Effects of drilling fluids on the strength properties of clay-bearing rocks
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ORIGINAL PAPER
Effects of drilling fluids on the strength properties of clay-bearing rocks Hasan Karakul 1 Received: 17 May 2018 / Accepted: 9 August 2018 # Saudi Society for Geosciences 2018
Abstract Certain drilling problems are frequently observed during different types of drilling (e.g. geothermal, oil, geotechnical). In order to maintain the stability of boreholes, analysis of the essential parameters affecting their stability, including induced post-drilling stresses, the geomechanical properties of the rocks encountered during drilling and the type of drilling fluid used, should be considered. Well pressure, which is a function of the density of the drilling fluid, should be kept at a level that will not create shear- or tensile-type failure around the borehole walls; however, the mechanical properties of rocks are not constant and can be varied due to the interaction between drilling fluid and rock materials especially for clay-bearing rocks. Despite being aware of these factors, to date, no comprehensive study has been conducted on these issues. In order to overcome these shortcomings, the variations in mechanical properties of two clay-bearing sediments, which were saturated with different types of drilling fluids that are widely used in drilling practices, were investigated. By considering the obtained geomechanical properties, the effect of drilling fluids was compared and evaluated in terms of degradation of mechanical properties. As the occurrence of excess pore pressures that can develop under undrained conditions around a borehole is essential to borehole stability, the distribution of excess pore pressures and plastic deformations was also investigated by a finite element-based borehole simulation under drained and undrained conditions. By considering the results of experimental and numerical studies, some recommendations were presented for understanding wellbore stability. Keywords Drilling fluid . Wellbore instability . Plastic deformation . Clay-bearing rocks
Introduction Maintaining the stability of boreholes drilled for various purposes (geotechnical, geothermal, oil) has become an important rock engineering topic. The relief of stresses caused by drilling induces strain, which enables contraction of the borehole diameter. The characteristics of this strain mainly relate to the elastic limit of the host rock. The instability of boreholes is related to plastic strains developed by exceeding the elastic limit of rocks. The rocks in which drilling is performed are usually saturated with a drilling fluid that is selected by the drilling engineers to create appropriate well pressure by considering the density of the fluid and its ability to prevent shearand tensile-type failure in the rock; however, this approach is based on the assumption that the rock has a constant strength,
* Hasan Karakul [email protected] 1
Department of Petroleum and Natural Gas Engineering, İzmir Kâtip Çelebi University, İzmir, Turkey
ignoring the interaction between the rock and the drilling fluid. But the geo
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