During primary cementing of coalbed methane (CBM) wells, it is necessary to consider slurry designs not typically encountered during conventional cementing operations. An important difference between coal seams and conventional reservoirs is the cleat system of coal. This unique petrophysical property of coal should be factored into the design to meet the basic tenets of primary cementing (i.e., zonal isolation and casing support). This paper presents cement design considerations, case histories, and best practices developed during the course of seven years of cementing operations in CBM wells in India. It also presents the cement bond evaluations that verify the conclusions. Formation damage, lost circulation, low-fracture gradients, shallow gas, and coal seam gas are the most frequent challenges encountered during cementation of CBM wells. Cement and cement filtrate loss can plug the cleat matrix and cause reduced well productivity, increased injection pressures, and ineffective stimulation operations. These challenges are exacerbated by the necessity for long cement columns that cover multiple coal seams. Cement operations must also provide excellent annular displacement efficiency to achieve the necessary annular fill and provide zonal isolation during the life of the well. Adequate compressive-strength development can be difficult to achieve in low-temperature CBM environments. Selection of cement and additives necessary for slurry design are governed by the ability to meet the operator's objectives at temperatures ranging from 45 to 80°C. The thixotropic properties engineered into the cement slurry help enable rapid gel-strength development once the slurry column is placed. This helps remediate lost circulation, reduce cement contamination into the coal cleat system, and reduce fallback. Best operational practices for preparing the wellbore for effective cementing, such as optimum flow rate, hole conditioning, and centralization, help ensure complete isolation of coal intervals with cost synergies achieved through efficient deliverance preparedness. A three-dimensional (3D) displacement simulator models the intermixing of wellbore fluids and corresponding changes in rheology. This simulator, which contains a built-in, free-fall algorithm, helps provide a more accurate estimation of fluid movement/flow patterns. It also simulates intermixing of fluids, which helps better predict equivalent circulating densities (ECDs) and frictional pressures. The 3D displacement simulation results and their agreement with cement bond log (CBL) evaluations help verify the effectiveness of controlling critical operational parameters and their effect on cement displacement efficiency. The combination of high-strength, low-density (HSLD) cement slurry, efficient field-blending procedures, and operational considerations helped enable successful cement operations in 200 CBM wells in the Sohagpur-West block, Madhya Pradesh. The unique advantages of the HSLD cement slurries include Reduced density, which helps prevent formation damage and lost circulationHigh compressive strengthGas-tight properties that help prevent annular gas migrationEliminating the need for stage cementing

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.

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Fluid systems used for servicing wellbores are usually a combination of particulate materials of varying specific gravity, particle size, aspect ratio, and reactivity, such as lightweight materials/weighting agents, clays, fibers, elastomers, polymers, resins, salts, and cementitious materials in water or oil media. These fluids are more commonly referred to as -complex fluids‖ and often exhibit a high degree of non-Newtonian and time-dependent behavior. To more efficiently and expeditiously perform well operations, it is beneficial to accurately probe the rheology of fluids (and their admixtures) under downhole conditions.A novel, helical-shaped stator-rotor assembly was designed and developed to work around measurement errors arising from sample inhomogeneity, particle separation, wall slip, and coring-related issues with commonly used geometries, such as those of a bob/sleeve and vane. The rotor blade arrangement is a double helix with cut flights, whereas the stator unit has blades that are manufactured by parting a coaxial double helix offset to the envelope of the rotor. Constant relative separation between the stator blades and rotor vanes is maintained in all planes to create shear geometries that enhance in-situ mixing. This was leveraged to conducting compatibility testing.Torque and rev/min data was collected for eight different Newtonian fluids with viscosities ranging from 10 to 1000 cp. The power number and impeller Reynolds number were plotted to derive functional relationships between these quantities in the laminar and turbulent regimes. Various complex fluids, including fracturing gels, viscoelastic fluids, oil, water-based muds, spacers, and cement slurries were tested on the helical mixer, a triangular impeller, and Couette geometries for comparative mathematical modeling.A unified algorithm and data analysis protocol featuring the four-parameter generalized Herschel Bulkley model is presented to derive rheograms and yield stress. A comparison of experimental results with computational fluid dynamics (CFD) simulations is also presented.