As the underbalanced-drilling (UBD) technique develops, the benefits for eliminating formation damage and improving productivity have been well received by Chinese national oil companies. Potential candidates for UBD reservoirs are tight sandstone and volcanic rock, as well as other low-permeability hard rocks. Not all candidates are ultimately suitable. Experience in China suggests that inappropriate application of underbalanced reservoir drilling may have an adverse effect on protecting reservoirs, which in turn increases drilling risks. To achieve successful application of the technique, it is essential first to fully evaluate the suitability of the reservoir for UBD. Furthermore, it has been observed that there is a lack of research when it comes to evaluation of reservoir suitability with respect to UBD opportunities in China. Most current basin studies in China focus on geographic distributions and reservoir lithology characterizations rather than geomechanical properties.

This paper addresses these gaps by presenting the result of a China UBD basins study. We have applied our reservoir-screening process to reservoirs in 382 oil and gas fields and 17 onshore basins of China to determine whether or not these reservoirs are practical for UBD. Candidacy is determined by evaluating a number of formation-damage mechanisms and related operational risks within the reservoir-screening process. The research then identifies formation-damage mechanisms of the UBD reservoir candidates and also presents a ranking from high-potential to low-potential reservoirs for the suitability of UBD, and establishes the highest production potential by application of the UBD technique.

Also, the objective of the research is to evaluate and rank the suitability of reservoirs for exploitation by UBD compared with conventional drilling and completion processes. Proper identification of UBD opportunities will minimize drilling risks for Chinese national oil companies.

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