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Summary Plug cementing is still considered to be a critical operation, and cases of failure eventually happen. A large annular gap and eccentricity, typical of these operations, are factors that may promote unstable flows, resulting in cement-slurry contamination. Deepwater conditions enhance chances of free fall, and, consequently, low displacement velocities can occur in the annulus. This article presents a parametric study of the role of rheological properties of fluids (drilling fluid, spacers, and cements slurries), string rotation, and flow rates (including free-fall effects) in the displacement quality of cement plugs. Analyses are based on two different simulation tools. Conventional cement-pumping software defines flow-rate profiles at the annulus entrance, accounting for free-fall effects, and computational fluid dynamics (CFD) simulates the interface propagation and contamination levels. The main issues addressed by the simulations are What are the maximum yield stresses that guarantee nonstagnation regions while circulating the drilling fluid?How can one optimize density and rheology hierarchy, which minimizes contamination and avoids channeling?What is the role of string rotation on the displacement efficiency? The compilation of simulation results into useful guidelines and procedures for displacing cement plugs in vertical, inclined, and horizontal offshore wells is presented.

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Summary Displacing fluids in downhole conditions and for long distances is a complex task, affecting several steps of well construction. Cementing gains relevance the moment that fluid contamination compromises cement-sheath integrity and consequently zonal isolation. Density and rheology design for all the fluids involved is essential to achieve operational success. Properties hierarchy and preferred flow regimes have been empirically defined and tend to provide reasonable generic results. Challenging operations, including ultradeep waters and their narrow operational-window scenario, require further knowledge of the physics involved to prevent undesirable events. This paper presents the in-house development of software for annular miscible fluid displacement that analyzes fluid displacement in typical vertical and directional offshore wells, for Newtonian and non-Newtonian liquids and laminar- and turbulent-flow regimes. The formulation proposed provides accurate results for a wide range of input parameters, including the cases in which the ratio of the inner radious to the outer radius of the annulus is small. The computational work is validated by unique results obtained from an experimental test rig where detailed displacement tests were conducted. Contamination degrees were measured after the displacement of a sequence of fluids through 1192 m of vertical well. Effect of fluid-density and rheology hierarchy, flow regimes, and displacement concepts was investigated. The results provide relevant information for the industry and fundamental understanding on displacement of Newtonian and non-Newtonian liquids through annular sections.

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This article details an extensive experimental study aiming the evaluation of friction losses resulting from the flow of 4 different drilling fluids in use in deepwater operations through pipe and annular sections, besides accessories such as tool joints, bit jets and stabilizers. After a data analysis process, it was possible to compile a set of equations for prediction of relevant hydraulic calculations, such as: hydraulic diameter for annular flows, friction factors for turbulent pipe and annular flows and discharge coeeficients for accessories. Introduction Drilling hydraulics consists on a fundamental step on well design and, in several conditions, may limit the feasibility of the construction process. Dynamic pressures should be maintained inside the operational window defined considering pore, collapse and fracture pressures, guaranteeing that no influxes, losses or rock instability issues occur during drilling. Besides, minimum flow rates are required to assure that an adequate drilled cuttings transport occurs. Downhole pressures are generated from two different origins: hydrostatic forces and friction losses. Hydrostatics depends on fluid density and vertical depth while friction losses depend on fluid density and rheology, flow rate, flow geometry and flow path. The following equations describe the correlation between the design variables and the downhole pressures.

In order to avoid mistakes and to save a great deal of time in analysis, an innovative methodology was developed that can analyze the well operations and rig characteristics involved to define the best emergency disconnect sequence (EDS) available. A solution was developed based on the characteristics of the rigs and blowout preventers (BOPs), and six variables were considered that directly affect the choice of EDS. All possible combinations of 64 scenarios were analyzed, and the priority of choice of the EDS was defined empirically. This paper presents an approach to EDS risk management and examples of exposure time (time without riser safety margin and shear capability) for the same well, which can be lowered from 13% to 0.1%. The impact of this reduction is related to the ability of the BOP to cut some of the heavy casings, in addition to improved availability of EDS modes. This implementation opened up many possibilities for the performance of risk exposure analysis, enabling comparison of several BOP configurations of contracted rigs and selection of the best options. This innovative approach allowed a better management of the rig schedules, prioritizing safety aspects and making it possible to allocate the fleet in a systematic way.

Plug cementing is still considered to be a critical operation and cases of failure eventually happen. Large annular gap and eccentricity, typical of these operations, are factors which may promote unstable flows resulting in cement slurry contamination. Deepwater conditions enhance chances of free fall, and consequently, low displacement velocities can occur in the annulus. This article presents a parametric study in the role of rheological properties of fluids (drilling fluid, spacers and cements slurries), string rotation, flow rates (including freefall effects) in the displacement quality of cement plugs. Analyses are based on 2 different simulation tools. Conventional cement pumping softwares define flow rate profiles at annulus entrance, accounting for free fall effects and CFD to simulate the interface propagation and contamination levels. The main issues addressed by the simulations are: What are the maximum yield stresses which guarantee that non stagnation regions while circulating the drilling fluid? How to optimize density and rheology hierarchy which minimizes contamination and avoids channeling? What is the role of string rotation on the displacement efficiency? The compilation of simulation results in useful guidelines and procedures for displacing cement plugs in vertical, inclined and horizontal offshore wells.