A new generation Wireline Formation Tester (WFT) was used to perform fluid sampling in a subject field as part of an appraisal drilling campaign targeting significant undeveloped conventional oil reserves. The field consists of free gas caps with associated saturated oil intervals that appear as thin rims around anticlinal flanks. Resistivity log analysis predicated a mud filtrate invasion volume of 89L. Approximately 150% of the expected invasion volume (128L) was pumped during sampling. Laboratory results showed that the sampled fluid was still mud filtrate, despite pumping an extra 50% of the estimated volume required for hydrocarbon breakthrough.
This paper presents a mathematical model for predicting filtrate invasion and post drilling WFT cleanup performance. A new methodology is developed to incorporate critical shear stress concepts into mud filter-cake thickness and filtrate invasion calculations. The mathematical model is used to provide input data for Eclipse reservoir simulator (E100) and an analysis of the field case. The model can also be applied as a predictive tool for filter-cake development, filtrate invasion and pump-out volumes required during fluid sampling. In addition, the model can be used to design optimal mud properties that minimize filtrate invasion and extra treatment of the invaded zone post-drilling. The mathematical model is integrated with reservoir simulation to model the cleanup process.
The mathematical model developed in this study predicted an invasion volume of 528L, which is significantly greater than what was estimated based on resistivity logs (89L). This implies that filtrate invasion volume in the field case was underestimated. A sensitivity study showed that filter-cake permeability was the greatest determinant of filtrate invasion volume, and that radius of filtrate invasion was the greatest control on mud filtrate cleanup during sampling. Reservoir simulation provided a good match between model simulation results and field data.