Patent attributes
A multistage cementing assembly is used for cementing casing in a borehole. A subsurface tool on the casing holds a subsurface plug therein using a temporary retainer. The subsurface plug is located in a subsurface location below one or more stage tools or cementing collars on the casing. First stage cement passing down the casing can pass through a passage of the subsurface plug to flow out of a float valve. A wiper plug passed through the casing behind the cement is then engaged with the subsurface plug. Pressure buildup can release the two plugs as a unit from the temporary retainer. Before reaching the float valve, a deformable ring on the subsurface plug can engage in a locator ring on the casing to indicate the plug unit's location during operations. Eventually, additional stages can be cemented using the one or more stage tools uphole on the casing.
