Slim-well construction helps to significantly reduce overall well construction costs. The first step of the successful completion process is achieving effective zonal isolation. Slimhole configurations experience higher shear rates at standard annular pump rates as compared to conventional configurations. The increase in shear rate could exacerbate intermixing of the fluids, diffusion, and fingering in the annulus, thereby leading to incomplete mud and filter-cake removal and poor cement placement. Real-world consequences of these phenomena can include interzonal communication, loss of production, remedial squeeze work, and even well abandonment. Whether a conventional or slim hole well, designing an effective zonal isolation job involves balancing two technical challenges: (1) providing adequate hole cleaning through effective erodibility shear stresses; and (2) reducing channeling of mud or spacer by minimizing intermixing at fluid interfaces, controlling diffusion and minimizing fingering in the annulus. This paper deals with the underlying interactions between fluids and the challenge of controlling the aspects affecting intermixing Intermixing lengths can commonly be as high as 500 feet for slimhole cementing jobs owing to the large difference between central and peripheral velocities of the annular fluids. In an attempt to minimize this problem, a three-dimensional (3D) numerical fluid-flow simulator was used to compare flow profiles in a low clearance 143/4-in. × 133/8-in. annulus to those in a conventional 16-in. × 133/8-in. annulus. Rheologies of the annular fluids were accurately modeled using the Herschel-Bulkley scheme. Using the same fluids and pump rates, numerical simulations in the slimhole configuration clearly showed worse displacement as compared to the standard configuration. The effects of the three rheological parameters viz., τ0, µ∞, and n of the fluids, pump rate and eccentricity on dynamic velocity profiles, displacement efficiency, intermixing lengths, and top of fluids were then studied to improve drilling-mud removal, and cement slurry placement. Results of various simulations are shared to reveal the properties and parameters that are needed to help achieve a competent cement sheath over the desired interval.
The current approach to designing a lost circulation treatment for drilling or cementing applications is primarily based on workflows designed by project teams using experience and knowledge from historical wells. However, the effectiveness of this approach depends on targeting operations with well conditions similar to historical jobs and the collective experience of the project team. Consequently, there is less flexibility to apply these workflows globally or to a job with different fluids placement strategies and/or different fluid treatments. In addition, dynamics of the wellbore during losses are influenced by multiple factors such as wellbore geometry, downhole fluid properties, free-fall (during cementing), details of the unknown loss zone geometry, as well as the features of the loss circulation material (LCM) (i.e. material, particle size distribution, density etc.). Thus, there is a need to create a robust and effective workflow based on engineering models that provide a platform to systematically design cement jobs to mitigate losses. Similar workflows may be adapted for controlling losses during drilling. This work presents a complete coupling of a wellbore hydraulics model with loss zone dynamics including LCM particle bridging and packing at the loss zone. The loss zone dynamics use circulating pressures from the hydraulic model, estimated geometry of the loss zone and characteristics of LCM laden fluids to determine the loss rate and the filter cake buildup. As fluid systems with different densities, rheology and LCM packages enter the loss zone during circulating and/or cementing, the extent of losses is governed by solving for a coupled problem of flow through (i) the wellbore and (ii) the loss zone with filter cake buildup. The first step in the analysis is to characterize the loss zone by calibrating the hydraulics model to match circulation pressures and loss rates observed prior to the addition of any LCM material in the wellbore fluids such as pre-job circulation before cementing. The integrated model described in this study was successfully verified and validated based on multiple field cases. These cases covered three types of loss zones – induced, natural fractures, and permeable zones. Pre-job circulation loss rates ranged from 30 to 240 bbl/hr. LCM designs include single and composite systems with multi-modal particle size distributions. LCMs were loaded in a wide range of fluid systems involving, but not limited to, spacer, lead and tail cement slurries. In all cases, the predictions from the model matched operational loss control within an accuracy of 10%. Furthermore, post job evaluations indicated that real time pumping pressures aligned with model predictions, indicating that the hydraulic response of the model matched that of the wellbore. The evidence gathered during the verification process proves the ability of the tool to tailor and design cement jobs to mitigate losses. Additionally, due to the integrated framework, the novel tool allows for tailoring of multiple operationally relevant variables such as flow rate, density, volume, rheology, and fluid recipe including LCM type and concentration across different loss zones. The framework also provides an estimated top of cement (TOC) which is one of the most important cement job objectives.
In the cementing industry, it is known that zonal isolation is dependent on a successful cement job. To achieve zonal isolation, effective mud removal is recommended (Crook et al. 2001). Efficient mud displacement can be achieved with the use of spacers and flushes before cement is placed. With an increasing need to successfully complete a well on the first try, accurately modeling the spacer rheology has become key. This paper presents results from a comparison of two rheological models. The Bingham plastic and Generalized Herschel Bulkley (GHB) models examined in this work are both used to model rheologies of spacers. For most field operations, the Bingham plastic model is used to model rheologies; however, studies have shown that using this model can lead to over-predicting yield point. The GHB model is used to model non-Newtonian fluids by incorporating the possibility that the fluid might or might not have a yield stress and that the shear stress might be a nonlinear function of shear rate. Spacers are used to separate one fluid from another. They are designed to be compatible with the mud being used in the well. This paper compares the performance of spacer and spacer/mud rheologies at different elevated temperatures (200, 300, and 350°F) with measurements from a Model 75 HP/HT viscometer. Results have been modeled in a computational fluid-dynamics simulator to illustrate the effects of each rheological model used to describe the well-completions fluids. The simulator modeled displacement and intermixing of wellbore fluids downhole, and these are investigated in the annulus for the simulated case.
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