Qualification and Use of a Mill-Premium Connection in a Geopressured Gulf of Mexico Application
Authors: Payne Mike L. 
Published: December 2007

Qualification tests were conducted on a new connection for use in a geopressured Gulf of Mexico application. Possible compaction of an unconsolidated reservoir under pressure decline made postyield axial strains in the well casings possible. Strain-limit tests were conducted in addition to conventional tests for structural load capacity and gas/thermal-cycle sealing. Unique approaches were also taken on the thread compound for the connection to alleviate the "trapped-dope" phenomenon. The connection was a mill-premium connection with tapered, interference-fit threads, a metal-to-metal seal, and a teflon seal ring. Testing and use of the connection were successful, including the unique strain-limit and thread-compound work. This paper describes project background, test design and execution, and field use of the connection.

Introduction

Extensive qualification testing was conducted on a new connection for use in a geopressured Gulf of Mexico application. The only previous use of the connection was a geothermal application, so this was the first high-pressure use. Because of expected compaction of the unconsolidated reservoir under pressure decline from production, subsidence of the pay sands could induce postyield strains in the well casings. For this reason, strain-limit tests were added to more conventional tests for structural load capacity (typical for drilling strings) and gas/thermal-cycle tests (typical for production strings).

In addition to the wider scope of the tests, unique approaches were taken regarding the thread compound used on the connection to avoid the trapped-dope phenomenon. The connection chosen for the qualification effort was a mill-premium connection, based on tapered, interference-fit threads with a metal-to-metal seal and a teflon seal ring. Testing and use of the connection were very successful, including the unique strain-limit and thread-compound work. This paper describes the project background, test design and execution, and field use of the connection, and should be of interest to engineers facing similar qualification tasks or selecting connections on a more simplified basis.

Project Background

Platform Development Project. In Spring 1985, Arco Oil and Gas Co. began procurement procedures on development drilling tangibles for a recently discovered field offshore Louisiana. The planned development for the field involved eight wells at about 12,000 ft [3660 m] total vertical depth (TVD). As shown in Fig. 1, the well programs called for 24-in. [61-cm] conductor, 18 5/8-in. [47-cm] surface, 13 3/8-in. [34-cm] protective, 9 5/8-in. [24.4-cm] protective, and 7-in. [17.8-cm] production casings. Estimated well conditions included shut-in bottomhole pressures of 9,600 psi [66.2 MPa] and bottomhole temperatures of 200 deg. F [93 deg. C]. Surface pressures would vary from 7,300 psi [50.3 MPa] flowing to 8,200 psi [56.5 MPa] shut in with flowing temperatures up to 160 deg. F [71 deg. C].

With setting depths for the 9 5/8-in. [24.4-cm] protective casing near 12,500 ft [3810 m] measured depth and for the 7-in. [17.8-cm] production casing near 13,500 ft [4115 m] measured depth, total footage for the eight-well development project was 100,000 ft [30 480 m] for the 9 5/8-in. [24.4-cm], 53.50-lbf/ft [780.8-N/m] P-110 and 108,000 ft [32 918 m] for the 7-in. [17.8-cm], 35-lbf/ft [510.8-N/m] P-110. Procuring tubulars on a project basis maximized economic leverage and discounts with the product suppliers.