ABSTRACT

Polish Oil and Gas Company (POGC) is in the process of appraising tight gas discoveries in the Polish part of the European Southern Rotliegend Basin. Encouraged by the results and experience of directional wells, it planned to assess the feasibility of drilling a horizontal well using underbalanced drilling technology. For this evaluation, an understanding of the geomechanical setting was needed, and thus constructing a field-specific geomechanical model was the underlying aim of the study. Two constitutive models were used in order to obtain a range of safe mud weights and to assess the effects of under- or near-balanced drilling operations on wellbore stability in the reservoir sands: an elastic and a poroelastic model. The two models did not agree, so we tried to assess the model quality based on which model best described the failures observed in the offset wells during drilling. The data was not sufficient to determine the best model, so we have two possible answers. If the elastic model is correct then a higher mud weight is needed to safely drill the horizontal well. If our assumptions about poroelasticity are correct, then the horizontal well can be drilled with much lower mud weights.

1. INTRODUCTION

Underbalanced drilling (UBD) is a valuable method for optimization of multiple drilling objectives including: minimizing formation damage caused by drilling fluid invasion, increasing the rate of penetration and reducing drilling time, increasing bit life, performing early detection of hydrocarbons, dynamic testing of productive intervals while drilling, and minimizing lost circulation (Finkbeiner et al., 2009).

The primary aim of this study was to determine the feasibility of underbalanced drilling in a horizontal appraisal well through the Permian age Rotliegend sandstone target in the Wielkopolska Province, onshore Poland. Deposition of the Upper Rotliegend occurred in arid and semi-arid continental conditions, and therefore the basin contains Aeolian, fluvial, and playa deposits. Continental Rotliegend sedimentation was terminated by the Zechstein transgression, which caused partial redeposition of poorly consolidated, porous, and permeable Aeolian sands. They are topped with the Zechstein Limestone deposits and then by the PZ1 evaporites (Wagner and Peryt, 1997).

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