A method of cementing a casing in a wellbore extending from an Earth's surface into a subsurface includes providing a tube having a bi-frustoconical shape. The bi-frustoconical shape is defined by an upper tube part having an inverted frustoconical shape, a lower tube part having a frustoconical shape, and a waist intermediate between the upper tube part and the lower tube part. The tube is positioned in an annulus formed between the casing and a wall of the wellbore from a surface opening of the annulus. The tube is urged in a direction down the annulus and along the casing until the tube lands on a collar radially projecting from an outer surface of the casing into the annulus.