Foamed cement is often used to seal the annulus of many surface casing strings in deepwater offshore wells. The objective of this paper is to highlight the benefits a foamed fluid placement simulator brings to the design and evaluation processes for foam cement jobs.
A dynamic compressible hydraulic model that simulates the flow of nitrified fluids during primary cementing operations is presented. For a long time, the industry had relied on models that used average density properties for the foamed fluid. Yet the foam quality can change from 70% at surface conditions to 10% or less at bottomhole conditions. In terms of density this ranges from 600 kg/m3 (5 lb/gal) to 1800 kg/m3 (15 lb/gal). The average density approximation is therefore too coarse. In reality, there is continuous changing of fluid density and rheology during placement of the foamed fluids due to the changes of pressure and temperature along the flow path. Both density and rheology variations can be taken into account in a compressible hydraulic model that simulates the flow of nitrified fluids during primary cementing operations.
Measurements taken on actual foam cement jobs are compared to the simulator's output. The actual data and the simulated output compare favorably. Discrepancies between the model and acquired data are also analyzed to improve the understanding of the actual operation. The inherent limitations of post job analysis are discussed. The absence of caliper or known geometry on surface and conductor casings becomes more obvious. The placement of sensors on flow lines, the absence of sensors after the foam cross, and the uncertainty on geothermal gradient are also sources of divergence between the model and the measurements as will be seen through case study in this paper.
The continuous improvement of the model and the simulator will contribute to more robust foam cement job designs. Robust foam cement job designs will assure the isolation integrity of the structural casing strings for deepwater wells and ensure safe operations while executing this type of jobs.