ABSTRACT

ABSTRACT: The paper presents experimental results on correlating far field applied stresses with near borehole stresses, displacements and breakouts stress. Laboratory tests were performed on 6" x 6" x 6" cubical rocks samples including Berea sandstone, chalk, limestone, and shale with a 0.93" diameter wellbore under uniaxial stress mode applied on the faces parallel to the borehole azimuth. The measured borehole strains and displacements were used to estimate the induced stresses at the borehole, and the borehole-failure stresses were correlated with the rock compressive strength. The results show that the stress magnitude measured at the borehole varies considerably with the type of rock being tested. The measured induced tangential compressive stresses at the borehole exhibited a non-linear function with the applied external stress. Therefore, tangential stresses around a wellbore may be over or under estimated when calculated based on the elastic Kirsch's solution. The applied boundary stress level at which tensile failure initiated at the borehole wall was also correlated with the measured Brazilian tensile strength. Similarly, the applied maximum far field stresses that caused shear failure, thus development of breakouts around the wellbore, were compared with the measured uniaxial compressive strength of the various tested rock formations.

1. INTRODUCTION

When drilling a borehole within a rock formation, the in-situ stress field is locally disturbed and a new form of stresses are induced around the borehole. The knowledge of the magnitudes and orientations of these in-situ and induced stresses is of fundamental importance in a wide range of applications in petroleum engineering, civil engineering, mining, as well as in geology and geophysics. In petroleum engineering stability of the borehole, due to the in-situ stresses and induced stresses around the borehole, is one of the major challenges to the industry. Stress concentration around the excavation of the wellbore walls may be large enough to overstress the rock, creating tensile and/or shear failures, depending on the magnitude of the far field stresses. The accurate estimation of far field in-situ stress is essential not only in drilling but also in production and reservoir management. In production, the knowledge of in-situ stress field is important in the hydraulic fracturing of oil and gas formations. In reservoir management, the change in the in-situ stress field must be known during reservoir depletion. Oil and gas production causes changes in the effective stress field, resulting in reservoir compaction and subsidence [1]. Moreover, changes in the effective in-situ stress field affect rock properties such as porosity and/or permeability, for example, compressive stresses tend to close pore space whereas tensile stresses tend to open them [2].

The induced stresses can extend to a few wellbore diameters away from the hole and are function of the far-field stresses. Therefore, it has been of great interest for rock mechanics researchers to look into the measurements and evaluation of stresses in rock around underground openings. Several investigators have made theoretical and model studies of stress and strain concentrations around the wellbore under subjected far field stresses, i.e. [3]. While the induced stresses and strains around the borehole have been modeled and predicated theoretically, laboratory and field experimental measurements and verifications of these dynamic and kinematic parameters have been limited, i.e., [4, 5].

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