Optimization of Downhole Fluid Sampling Using Focused Probes

Acquisition of fluid samples using wireline formation testers is an integral part of reservoir evaluation and fluid characterization. The use of focused probe technology, in which the flow of mud filtrate to the main sampling port is shielded by one or more guard ports, has proved to be particularly effective in acquiring clean formation fluid samples with short pump-out times. To both improve operational efficiency for the current focused probes and optimize the design of next-generation tools, it is necessary to understand how to optimally operate the tools in different sampling environments. The objective of this study is to investigate optimal sampling strategies for focused tools in presence of formation and fluid property uncertainty. In particular, the pump rates for the sample and guard ports in a focused tool can be manipulated, allowing for optimization of pump rate profiles to maximize overall sampling efficiency. We use a numerical forward model of the filtrate cleanup process coupled with optimization. We study the problem of pump rate profile optimization in different sampling environments and compare the results against two operating strategies commonly applied in the field. The optimization results show that significant sampling time savings are possible (for each sampling station) compared with a default fixed-rate strategy, especially in environments characterized by a high viscosity contrast between the formation fluid and mud filtrate. These savings translate directly into rig time savings for the operator. In general, the results in the paper provide guidance on optimal focused-sampling operation in different environments.