In SAGD wells, the gap between the oil sand and the sand control liner closes or collapses over time due to such factors as the oil sand thermal expansion, the melting of bitumen and the ensuing loss of the apparent bonding between the grains. The result is the buildup of effective stresses and the gradual compaction of the oil sands around the liner. Current practices for the sand control design do not account for the effect of time-dependent effective stress variation around the liner on the sand control performance. In this paper, we outline the results of an experimental study on the effect of near-liner effective stress on the performance of slotted liners. This study builds on existing experimental procedures and investigates fines migration, sand production and clogging tendency of slotted liner coupons in large-scale unconsolidated sand-packs. Sand-packs with controlled properties (grain size distribution, grain shape, and mineralogy) are packed on a multi-slot sand control coupon in a triaxial cell assembly. Varying levels of stress are applied to the sand-packs in directions parallel and perpendicular to the multi-slot coupon. For each stress level, brine is injected into the sand-pack from the top surface of the sample towards the coupon. Test measurements include pressure drops across the sand-pack and the coupon as well as the produced sand/fines mass for each stress level. Post-mortem analysis is performed to measure fines/clay concentration along the sand-pack as a direct measure of fines migration. Experimental results show that under the subsequent increase in effective stresses, sand-packs experience considerable deformations in directions parallel and perpendicular to the multi-slot coupon; which result in a drastic drop in the porosity and retained permeability. Test results show that the maximum reduction in permeability occurs in the vicinity of the multi-slot coupons due to the fines accumulation and the higher compaction in that region. In comparison to experiments with no confining stress, the application of confining stress results in lower retained permeability in the sand-packs as well as reduced sand production. This paper presents, for the first time, the effect of near wellbore effective stress on clogging tendency and sand retention characteristics of slotted liner completions. The tests allow the assessment of the adequacy of the use of existing design criteria over the life cycle of the well under variable stress conditions around the liner.
Sand production from a poorly consolidated reservoir could give rise to some severe problems during production. Holding the load bearing solids in place is the main goal of any sand control technique. The only sand control techniques that have found applications in steam assisted gravity drainage (SAGD) are some of the mechanical methods, including wire wrapped screens, slotted liners and more recently, punched screens. Slotted liner is one of the most effective mechanical sand control methods in the unconsolidated reservoir exploitation, which has proven to be the preferred sand control method in the SAGD operations. The main advantage of the slotted liners that makes them suitable for SAGD operations is their superior mechanical integrity for the completion of long horizontal wells. This study is an attempt to increase the existing understanding of the fines migration, sand production, and plugging tendency for slotted liners by using a novel large-scale scaled completion test (SCT) facility. A triaxial cell assembly was used to load sand-packs with specified and controlled grain size distribution, shape and mineralogy, on multi-slot sand control coupons. Different stress levels were applied parallel and perpendicular to different combinations of slot width and density in multi-slot coupons, while brine was injected from the top of the sand-pack towards the coupon. At each stress level, the mass of produced sand was measured, and the pressure drops along the sand-pack and coupon were recorded. Fines migration was also investigated by measuring fines/clay concentration along the sand-pack. The current study employed multi-slot coupons to investigate flow interactions among slots and its effect on the flow performance of liner under typically encountered stresses in SAGD wells. According to the experimental observations, increasing slot width generally reduces the possibility of pore plugging caused by fines migration. However, there is a limit for slot aperture beyond which the plugging is not reduced any further, and only a higher level of sanding occurs. Test measurements also indicated that besides the slot width, the slot density also influences the level of plugging and sand production and must be included in the design criteria.
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