1. Field of the invention
This invention relates in general to wellhead equipment for oil and gas wells, and in particular to an emergency casing hanger seal.
2. Description of the Prior Art
In a well of the type concerned herein, a wellhead housing has a landing profile or shoulder within its bore. When running casing conventionally, a casing hanger is installed on the upper end of the string of casing. The casing hanger lands on the landing shoulder in the bore of the wellhead housing.
After cementing, a seal is positioned between the casing and the wellhead housing. The seal locates between machined surfaces on the wellhead housing and the casing hanger. A tubing hanger may be installed over the uppermost casing hanger. The tubing hanger is normally held by lockdown screws if the wellhead housing is located at the surface. The tubing hanger secures to the tubing which extends into the well. A tubing hanger seal locates between the tubing hanger and the wellhead housing.
Occasionally, the casing will not proceed smoothly to the bottom of the well. When this occurs, the casing hanger will not be properly positioned to land in the wellhead housing. Generally, when this happens, the casing cannot be retrieved to the surface and becomes stuck. In that case, the casing must be cut above the landing shoulder after cementing. In the prior art, the casing is supported by slips in the wellhead housing.
A problem exists in sealing against the casing stub, because the casing does not have a smooth machined surface for receiving a seal. The casing outer diameter has a high dimensional variation. The outer diameter may be slightly oval shaped. The surface of the casing may have many defects, such as rust, pock marks and tong marks. Various seals have proposed for sealing against the casing stub. However, improvements in locking the seal in the wellhead housing are desired.
Another desirable feature would be a means that would prevent the casing from moving upward due to temperature increase in the well during production. Movement of the casing could have damaging effects on the seal. Also, it would be desired to have a tubing hanger lockdown that did not employ lockdown screws, and would allow the seal to be removed without removing the lockdown.