Laboratory testing of drilling/drill-in/completion fluids for evaluating formation damage potential requires return permeability tests. It is shown that the apparent shear yield stress of the filter cakes developed by these fluids at the wellbore interface determines the flow initation pressure and the near wellbore return permeabilities during onset of production.
Traditional bentonite clay based and recent non-damaging sized calcium carbonate polymer fluids were chosen to represent the wide variety of water-based drilling and completion fluids used in the field. A standard API filter cell was used with an over-balance of 100 psi and a filtration time of 16 hours for preparing the filter cakes. A constant strain rheometer was used to measure the filter cakes' apparent shear yield stresses. Linear strain and dynamic strain sweep tests were successfully conducted to get the estimates of the apparent shear yield stress of the prepared filter cakes.
Filter cakes prepared with bentonite muds showed an apparent shear yield stress of 800 Pascals (0.12 psi) while filter cakes of typical sized calcium carbonate drill-in fluids showed smaller apparent shear yield stress values between 250 and 450 Pascals (0.06 psi) varying with the median diameter of the calcium carbonate particles. The higher apparent shear yield stress of bentonite filter cakes was attributed to the smaller size of bentonite clay particles (< 2 microns) compared to the drill-in fluid sized calcium carbonate particles (> 2 microns). Two models were developed that used the apparent shear yield stress of these filter cakes and predicted the flow initiation pressures (FIP) and the return permeabilities. The models matched reasonably well with the experimentally measured FIP and return permeability ratios.
A novel approach for measuring and utilizing the apparent shear yield stress of filter cakes is presented that predicts FIP's and return permeability ratios during the onset of production. A new protocol is suggested that measures the filter cakes' apparent shear yield stresses to model the FIP and the return permeabilites for a given drilling and completion fluid.