Abstract: The unreasonable formation evaluation often causes inappropriate choices of underbalanced drilling, resulting in some complex situations and severe damages of reservoir. Also, for its large equipment and high moving cost, significant economic loss will be made. Based on large amounts of underbalanced drilling site statistics, this paper presents the evaluations of underbalanced drilling adaptability to formation, including contents and procedures.

Key words: underbalanced drilling, formation fluid, hole stability, geologic condition, drilling process