Economic, Safety, and Performance Considerations for Safety System Design for Large-Bore Gas Completions
Authors: Aldridge D.S. , Shaw B.S. , Shirazi S.A. 
Published: October 2008

In recent years, it has become increasingly apparent that natural gas is more readily accepted as a viable, primary form of energy. Often cleaner than traditional alternatives, the economic and environmental advantages of this fossil fuel in today's global marketplace are modifying the focus of the oil and gas industry. Discovery of large gas and gas/condensate reserves presents the operator with the challenge of safe, expedient and economic recovery of these resources.

The high flow rates that are desired from these reservoirs dictate the utilization of completion tubulars larger than those traditionally used. Production tubulars larger than 5.5" OD are becoming more commonplace in those completion designs accommodating high flow rates. With higher flow rates, it becomes extremely important to have the ability to predict the different types of aerodynamic forces on completion equipment. Accurate prediction models enable the designer to determine whether aerodynamic effects will complement or hinder the operation of completion equipment.

This paper examines the aerodynamic effects through a surface-controlled subsurface safety valve (SCSSV), discusses modeling and testing techniques, and presents conclusions based on full-scale test data and Computational Fluid Dynamics (CFD) modeling.

Introduction

A tubing retrievable surface-controlled subsurface safety valve (SCSSV) is a fail-safe (fail-closed) device that is an integral part of the tubing string. The SCSSV is held in the open position by application of hydraulic pressure at the surface, which is transmitted via a capillary to the SCSSV. In simplistic terms, the pressure places a force on a hydraulic piston, which moves downward, compresses a spring, and pushes a flapper (biased in closed position by a second spring) out of the flow path to permit well production. Upon loss of signal, the spring (and well pressure) force the piston upward and allow the flapper to swing into the flow path, thus closing the safety valve and preventing well production. It is imperative that the SCSSV, the most important, or often the sole safety device, close upon loss of signal in all scenarios regardless of fluid velocity or tubing pressure.

Most of the recent large gas discoveries are offshore, often far from existing production and transportation infrastructure. The economic benefits of many such discoveries can be realized only if gas production rates are very high and recovery costs are minimized. While producers always seek to minimize the number of wells drilled and to maximize the production rate from each well, these considerations are even more important in the case of remotely located reservoirs. Fortunately, many recent gas discoveries have characteristics that permit extremely high production flow rates without damage to the geologic formation. In such cases, very large bore production tubulars, 7" and larger, are used to maximize the recovery rate.

Currently, the size of the tubular employed is limited by the availability and performance of surface and subsurface completion equipment. If field-proven equipment is not available in the tubing size desired, a mixed-size tubing string might be considered. As mentioned above, one of the key elements of the production string is the SCSSV.