Abstract
Drilling fluids left static in the well during its operational life tend to form sediments. When the time comes for well Plugging and Abandonment (P&A), these sediments may hinder casing removal, making the operation more time consuming and expensive. But the sediments may also be an advantage for P&A, as dense and impermeable sediments may be used as a barrier material. A better understanding of the sediment formation would help improving the efficiency of P&A operations. We have investigated this issue by observing experimentally columns of oil-based drilling fluid at rest for 500 days. A particle-free dark layer forms at the top and grows until it reaches a final height between 300 and 500 days. We note that the severity and velocity of the dark layer growth are promoted by a larger height of the fluid column and depend on the drilling fluid composition (oil-brine ratio and barite content). We study the composition of the sediment, i.e., the lower section of the column containing particles, at different points and note that the droplet-particle ratio remains constant in the sediment. On the other hand, the oil content is largest at the top of the sediment. We show that these results cannot be explained by classical sedimentation models but show that the particle migration can be modelled as a saturated porous medium under compaction.