This paper reviews the project to develop a wireline retrievable, surface controlled, subsurface safety valve system capable of being set at great depth.
Following alternative system reviews, four valves were selected for further evaluation and possible development. Only one of them, the electric solenoid safety valve system, has been fully developed and field proven.
In addition to the development of the electric solenoid safety valve and its respective nipple, the special cable that was developed for the system, the high pressure wellhead pack-off arrangement for cable passage through the wellhead and the electric safety valve control system are discussed.
The field experience with this safety valve, in two gas well completions, the wider application in Shell's operations and possible future developments are described.
Subsurface safety valves (SSV) presently available fall into two classes. Surface controlled subsurface safety valves (SCSSV) which, as their name Implies, can be directly controlled from the surface, and subsurface controlled subsurface safety valves (SSCSV) which can only be controlled indirectly by changes in flow rate through the valve.
Setting as SSV deep (at or below the packer) is desirable because of the increased scope of well control that deep setting provides. Not only is the tubing under control, the potential deep leak points from the tubing to the annulus are also under control. In the past, when a deep setting valve was required, an SSCSV was usually chosen because there was no inherent depth limitation. The trade-offs to be considered when choosing a deep set SSCSV are:
the necessity to impose a flow restriction in the order of 10 to 20% on the well potential;
increased operational difficulty of checking for correct functioning.
A conventional hydraulically controlled SCSSV, though more suitable for high flowrates is not practical at packer depth because of the limitation caused by the hydrostatic head of the control fluid. So, the available choices were a deep set SSCSV which created a production restriction or an hydraulically controlled SCSSV that had to be set relatively shallow.