Practice and Understanding of Large-Size Casing Cementing in BZ-X High Risk Well

BZ-X is a high pressure natural gas pre-exploration well in BZ block of Tarim oilfield. In the third section of Ф431.8 mm borehole, the density of water-based drilling fluid is 1.78 g/cm3, multiple sets of high-pressure gas layers and leakage layers are drilled from top to bottom. The safety density window is narrow, malignant leakage and gas invasion occur for many times, and the overflow and leakage conversion are frequent. The drilling of the third section was completed by carrying out plugging and killing simultaneously, and then put in the Ф339.7 mm technical casing for cementing. During the drilling period, the cumulative leakage of drilling fluid was 7400 m3, and a large section of pure gas column was seen many times at the wellhead during the aftereffect of cyclic drainage, so the risk of well control was high. In view of the complex downhole situation of overflow and leakage, the settlement isolation plugging technology is adopted to plug the malignant leakage layers at the bottom of the well. The safe running of casing is ensured by optimizing the drifting measures, tripping measures and designing the casing running speed. By optimizing the cementing method, strengthening the wellbore preparation, optimizing the casing string and slurry column structure, improving the centering degree of the string, increasing the friction difference of slurry, large displacement and other measures, the cementing technology of “one-stage positive injection + in-pipe reverse extrusion + two-stage cementing” is matched to ensure the cementing quality. The logging results show that the leakage layer and active gas layer are effectively sealed, and the qualified rate of cementing quality is 87%, which achieves the goal of “wearing shoes and hats to prevent annular pressure”. The successful cementing of this well can provide reference for similar large-size casing cementing operations in high-risk wells in the future.