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

Abstract

Summary. Gravel packing is unattractive as a way to protect against the effects of sand production in subsea wells because 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 must be designed and operated so that they can tolerate sand production. An experimental study was carried out on sand transport and deposition in multiphase flow in modeled subsea flowlines to address the problem of sand collection in horizontal trunklines, which could lead to reduced line throughput, pigging problems, enhanced pipebottom erosion, or even blockage. This study led to the definition of a new model for sand transport in multiphase flow, which was used to establish the risk of sand deposition in trunklines connecting a subsea development to a nearby production platform. Introduction Several small North Sea discoveries can be developed only as subsea satellites of existing production platforms, given current oil and gas economic constraints. These constraints also dictate simple underwater manifold centers and simple, minimum-maintenance subsea completions, with high productivity per well. Conflicting demands arose during the development of several small offshore fields when calculations based on in-house sand failure criteria indicate-d that the wells were potential sand producers. For various reasons (e.g., completion complexity, loss of production potential, or interference with anticipated acid stimulation), the use of sand exclusion techniques during completion of these wells was considered undesirable. Thus, 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 several Kilometers from the sandface without damaging the intervening production equipment or the processing facilities on the platform. In one particular development producing oil with a low GOR (so velocities in the trunklines carrying the oil to the platform generally would be low), sand deposition was considered the main risk. Such deposition and the formation of (moving) sand beds at the pipe bottom had caused several problems: reduced line efficiency owing to (partial) blockage of the lines and increased frictional pressure loss, enhanced pipe-bottom erosion/corrosion owing to a high concentration of solids at the bottom and the formation of corrosive cells beneath the sand beds, and malfunctioning of equipment owing to sand deposition 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 flowlines and the vertical riser section to the platform's production deck. Although the flow of sand/water slurries has been studied extensively (e.g., for hydraulic engineering purposes, data on three-phase sand/gas/liquid flow are unavailable in the literature. Therefore, a project was begun to collect such data in the laboratory and translate the results to field conditions. This paper describes the experimental methods used to study sand transport and deposition. The results and their interpretation in terms of the relations between dimensionless parameters are discussed. Finally, this paper shows how these relations can be applied to field conditions and their consequences on design and operation of subsea trunklines. Experimental Methods The experiments were carried out in an air/water/sand test loop with a 0.070-m ID. Fig. 1 shows the layout of this loop schematically. Three methods were used to monitor the transport and deposition of sand injected into the air/water flow.Visual Observation. A transparent test section was included in the loop some 50 diameters from the sand injection point to minimize entrance effects. This allowed sand transport modes to be determined and sand-bed widths to be estimated by direct visual inspection.Sampling. A full-bore sampling probe was installed in the riser section. This allowed total sand transport through the system to be determined by passing the suspension over a 100-um filter.Acoustic sand detection. A clamp-on sand sensor was mounted at the bottom of the last horizontal section to study the mechanics of sand-grain movement under multiphase conditions. Four liquid/gas/solids systems were studied with this equipment. Series A - air/water with sand-grain size (0. 15 to 0.30 mm, input gas volume fraction of 0% to 20%, liquid velocity between 0.1 and 1.2 m/s, pressures slightly over atmospheric, room temperature of about 20C). Series B - similar to Series A, but with larger sand grains (0.69 mm) to simulate the effect of clustering of smaller grains. Series C - water viscosified with carboxymethyl cellulose EHV, giving it an effective viscosity of about 7 mPa - s, and a grain size of 0.15 to 0.30 mm. This system was used to investigate the effect of viscosity on sand transport (the viscosity may vary considerably over the trunklines as a result of liquid cooling). Series D-air/water with surfactant added to the water to reduce the surface tension from 0.064 to 0.028 N/m (further reduction was impractical in view of the increasing tendency to foam), with a grain size of 0.15 to 0.30 mm. This system was used to study the effect of surface tension, which is considerably higher in an air/water mixture than in an oil/gas mixture, Some 270 tests were carried out, resulting in a large volume of data over a wide variety of parameters. Results and Discussion To approach the vast amount of data systematically, the (qualitative) observations on the sand transport mode will be discussed first. The measured sand transport then will be analyzed in terms of the appropriate dimensionless quantity. Finally, some data on particle movement at low flow rates will be discussed. Sand Transport Modes. Three sand transport modes and two fluid flow modes can be distinguished visually. The sand transport modes are as follows. 1. Stationary bed. At the lowest liquid velocities, the injected is deposited at the bottom of the pipe. This leads to local sand buildup as injection continues. 2. Moving bed. Above a certain critical velocity (which is a function of pipe ID, grain size, and liquid and solid density and viscosity), the grains start to move, initially as dunes, at higher velocities as a continuous sand bed. SPEPF P. 237^

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