellent fracture plugging capacity and formation pressure-bearing capacity. This drilling fluid exhibits remarkable effects in lost circulation control and wellbore stabilization in drilling ultra-high temperature fractured formations. In field application of this drilling fluid in the well K2-3 (well depth 7170 m, and bottomhole temperature 199.5 ℃), “zero mud loss” was achieved in drilling the bedrock section, and no downhole complex situation caused by lost circulation or drilling fluid properties occurred throughout the whole drilling process, with a complex time efficiency of zero. The application of this drilling fluid technology significantly reduced the non-productive time due to lost circulation, providing reliable technical support for the safe and efficient drilling of ultra-deep wells in this area.