Abstract

The Cannonball Field is a one Tcf gas condensate development offshore Trinidad producing at a sustained rate in excess of 800 MMcf/D from three wells. The completion design selected was 7" inch production tubing with an open-hole gravel pack. The initial well (CAN01) has produced at 333 MMcf/D. These rates are higher than typically experienced which has raised concerns concerns about the resultant potential for metal erosion. As a result, a rigorous erosion study was initiated. The objective was to quantitatively evaluate erosion at various rates over the life cycle of the well to appropriately design the completion and select the appropriate materials.

The erosion nodes within the completion - changes in flow direction (e.g. a tee such as in the wellhead) and/or flow constrictions - were identified as: the tree; a landing nipple profile near the surface; and a formation isolation device (FID) positioned in the gravel pack assembly. The key parameters were defined as particles of sharp sand, with a diameter of 50 microns, at a concentration of 0.1 lbs/MMcf. Erosion rates were calculated using the erosion model - Sand Production Pipe Saver (SPPS) - developed by the Erosion/Corrosion Research Center, University of Tulsa, USA. Erosion rates were calculated over the life cycle starting at initial rates of 280 and 400 MMcf/D. Erosion rates were also calculated with and without a liquid film (a protective layer on the pipe wall that can reduce the erosion rate). Erosion results (without a liquid film) at all nodes exceeded BP's erosion limit; however, the erosion results with a thin liquid film were mostly below the company's erosion limit. Determination of the presence and thickness of the liquid film was critical. A multi-phase pipeline simulation calculated that a sufficient liquid film would exist at all critical areas. Erosion of the tree was further assessed by computation fluid dynamics (CFD) models, which identified several hot spots; thus, additional cladding of all flow-wetted surfaces and rounding of the outlet corner was required. The Cannonball completion design, including the tree, was determined to be capable of sustained rates up to a maximum 400 MMcf/D. The three (3) well development, where initial rates have been as high as 333 MMcf/D, has been on production for several years without any erosion issues.

Introduction

Trinidad's gas production has increased dramatically over the past 10 years. In 1996, local gas production exceeded oil production for the first time as the twin island Caribbean state of Trinidad and Tobago moved from a predominantly oil producing country to a major gas producer. The gas growth has been driven by an increase in local demand and construction of a liquefied natural gas (LNG) infrastructure, which now includes four Trains. BP Trinidad and Tobago LLC's (bpTT) share of the gas supply to the local market has grown from less than 350 MMcf/D in 1994 to over 2 Bcf/D by mid-2007 with production coming predominantly from several prolific gas fields located off Trinidad's East Coast.

The Cannonball field is located approximately 35 miles off the southeast coast of Trinidad in 240 ft of water (Figure 1). The discovery well, Ironhorse-1 ST1, was drilled in 2002. In 2005, a minimal structure (nine slot four pile) production platform was installed and three development wells were drilled and completed with a jack-up cantilever drilling rig. Initial production commenced on March 12, 2006 following pipeline hook-up and commissioning. The Cannonball field was brought on production at a sustained rate in excess of 800 MMcf/D. A previous paper 1 presented a detailed review of the design, engineering assurance, installation and performance of the Cannonball completions.

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