Research On The Calculation Model Of Displacement Interface Boundary And Shape During Cementing In Eccentric Annulus

During cementing, the displacement interface boundary describes the retention positionof annular drilling fluid and the displacement interface shape reflects the intermixing extent oftwo-phase fluids caused by late-flow. There is no doubt that the retention and late-flow ofdrilling fluid can result in the decrease of annular isolation ability. Therefore, decreasing theretention and late-flow extent of drilling fluid by optimizing the displacing parameters on thebasis of analyzing displacement interaface boundary and shape is the premise to obtain goodcementing quality. Unfortunately, the present cementing theory can't involve the study ofdisplacement interface position on the annular walls. Moreover, it can't take the effect ofdriving force generated by density difference which changes with azimuthal angle and annularradius on the dispalcement interface shape into consideration. In order to solve the problem,this paper conducts a research on the calculation model of displacement interface boundaryand shape during cementing in eccentric annulus, which mainly includes that:(1) Herschel-Bulkely model is chosen to describe the rheological behavior of annularslurry and drilling fluid. Then the flow characteristics of Herschel-Bulkely fluid in theeccentric annulus is analyzed. Finally, the overall range of drilling fluid retention is obtainedand the distribution model for fluid flow rate in different azimuthal angle and annular radius isestablished.(2) A model for calculating the boundary position of drilling fluid retention layer isestablished with the method of mechanism analysis on displacement interface element, whichreveals the mechanism of drilling fluid retention during slurry displacing in eccentric annulus.On the basis of the model, the distribution in different azimuthal angle of drilling fluidretention on casing and well walls is obtained. In order to insure the drilling fluid to beadequately displaced, the non-retention model is established, which provides a theoreticalbasis for the adjustment of drilling fluid and slurry rheological parameters and displacingparameters that can achieve non-retention with no doubt.(3) By taking the change of driving force generated by the height difference betweentwo-phase fluids along annular azimuthal angle and radius, the calculation model includingdiffusion effect for displacing fluid concentration and the description model excluding diffusion effect for displacement interface shape are established respectively. Obviously, thetwo models indicates the flow behavior of displacing fluid in the azimuthal and axialdirections, therefore, which can the overall shape of displacement interface in the annulus atdifferent time.(4) The formation mechanism of steady displacement interface under the effect ofazimuthal flow is analyzed. Moreover, the description models for steady displacementinterface in concentric and eccentric annulus are established respectively. The two models canillustrate the change rule of steady displacement interface shape in the vertical and deviatedwells. Finally, the matching relationship between casing eccentricity, deviation angle anddensity difference is obtained. Besides, it is verified by the numerical simulation results.The retention mechanism and late-flow law of annular drilling fluid are revealed bystudying on the displacement interface boundary and shape during cementing in eccentricannulus. As a result, it improves the two-dimensional description model for displacementinterface shape and enriches the cementing theory to some extent. Furthermore, the matchingrelationship between casing eccentricity, derivation angle and density difference is obtained inthis paper, which provides a theory basis for the optimization design of displacing parameterswith no doubt. Obviously, the improvement of cementing quality is beneficial for prolongingthe oil and gas wells life-span and enhancing the implementation effect of zonal fracturing.