Effect of Near Wellbore Effective Stress on the Performance of Slotted Liner Completions in SAGD Operations

摘要

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.