Oil Base Drilling Fluid Technology for Drilling Broken Ordovician Formation in Shunbei Block

Several ultra-deep wells were drilled in the Shunbei Block with water base drilling fluids, resulting in borehole wall collapse and mud losses in the Ordovician formation. Based on the analyses of the mechanisms of borehole wall collapse, a Gemini type high temperature emulsion stabilizer and a branched flow pattern additive with three heads and two tails have been developed. Using microencapsulation treatment method, a temperature-sensitive expandable lost circulation material was developed. This lost circulation material, when excited at 156 ℃, can expand to a volume that is 5.37 times of its original volume. With these additives, a high temperature oil base drilling fluid was formulated. This drilling fluid has low viscosity, high gel strength and strong sealing capacity. Laboratory evaluation results showed that this drilling fluid can be used at 180 ℃. The sizes of the emulsified droplets are in a range of 1.2-26.9 μm. The HTHP filter loss, plastic viscosity and yield point/plastic viscosity ratio of the drilling fluid are 2.4 mL, ≤40 mPa.s and 0.31-0.40 Pa/mPa·s, respectively. The plastic viscosity of the new oil base drilling fluid is 10%-15% lower than that of the conventional oil base drilling fluids, while the yield point of the new oil base drilling fluid is 15%-25% higher than that of the conventional oil base drilling fluids, demonstrating an excellent low viscosity high yield point characteristics and capabilities of plugging microfractures by size matching. Application of the new oil base drilling fluid in the well Shunbei-55X showed that the average percent hole enlargement in broken formations was only 7.77%. Apart from a momentary mud loss into a cave during drilling, there was no other obvious mud losses encountered. Use this new oil base drilling fluid, collapse of the Ordovician formation during drilling was avoided, mud losses into the fractured reservoir were reduced. It also helps set a record of the deepest onshore directional well in Asia and develop the oil and gas resources in Shunbei Block with high speed and high efficiency.