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
Steam injection is a fundamental method for enhanced oil recovery (EOR) wherein the wells are drilled and steam is injected to heat the crude oil in the formation to help reduce viscosity and improve oil recovery. The best practices for zonal isolation in the Baghewala field in India are discussed in detail. For desired zonal isolation, cement should maintain long-term stability once placed. Pressure and temperature changes can cause failure within the set cement. An effective annular seal designed in these wells will be exposed to high temperature, cyclic stresses, and potentially corrosive environments caused by the injection of high-temperature steam into the wellbore. The design stage should go beyond cement placement, compressive strength, and gas-migration prevention. The solution is rooted in the synergy between diagnostic tools and engineered cement systems. Such design methodology was developed to address the challenge of the loss of zonal isolation caused by changes in the wellbore that can stress the cement sheath and cause destabilization at any point during the life of the well. Specific to the cyclic steam injection in the Baghewala field, a cement system was designed to address the following challenges: Achieving cement returns to surface without fallbackHelping ensure the cement system is thermally stable at the maximum expected temperature (i.e., no strength retrogression, radial cracking, debonding, etc.)Achieving a good cement bond throughout the open and cased holesHelping prevent wellhead growth during steam injectionEncountering different types of formations (halite, anhydrite, sand, and carbonate) in this fractured reservoirUsing a cement slurry designed to avoid strength retrogression of set cement during exposure to high-temperature injection Proprietary 2D hydraulics, finite element analysis (FEA), and 3D displacement simulators are analytical tools designed for simulations or investigative modeling to simulate fluid properties and slurry placement during operations. Additionally, they help reduce the need for costly remediation and positively affect the long-term cement sheath integrity by assessing and addressing issues before they become problematic. More than 10 wells were successfully constructed with no issues reported during/after cyclic steam injections, which is endorsed by excellent cement bond logs. This helped minimize multiple potential risks and ultimately maximize production. The cement design technology associated with FEA simulations and analysis provides the ability of the set cement to expand and contract in sync with the well's fluctuating temperature, reducing stress on the cement sheath. This occurs because these cement systems have approximately the same thermal expansion properties as the steel casing. Unlike conventional cement, particles can expand and contract thermally, thus reducing cement-sheath stress six times more than conventional systems.
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