Sand Settling in a Subsea Production System
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 132909, ’Sand Settling and Locations of High Erosion in Subsea Production System,’ by S.A. Shirazi, SPE, and B.S. McLaury, University of Tulsa; E. Sow, SPE, BG Group; and M. Butter, SPE, A. Folefac, SPE, and S.A. Hassaballa, Rashpetco, originally prepared for the 2010 Trinidad and Tobago Energy Resources Conference, Port-of-Spain, Trinidad, 27-30 June. The paper has not been peer reviewed.

When higher-than-expected sand volumes were produced from Gas Field 1 after a second well group was brought on line, it was believed initially that the sand production came from these new wells. Sand-transport studies demonstrated that the sand was produced by the first well group, rather than the second, but that the sand was not being transported through the main export lines until after the second well group came on line and gas velocities through the flowlines increased.

Introduction

The subsea wells in Gas Field 1 are completed with downhole sand control because of the unconsolidated nature of the reservoir sands. The gravel-pack completions are designed to stop sand but allow some fines production to prevent plugging of the gravel packs and screens and, hence, loss of productivity. Some solids production was expected over the life of Gas Field 1. However, after a second well group was brought on line in 2005, higher-than-expected sand production was observed. The first effect of sand production was the erosion of a valve downstream of a 36-in. slug catcher 6 months after the first well of the second group came on line.

Because the produced solids may cause further erosion damage at locations of high production velocities, it was essential that the sand production be investigated and acceptable erosion rates be determined. A fines-management program was put into place following the first detection of solids production.

Sand-Settling Calculations

Studies were conducted to determine if there is a potential for sand to settle inside flowlines and transport pipelines. Calculations were performed for producing wells in the first group and second group and for the transport and export lines. Initial calculations were conducted in the production system for the second group of wells because sand production was observed first soon after these wells came on line. Earlier calculations showed that sand was not settling anywhere in the wells or in the subsea infrastructure from the wells in the second group to onshore. Therefore, the calculations focused on wells from the first group that had been produced since 2003.

The first step in the sand-settling calculations was to gather historical production information and determine gas- and liquid-flow rates and the appropriate flow regimes in various pipe sections. The findings indicated that there are primarily two possible flow regimes in horizontal sections—stratified flow and slug flow. Experience with flow loops has indicated that sand settling is more prominent in stratified flow than in the slug-flow regime. Therefore, calculations assumed a stratified-flow regime. To make sure that sand can be transported even if a liquid film does not exist in the pipe, calculations also were performed for a mist/gas-flow regime.