Traditional cementing operations involve displacing fluids (spacers/flushes/slurries) down through the bore of the casing and out the shoe on the bottom of the casing string. From that point, the cement is lifted into the annular space between the outside of the casing and the wellbore (formation wall) to the desired level referred to as the top of cement. The goal is for the cement to completely fill this space and achieve a hydraulic seal for successful zonal isolation. Conversely, in reverse circulation cementing operations, cementing fluids are placed down through the annulus and into the shoe at the bottom of the casing. Lower hydraulic horsepower is needed for reverse circulation cementing and the operation requires less rig time than with traditional cementing operations.
Reverse circulation cementing operations rely on gravity forces and density differences to aid in the fluid-flow process. Backpressure may be required to control the slurry free fall and placement time is shortened. Because only the lead portion of the cement is exposed to bottomhole temperatures, the thickening time can be customized and the amount of retarder can be staged and/or reduced resulting in a relatively faster set time. This helps control fluid migration and plastic formation movement, reducing the potential for production zone invasion of the slurry during placement.
Reverse circulation cementing is a viable option for the following situations:
Cementing past weak formation that can tolerate only very low equivalent circulating densities.
When up-hole porosity or water table is present requiring rapid compressive strength development.
When the probability of seepage loss from the cement system could create blockage in the producing zone.
Placing large slurry volumes with long displacement times and wide temperature differentials
Temperature restrictions such as deep high-temperature zones or shallow low-temperature zones.
Standard float equipment is not used for reverse circulation cementing operations and additional well conditioning as well as larger spacer and shoe track volumes may be required. The surface operations require different iron configuration as compared to conventional cementing. Specific tools and/or methodologies should be implemented for estimating the top of cement, including use of an indicator to determine when cement is entering the casing or pipe. The cement within the casing may need to be drilled out.
Experienced personnel and careful planning is imperative for successful reverse circulation cementing operations. Halliburton’s iCem® service is the only simulator in the industry that can run reverse circulation cementing models to assess specific variables and help answer questions during the planning stage of the operation.