This paper presents simultaneous prediction of pressure response and sample contamination profile in formation testers. This study is based on numerical simulation of snorkel (probe) flow aperture formation testers to capture hemi-ellipsoidal pressure diffusion and saturation changes around the snorkel and wellbore during formation testing.
For this purpose, formation tests with snorkel aperture were modelled using a three-dimensional (r-θ-z), single well, radial compositional numerical simulation model. The model incorporates dynamic and static mud filtrate invasion due to mud hydrostatic pressure; static mud cake; capillary pressure; filtrate-reservoir fluid miscibility and non radial symmetrical flow that characterises a snorkel flow around a wellbore.
The model was used to determine formation permeability/mobility using steady-state pretest and formation rate analysis (FRA) methods from simulated pressure response; reservoir radial and vertical permeabilities using simulated extended snorkel pump-out; and contamination prediction for water and oil based mud using simulated fluid sampling profile.
The study shows that a robust numerical simulation model that correctly captures the phenomenon that affects the dynamics of formation testers; predicts pressure diffusion and saturation changes around the formation tester in a wellbore during formation testing. This model is required for simultaneous prediction of pressure response and sample contamination in formation testers. This model can then be used for prediction of formation permeability; prediction of reservoir fluid sample contamination; computing geometric (shape) factors for formation testers; validating formation test interpretation results; and reservoir characterisation.
Sensitivity on numerical model grid size is important to determine the optimum grid size required for simultaneous prediction of pressure response and sample contamination in formation testers.
Currently in the industry, test design and simulation of snorkel-aperture formation testers are usually carried out using analytical pressure transient models and empirical sample contamination prediction models separately. Analytical pressure prediction models, often based on single phase or single phase equivalent, cannot be used for contamination prediction. This study has used numerical simulation to predict pressure transient and sample contamination profile, simultaneously.