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Over the last decades, drilling through fractured and unstable formations has proven to be challenging, specially in terms of wellbore instability. Stability of boreholes in these scenarios requires a careful consideration of in-situ stresses, rock strength, pore pressure, wellbore fluid pressure and drilling fluid characteristics. This issue is more critical and complex in fractured rocks, because of the need to consider more parameters than in other kinds of rocks. In fractured formations, fractures play an important role and rock wedges movement controls the instability. Also, fractures fillings are of paramount importance. In spite of this fact, there are only a few studies on stability of fractured rocks and their strengthening methods. The present paper aims to provide insights into wellbore stabilization in fractured rocks. A broad literature review was conducted on existing formation strengthening methods and their effectiveness for fractured formations. The work concludes with some recommendations for these kinds of formations and proposed topics for additional research and development. Introduction Wellbore instability problem Nowadays, wellbore instability is one of the most important challenges in drilling in through deeper fractured formations. Overall, wellbore instability is an industry-wide problem affecting both exploratory as well as development drilling, with global annual costs estimated in excess of $2 billion. This problem may cause reduction of wellbore diameter by plastic squeezing of rock or its enlargement by caving and spalling, resulting in excessive volumes of cuttings, borehole fill after collapse, increase in required circulation pressure, stuck pipe and drill string failure. Borehole instability is primarily a function of rock response to the stress concentration induced around the well during drilling. If the strength of the rock is higher than these stresses, the borehole will be stable; otherwise rock will yield and may collapse, detach, or converge according to factors such as post-failure behaviour of rock, residual strength of fractures, and the fluid properties. Generally speaking, factors affecting borehole instability include: in-situ stresses, natural fractures, rock properties (type, strength, porosity, permeability, thermal characteristics, swelling potential, pore distribution), formation temperature, native pore pressure, near-wellbore pressure, pore water chemistry, mud type and chemistry, bottomhole pressure, time of exposure, circulating rate, well trajectory and size, drilling vibrations, etc. Wellbore stability in fractured formations A natural fracture is defined as a macroscopic planar discontinuity resulting from stresses larger than the rupture strength of the rock. These natural fractures can have a positive, neutral or negative effect on fluid flow1. Aguilera (1995) believes that virtually all reservoirs contain some natural fractures; however, if the effect of these fractures on fluid flow and stability of well is not negligible, it can be treated as a "fractured formation."2 The mechanisms of borehole instability in these formations are completely different from the ones in shaly formations. Failure of a wellbore frequently occurs in fractured zone and sometimes obstructs any further drilling. There is a number of techniques to improve drilling conditions in fractured formations, but a highly effective method is still lacking3. Santarelli et al. in 19924 performed one of the earliest research works in the field of wellbore stability of fractured formations. They applied discrete element modeling to some drilling data gathered from deep and heavily fractured-layers of basalt and tuff with the aim of identifying the mechanisms of wellbore instabilities. They noticed that these instabilities probably resulted from the opening of joints in the borehole wall and the subsequent invasion of a limited region of the fracture network by the drilling fluid. Consequently, the loosening of the corresponding blocks made it possible for the drilling fluid to erode them from the wall. They concluded that an increase in mud density can play a major destabilizing role because of the opening of radial cracks.
Over the last decades, drilling through fractured and unstable formations has proven to be challenging, specially in terms of wellbore instability. Stability of boreholes in these scenarios requires a careful consideration of in-situ stresses, rock strength, pore pressure, wellbore fluid pressure and drilling fluid characteristics. This issue is more critical and complex in fractured rocks, because of the need to consider more parameters than in other kinds of rocks. In fractured formations, fractures play an important role and rock wedges movement controls the instability. Also, fractures fillings are of paramount importance. In spite of this fact, there are only a few studies on stability of fractured rocks and their strengthening methods. The present paper aims to provide insights into wellbore stabilization in fractured rocks. A broad literature review was conducted on existing formation strengthening methods and their effectiveness for fractured formations. The work concludes with some recommendations for these kinds of formations and proposed topics for additional research and development. Introduction Wellbore instability problem Nowadays, wellbore instability is one of the most important challenges in drilling in through deeper fractured formations. Overall, wellbore instability is an industry-wide problem affecting both exploratory as well as development drilling, with global annual costs estimated in excess of $2 billion. This problem may cause reduction of wellbore diameter by plastic squeezing of rock or its enlargement by caving and spalling, resulting in excessive volumes of cuttings, borehole fill after collapse, increase in required circulation pressure, stuck pipe and drill string failure. Borehole instability is primarily a function of rock response to the stress concentration induced around the well during drilling. If the strength of the rock is higher than these stresses, the borehole will be stable; otherwise rock will yield and may collapse, detach, or converge according to factors such as post-failure behaviour of rock, residual strength of fractures, and the fluid properties. Generally speaking, factors affecting borehole instability include: in-situ stresses, natural fractures, rock properties (type, strength, porosity, permeability, thermal characteristics, swelling potential, pore distribution), formation temperature, native pore pressure, near-wellbore pressure, pore water chemistry, mud type and chemistry, bottomhole pressure, time of exposure, circulating rate, well trajectory and size, drilling vibrations, etc. Wellbore stability in fractured formations A natural fracture is defined as a macroscopic planar discontinuity resulting from stresses larger than the rupture strength of the rock. These natural fractures can have a positive, neutral or negative effect on fluid flow1. Aguilera (1995) believes that virtually all reservoirs contain some natural fractures; however, if the effect of these fractures on fluid flow and stability of well is not negligible, it can be treated as a "fractured formation."2 The mechanisms of borehole instability in these formations are completely different from the ones in shaly formations. Failure of a wellbore frequently occurs in fractured zone and sometimes obstructs any further drilling. There is a number of techniques to improve drilling conditions in fractured formations, but a highly effective method is still lacking3. Santarelli et al. in 19924 performed one of the earliest research works in the field of wellbore stability of fractured formations. They applied discrete element modeling to some drilling data gathered from deep and heavily fractured-layers of basalt and tuff with the aim of identifying the mechanisms of wellbore instabilities. They noticed that these instabilities probably resulted from the opening of joints in the borehole wall and the subsequent invasion of a limited region of the fracture network by the drilling fluid. Consequently, the loosening of the corresponding blocks made it possible for the drilling fluid to erode them from the wall. They concluded that an increase in mud density can play a major destabilizing role because of the opening of radial cracks.
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