Validation and Verification of Large-Bore Tubing Retrievable Safety Valves for Gas Wells

Abstract

This paper discusses validation and verification considerations for large-bore sub-surface safety valves, particularly with respect to use in natural gas completions producing high flow rates to 450 MMSCFD and beyond. With natural gas wells, it is generally desirable from an economic standpoint to use larger production tubing than typically used for conventional oil wells. The larger sub-surface safety valves used in gas wells are exposed to unique conditions during operation. An estimate of the expected production rate of a given gas well is written into the statement of requirements for completion equipment. Validation and verification of the valve design to meet these requirements is a great challenge for equipment designers and test agencies, particularly for high flow rate analysis and testing. Only a small number of facilities have the capability to meet the volume and pressure requirements for testing the closure performance of large-bore safety valves against high gas flow rates. Production gasses flowing through the tubing string encounter discontinuities in the form of stepped changes in diameter at each tubing connection. As these large-bore safety valves are in the full-open position, changes in the diameter from entry to exit are only slightly larger than those at the tubing connections. During closure, and especially closure while flowing, the transient state of the valve provides a significant discontinuity to the flow path in the immediate vicinity of the closure mechanism. Discussion of valve design utilizing various analysis models is provided to understand how these high rate flow characteristics affect valve performance. In addition to unique analysis methods, these large-bore valves also require physical validation to ensure reliable performance in high gas flow rate conditions. Validation testing for these conditions is highly desirable and lends additional confidence in the performance of these products in emergency high flow rate scenarios. By including the proper design and testing protocols as shown in an actual case history, these large-bore valves capable of high flow rate applications are shown as fit for purpose in the most demanding of gas fields.