The impact of shale properties on wellbore stability

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Zhang, Jianguo

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Most wellbore instability problems occur in shales due to their unique properties. Shales are highly laminated, have a very low permeability, and a high cation exchange capacity. This dissertation investigates how these properties impact wellbore stability in shales. The stress distribution around deviated wellbores in laminated shale/sand sequences is analyzed to show that failure can occur either along or across bedding planes, depending on the well trajectory. It is pointed out that both in-situ stresses and rock strength anisotropies should be considered in order to improve wellbore stability. A model to predict pore pressure distribution within the shale sample during a typical compression test is developed. Due to their low permeability, pore pressures often build up in shales during axial loading, and can greatly influence the measured shale strength. The effect of strain rate and permeability on pore pressure build-up, and thereby the compressive strength of shale is assessed. Experimental results show that the deviatoric strength for soft Pierre I shale decreases, while for highly compacted Arco shale its strength increases with increasing strain rates. The reasons for these observed phenomena are analyzed, and their impact on drilling operations is briefly discussed. A new Gravimetric–Swelling Test (GST), for quantitatively determining water and ion movement during shale/mud interaction is developed. Results show that water movement is controlled not only by osmosis, but is also influenced by ionic diffusion and capillarity. Experimental results are also presented to show how the compressive strengths and acoustic velocities of shales change when they are exposed to water-based fluids. By combining these tests with the results from GST, it is clearly shown that these different effects correlate well with the movement of water and ions into or out of the shales. Finally, the changes in shale properties observed in this dissertation are used to study wellbore stability in shales by taking into account 3-dimensional earth stresses around the wellbore as well as chemical and thermal effects. Results from this study indicate that for low permeability shales, chemical and thermal interactions between the shale and water-based fluids play an important role.



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