Numerical Simulation of Improving Cementing Displacement Efficiencyunder Narrow Safety Density Window

摘要

As for the thick salt-gypsum formation and the target strata in the Kuqa piedmont zone of Tarim basinin western China,The biggest challenge in well cementing is the narrow safety density window (only0 g/cm~3-0.1 g/cm~3 or 0ppg-0.833ppg)and the unsatisfactory leakage blocking technology under pressure(the improvement of formation pressure-bearing capacity is less than 0.1 g/cm~3 or 0.833ppg).The limitedflow rates (0.6m/s-1m/s lost still)during cementing operation are distinctly insufficient for displacementefficiency.In response to this challenge,a numerical simulation study has been carried out using thecomputational fluid dynamics (CFD)software ANSYA Fluent to explore practical guidelines in theseformations.The models we build are annulus with a 176.7mm (6 5/8”bit expanding 5%)outer diameter for thehole and a 139.7mm(5 1/2”)inner diameter for the casing,the length is 11 meters or 22 meters,and theeccentricities are 0.3,which means a sufficient centralizer according to our practical experience.The 11-meter and 22-meter models have one million and two million grids,respectively.In the thin boundary layerarea four thin boundary layers were constructed.The species model and the viscous laminar model havebeen applied,and the velocity inlet and the outflow outlet have been set as the boundary conditions.Both themud,the spacer and the cement slurry are non-Newtonian Herschel-Buckley(H-B)fluids,whose rheologicalproperties are determined by the flow index n,consistency coefficient K and yield point YP.By analyzingthe interface characteristics of the mixing slurry,distribution of mud mass fraction and wall shear stress atdifferent inlet velocity and displacing time during the displacement process,the effects on the displacementefficiency for each individual factor,such as casing centralizer,flow rate,and rheological properties,havebeen analyzed and summarized.According to these simulations,a series of guidelines have been drawn.A sufficient casing centralizationis the most important approach to improve the displacement efficiency.Flow rate and wall shear stress alsoplay an important role,but they are limited by the safety density windows of the formations.Under thecircumstances,the most practical way to improve the displacement efficiency is to optimize the rheologicalparameters of the mud-spacer-cement slurry (for instance,to decrease the consistency coefficient K and yield point YP of the spacer)to achieve a larger friction gradient difference and get a better displacementefficiency eventually.These guidelines,which have been applied in the X-well of Kuqa piedmont,have shown a remarkableachievement in cement sheath evaluation across the primary zone of interest (qualification rate improvedfrom an average 49% to 80.3%),and therefore are practical in the formations with narrow safety densitywindow.