An Innovative Approach for Formation Fluid Typing with API and GOR Assessments in Real Time from Mud Gas Data

Repsol Sinopec Brasil has refined the operational methodology to quantitatively determine methane to pentane composition from mud gas, establishing good matches with the laboratory PVT gas data.

Deriving hydrocarbon reservoir fluid properties, from mud-gas data while drilling, is an attractive perspective. The proposed methodology includes an in-house mathematical model with:

• Two parameters (alpha and beta) to describe the composition of the hydrocarbon fluids, except those that are altered by secondary processes.
• Mud gas ratios and delineation of alpha-beta regions to predict fluid type, API and GOR in ranges adequate for operational decisions. Prediction is enhanced by tuning to regional data.

Two examples from offshore Brazil are presented as proof of concept, a gas reservoir from the “post-salt” turbiditic deposits and a rich gas condensate reservoir from the “pre-salt” carbonates. Real time evaluation of fluid typing, GOR and API is illustrated by a “post-salt” appraisal well.

This new approach has a significant impact at the exploration and appraisal drilling stages. Early fluid type identification with API and GOR assessments can be integrated into the objectives of the fluid sampling program from wireline. More important, it helps to properly design the well testing at an early operational phase.