Pressure propagation and flow restart in a pipeline filled with a gas pocket separated rheomalaxis elasto-viscoplastic waxy gel

Abstract

A clogged waxy crude oil pipeline, entails a high pressure to de-structure the gelled oil and resume its normal operation. Our earlier work, which utilized generalized Newtonian fluid based model, demonstrated that the presence of a gas pocket in-between the gel plugs, delays pressure propagation in the downstream gel plug [L. Tikariha, L. Kumar, Pressure propagation and flow restart in the multi-plug gelled pipeline, Journal of Fluid Mechanics. 911 (2021)]. This delay in pressure propagation is effectively utilized for sequential gel degradation. In the present work, we consider gel as a rheomalaxis elasto-viscoplastic (REVP) fluid, capable in predicting flow stoppage and pressure signal stoppage. The governing equations for mass, momentum balance, and phase equation are solved together with the REVP rheology model. The propagation of pressure in a REVP gel-filled pipeline is directly related to the gel material's elastic response. For a REVP gel, the flow-restart in a gelled pipeline depends on the conditional statement γ>γs (i.e. mean gel deformation overcoming the yield strain). In the case of a single gel plug, no-pressure propagation is predicted at intermediate compressibility, whereas pressure propagation without flow restart is predicted at low compressibility. In these cases, the presence of a small gas pocket instigates the gel breakage process, resulting in a flow restart. The gas pocket delays the pressure propagation in the second gel plug, resulting in a pressure gradient higher than yield stress across the first gel plug. The first gel plug, which deforms under a high-pressure gradient, compresses the gas pocket. This results in a larger mean deformation in the first gel plug, and it overcomes the threshold yield strain limit (γs). At strain values larger than yield strain the gel structure breaks, resulting in a lower elastic strength in the first gel plug. The lower elastic gel strength ascertains a lower pressure gradient in the first gel plug, allowing a largely unattenuated pressure to propagate in the second plug. The propagation of unattenuated pressure in the second gel plug results in a pressure gradient higher than the yield stress. This causes sequential gel breakage, resulting in a successful flow restart in a pipeline longer than a critical length. The present study also demonstrates that the pressure propagation and initial transient flow depend on the gel properties (i.e. gel strength and compressibility) and the number and size of gas pockets in the gel. The insights gained from the present work provide a cost-effective solution to mitigate the flow restart problem.