Abstract
Results from sand retention tests (SRT) performed with different screens and test sands can show significant differences in perceived production performance given by the permeability of the retained sand layer as well as sand control behavior. However, this paper presents a new sand retention test set up that exhibits no plugging tendencies for various screen designs when using non-uniform test sands with high content of fines material (Uc=7.5, fines = 18%).13 screen designs, 12 metal mesh and one wire wrapped screen, with screen gauges varying from 200 to 275 microns were tested. The results show insignificant differences when it comes to sand control as well as the permeability of the sand layer calculated from the pressure drop across the screen and the sand layer retained by the screen.
This work supports critical reviews made by other authors of the various laboratory testing procedures used in the industry and confirms that interpretation of screen performance is often incorrect with these set-ups. The new test set up was developed to enable screen testing under more realistic conditions. Emphasis was placed on low velocities to avoid turbulent, non Darcy effects as well as avoiding the use of fluids containing polymers. High flow rates tend to cause rapid and efficient localization of the sand grains around the sand screen filter openings prior to the establishment of the sand layer. A set up where the total pressure drop is dominated by the turbulent part can readily lead to inaccurate determination of the true permeability and consequently erroneous comparison of screen behavior. With the low approach velocities used in this setup, sand layers of measureable thickness can develop before the pressure rating of the cell is reached. The sand layer permeability is calculated by using Darcy ’ law based on the thickness, flow rate, fluid viscosity and pressure drop. The measured volume of sand produced through the screens in relation to the total volume pumped was used to define their sand control behavior.
The flow rate used in this study was 160 ml/min which gives an approach velocity of 0.00053 m/s. Although this is sufficiently low to ensure the absence of non Darcy effects in the resulting sand packs around the screens, it still equates to relatively high rates in the field. For instance, this test rate represents a production rate of 36000 Sm3/d through a 2000 m long 6 5/8" screen section which is a normal length on some fields in the North Sea. In the storage cell where the test sand slurry is kept prior to injection into the sand screen test cell, the sand is suspended in brine with a matching density. In this way, a viscous carrier fluid is not required, thus eliminating any possible test artifacts based on undesired polymer effects.
The results show that screen plugging with porous test sands is not an issue for any screens given that sensible apertures are chosen that give retention whilst allowing the production of any mobile fines present. The test results confirm that other SRT set-ups with inherently high approach velocities will often misinterpret the pressure response as plugging and thereby rank the various screens incorrectly.
Based on these results, and providing that the various screen designs give sand control, the choice of optimum sand screen for a given application should be based more on mechanical properties, cost and contractual issues, other functionality (such as ICD or shroud) rather than "plugging" potential that is erroneously ascribed to different screens in standard SRTs. Screen plugging from other factors during installation and service in the field is, however, possible and should be minimized by good design and operational practice.
