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 circulation
High compressive strength
Gas-tight properties that help prevent annular gas migration
Eliminating the need for stage cementing