Design Challenges Due to Wax on a Fast Track Deepwater Project

Abstract

Abstract Flow assurance threats, such as hydrates, waxes, and asphaltenes, can be identified and controlled if oil and gas production and transportation systems are designed accordingly and proper operating procedures are implemented. To this end, prevention of wax deposition is a key component of good subsea deepwater system design. Wax deposition can form a blockage and impede flow, causing weeks of lost production and an operating nightmare. This paper discusses the challenges associated with the design of the Cottonwood deepwater subsea tieback in Gulf of Mexico Garden Banks 244 (GB-244). The subsea infrastructure for Cottonwood consists of two wells at a depth of 2,118 ft located in block GB-244 and connected via a flow line loop of two 6-inch, 17.4-mile lines running parallel to the host facility located in East Cameron Block 373 (EC-373). An 8-inch, 20-mile export pipeline transports the condensate from EC-373 to a platform in Garden Banks 72 (GB-72). Production from the field is predominantly natural gas with condensate and formation water. There was a lack of reliable fluid property data for the development, but potential wax deposition problems were expected based on known operations experience with comparable neighboring fields. In addition, this project required a fast-track subsea development project, creating challenges in design and implementation. This paper highlights the flow assurance challenges that were studied to develop operational strategies to enable prevention of wax deposition during initial production while maintaining the flexibility to accommodate modifications that might be advisable after actual fluid property data is available for review. In short, this paper presents the wax-related measurements and the effective strategies developed for wax control and remediation during start-up and ongoing operations. Introduction Flow assurance in subsea systems is one of the main issues in the design of deepwater field developments. Flow assurance efforts focus on preventing solid deposits from blocking or restricting the rate of flow from the well. The principal solids of concern are wax and hydrates. Scale and asphaltenes can also be a concern. For a given reservoir fluid, these solids precipitate at certain combinations of pressure and temperature. Precipitated solids are often carried downstream slurried in the fluid, but precipitated solids can also deposit on the walls of the production equipment, which ultimately causes high pressure drops, plugging, and flow stoppage. Control of this deposition - via prevention and/or mitigation -is the essence of flow assurance. Wax in hydrocarbons is comprised primarily of paraffin, which is a white, odorless, tasteless, chemically inert compound composed of saturated hydrocarbons. The linear paraffins are easily measured by high-temperature gas chromatograph (HTGC). The HTGC technique measures the amount of each n-alkane in the sample. The sum of the n-alkanes greater than 20 carbons is reported as the n-paraffin or wax content of the hydrocarbon. In general, the amount of wax decreases with decreasing API gravity. Wax varies in consistency from that of petroleum jelly to hard wax, with melting points from near room temperature to over 210 oF. Wax has a density of around 50 lb/ft3 and a heat capacity of around 0.081 Btu/(ft.h.oF).