Maintaining Late-Life Production From Long Offset Subsea Gas Wells

Abstract

Abstract The ability to maintain late-life production from long offset sub sea gas wells is very much dependent on the well stream's ability to reduce flow line pressure by effectively sweeping liquids from the flow line. This paper reviews the application and results of utilizing a gas ejector on long offset sub sea gas wells to enhance recovery. Although this document focuses on a sub sea application of this technology, gas ejectors can also be utilized on any low pressure well. The use of this device should be seriously considered when applicable. Its interchangeable internals allows the same device to be utilized through a wide range of productions and pressure regimes and can be relocated as needed. Post-installation data has shown that the installation of the gas ejector has performed as per the stated design and is a technically feasible and cost effective means to recover additional reserves from low pressure wells. It is particularly attractive where an existing high pressure gas source is available. The authors consider this the first application of this type in the Gulf of Mexico and since installation have received inquiries about potential applications for other Gulf of Mexico assets. Introduction The Gemini field is located in Mississippi Canyon Block 292 in 3400 feet of water. See Figure 1. The field was discovered in 1995 and production commenced in 1999. The original development consisted of three sub sea wells tied back to the Viosca Knoll host facility via two 27 mile long flow lines. Since first production, reservoir pressure and production rates have decreased to the point where the reservoir's natural ability to adequately sweep the flow line of liquids no longer exists. As a result it was predicted that the field would cease production unless a method to lower flow line pressure and reestablish a means to sweep the flow lines was developed. Several options were considered and a gas ejector was selected as the preferred alternative. This paper describes preinstallation operations, other options considered and design considerations specific to the Gemini facilities. In addition pre- and post-installation production and pressure history are presented. Pre-installation Operations Prior to the installation of the gas ejector, the Gemini wells flowed through a sub sea manifold where they could be routed into either of two 27 mile long 12" flow lines. Originally, production volumes required the utilization of both flow lines, but, eventually the declining production created the need to flow both wells into the same line. This continued until the pressure and corresponding velocity in the single line became inadequate to effectively sweep the liquids from the line. The resulting liquid hold-up in the long line caused excessive pressure drop and high manifold pressure, causing the wells to load-up with fluid.