As for the thick salt-gypsum formation and the target strata in the Kuqa piedmont zone of Tarim basin in western China, The biggest challenge in well cementing is the narrow safety density window (only 0 g/cm3-0.1 g/cm3 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/cm3 or 0.833ppg). The limited flow rates (0.6m/s-1m/s lost still) during cementing operation are distinctly insufficient for displacement efficiency. In response to this challenge, a numerical simulation study has been carried out using the computational fluid dynamics (CFD) software ANSYA Fluent to explore practical guidelines in these formations.
The models we build are annulus with a 176.7mm (6 5/8 " bit expanding 5%) outer diameter for the hole and a 139.7mm(5 1/2") inner diameter for the casing, the length is 11 meters or 22 meters, and the eccentricities 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 layer area four thin boundary layers were constructed. The species model and the viscous laminar model have been applied, and the velocity inlet and the outflow outlet have been set as the boundary conditions. Both the mud, the spacer and the cement slurry are non-Newtonian Herschel-Buckley(H-B) fluids, whose rheological properties are determined by the flow index n, consistency coefficient K and yield point YP. By analyzing the interface characteristics of the mixing slurry, distribution of mud mass fraction and wall shear stress at different inlet velocity and displacing time during the displacement process, the effects on the displacement efficiency for each individual factor, such as casing centralizer, flow rate, and rheological properties, have been analyzed and summarized.
According to these simulations, a series of guidelines have been drawn. A sufficient casing centralization is the most important approach to improve the displacement efficiency. Flow rate and wall shear stress also play an important role, but they are limited by the safety density windows of the formations. Under the circumstances, the most practical way to improve the displacement efficiency is to optimize the rheological parameters 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 displacement efficiency eventually.
These guidelines, which have been applied in the X-well of Kuqa piedmont, have shown a remarkable achievement in cement sheath evaluation across the primary zone of interest (qualification rate improved from an average 49% to 80.3%), and therefore are practical in the formations with narrow safety density window.