52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: In this study, pore-scale dynamic mud invasion in a radial system was investigated for different rock samples. The rocks were selected to generate the mud invasion patterns, filter cake permeability profiles, and change in stress profiles of the lithologies they represent. Each rock was cut into a thick-walled cylindrical shape to simulate drill pipe rotation inside the inner diameter. In the experimental design, lost circulation material (LCM) concentration and rotary speed were varied based on the results of preliminary experiments with ceramic filter tubes. Water based mud (WBM) was formulated with calcium carbonate, and the viscosity profile of the fluid with change in temperature was also determined. The results from the experiments revealed different dynamic mud invasion and filter cake permeability patterns for different rock samples. These patterns were controlled by the rock porosity, permeability, temperature, and rotary speed. Increasing the concentration of calcium carbonate beyond a certain threshold may not always reduce pore-scale dynamic mud invasion at elevated conditions. Analytical results showed that rock permeability and filter cake permeability profile largely control the changes in wellbore stress profile. This approach can be used in the planning and well design process for drilling similar lithologies.
Drilling fluid lost circulation and mud invasion still pose a wellbore stability challenge to drilling operations. Drilling through depleted reservoirs (low pore pressure and fracture gradient) or naturally fractured formations are typical scenarios where lost circulation events and mud invasion are almost inevitable. Deep invasion of fine mud particles (barite, polymers, and drill cuttings) and mud filtrate can alter the formation permeability in what is referred to as formation damage (Civan 2007). Current industry practice relies on either preventative or remedial lost circulation material (LCM) treatment in mitigating mud invasion. These have been described by several literatures as highlighted by Alsaba (2015) and Salehi (2012). Ezeakacha and Salehi (2018) also highlighted some of the previous experimental and field studies with successful reduction in mud invasion from filter cake evolution. Salehi and Kiran (2016) reported that a positive change in effective stress can occur when a permeable wellbore is exposed to a low-permeability filter cake. This can result to increase in the fracture gradient, and it is referred to as mud cake wellbore strengthening. However, the complexity in mud invasion, filtration, and filter cake evolution is contributed to by the interbedded nature of lithologies, rock minerology, and other operational control factors.
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