Theoretical Study On Carrying And Flowback Capacity Of Workover Fluid Contaminated By Calcium Ions

Presently, in process of well workover, the carrying capacity of operating fluid is one of the main factors that will determine the results of casing extraction; nevertheless, the fluid calcium contamination directly brings down the carrying capacity. The major cause is that the suspension system of workover fluid is contaminated by calcium ions released from well cementing section, when wall rocks and cement sheathes are broken by milling bit. The fluid initial performance parameters are changed, those some of which are increased, just as the viscosity, the density, the shearing force etc, will in the end lead to fluidity loss. Therefore, it is important to make a project on studying the mechanism of workover fluid calcium contamination, as well as the fluid carrying capacity.From the experiment results, the variation trends of rheological property and filtration property are tested from time to time, and the mechanism of calcium contamination is studied. The viscosity number and the shear value change obviously among all variable parameters, which also affect fluid carrying capacity greatly, meanwhile, the viscosity is prominent in parameters calculation. Accordingly, in this thesis, a new parameter describing the calcium destructiveness is put forward which will be used to modify viscosity value. All the variation of performance parameters are followed-up tested in field trial, recording the fluid condition contaminated by the calcium ions. From calculating and analyzing of calcium contamination fluid annulus flow, the Power Low fluid model is built to express the operation fluid reheology model. The effective viscosity of workover fluid is computed and the generalized Reynolds number is determined. And then the annulus flow pattern is recognized as laminar flow. The annulus friction loss is researched, and the pressure gradient is built. The rock debris motion equation is built with mechanical analyzing. The debris sedimentation rate is solved. The factors affecting debris annulus moving is analyzed. Based on borehole cleaning norm, the critical annulus velocity model is established. The numerical calculation has been done on field casing extraction operation parameters. The variation trend of the new parameter is determined, from which the carrying capacity of workover calcium contaminating fluid is rationally evaluated. It supplies a gap with new method in calculating annulus velocity and estimating carrying capacity.