ABSTRACT:

Production of hydrocarbon causes changes in pore pressure and effective stresses acting on the reservoir rocks. This will be followed by reservoir compaction, surface subsidence and may lead to fault reactivation, casing or wellbore failure and closure of micro-cracks. Nonetheless, acquiring a good understanding of rock strength, pore pressure and in-situ stress will be critical to successful horizontal drilling and hydraulic fracturing. Bakken Formation of Williston Basin, North Dakota, which is identified by three distinct members, is a huge unconventional, self-sourced, naturally-fractured reservoir. It is one of the least studied sedimentary rock units in the basin. The over pressured nature of this formation have made the study of its geomechanical properties even more important. Natural fractures are also considered as another major source of problems in this reservoir. To investigate these problems, Mechanical Earth Model (MEM), a numerical representation of the reservoir properties, was built. This enabled to predict the alterations and changes of the geomechanical properties in the reservoir. The results show that the lower and upper members are mechanically transverse isotropic whereas the middle member is isotropic. Besides, the numerical geomechanical modeling demonstrate that the elastic anisotropic characteristics of the upper and lower members will result in elastic failure of the region around the wellbore following a shear failure phase but the elastic isotropic middle member will mostly stay in the shear failure state.

1. INTRODUCTION

Unconventional reservoirs, especially tight shales, are becoming an important target in hydrocarbon exploration. Oil companies cannot neglect the huge amount of their recoverable reserves. Since the very low matrix permeability and primary porosity, conventional techniques in drilling, production and characterization may not be applicable to these reservoirs; new stimulation methods should be applied. Successful field development and enhanced recovery operation need a comprehensive understanding of geomechanical properties of the reservoir.

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