Sand Transport and Deposition in Horizontal Multiphase Trunklines of Subsea Satellite Developments

Abstract

SUMMARY Gravelpacking is unattractive as a means of protection against the effects of sand production in subsea wells, since it involves additional completion costs, loss of productivity and difficulties in subsequent recompletion/well servicing operations. On the other hand, omitting gravel packs means that subsea developments have to be designed and operated so that they are capable of tolerating sand production. An experimental study has been carried out on sand transport and deposition in multi-phase flow in modelled subsea flow lines to address the problem of sand collection in horizontal trunk lines, which could lead to reduced line throughput, pigging problems, enhanded pipe-bottom erosion or even blockage. This study led to the definition of a new model for sand transport in multi-phase flow, which has been used to establish the risk of sand deposition in trunk lines connecting a subsea development to a nearby production platform. INTRODUCTION A number of small North Sea discoveries can only be developed as subsea satellites of existing production platforms, given current oil and gas economical constraints. These constraints also dictate simple underwater manifold centers and simple, minimum-maintenance subsea completions, with a high productivity per well. Conflicting demands arose during the development of several small offshore fields when calculations based on inhouse sand failure criteria indicated that the wells were potential sand producers. For various reasons (e.g. completion complexity, loss of production potential, inteference with anticipated acid stimulation), the use of sand exclusion techniques in the completion of these wells was considered undesirable. This meant that these developments had to be designed to be "sand-tolerant", I.e. sand produced by the wells had to be collected in separators at the production platform located several kilometers away from the sandface, without causing damage to the intervening production equipment or to the processing facilities on the platform. In one particular development producing oil with a low GOR (so that velocities in the trunklines carrying the oil to the platform would generally be low) deposition of the sand was considered to be the main risk. Such deposition and the formation of (moving) sandbeds at the pipe bottom had been seen to give rise to such problems as;Reduced line efficiency due to (partial) blockage of the lines and increased frictional pressure loss [1 ],Enhanced pipe-bottom erosion/corrosion due to a high concentration of solids at the bottom and the formation of corrosive cells beneath the sandbeds,Malfunctioning of equipment due to deposition of sand at critical parts. These problems can be avoided by ensuring that the capacity of the lines to transport sand to the platform exceeds the total sand production of the wells connected to the lines. This requires knowledge of the flow of gas/liquid/solid mixtures through horizontal flow lines and through the vertical riser section to the platform's production deck. Whereas the flow of sand/water slurries has been studied extensively, e.g. for hydraulic engineering purposes [2], data on three-phase sand/gas/liquid flow were not available in the open literature.