Drilling fluid is the mixture of base fluid and special chemicals. The system is designed to meet operational requirements. These complex fluids can carry drilled cuttings to surface, provide enough force or pressure to the formation and have adequate holding capacity in pump-off period to prevent particle precipitation. These necessities are controlled by fluid rheology. The art of flow or flow science, i.e. rheology, examines the deformation and flow behavior of the fluid. Ideal viscous flow through ideal elastic deformation is analyzed in this branch.

Increase in energy demand and depletion of shallow hydrocarbon reserves has driven the industry to explore deeper reservoirs. Thanks to the technical developments in the drilling industry, operations can go further, especially in offshore wells. Non-aqueous conventional drilling fluid systems (NAS), synthetic based mud (SBM) or oil based mud (OBM), are favorable due to lubricity effect, high inhibitive characteristic and temperature-rheological stability in deep formations. Despite the advantages of NAS over the water based systems (WBM), their flow characteristics vary with pressure due to compressibility. Mezger stated that “For most liquids, the viscosity values are increasing with increasing pressure since the amount of free volume within the internal structure is decreasing due to compression, therefore the molecules are more and more limited in their mobility. This increases the internal frictional forces and the flow resistance”1. By considering the primary well control requirements, drilling fluid equivalent pressure in both static and dynamic conditions can overbalance the fluid pressure in the rock pores and cannot exceed the inelastic strength of the medium in conventional, over-balanced operations. Mud playing pressure window shrinks as the depth is increased and is even not tolerable in ultra-deep offshore wells. Incompetent fluid formulation and hydraulic design led to detrimental and vital problems. In literature, limited research on high pressure fluid deformation behavior and rheological experimental data are found to understand the flow behavior and rheological changes in down hole conditions2. Generally rheological measurements are taken at the surface conditions and extrapolated to the down hole conditions that cause the perversity in real high pressure deformation behavior.

This study is conducted to examine the high pressure effect on NAS rheology and to compare the experimental results to the conventional WBM. High Pressure, High Temperature (HPHT) Anton Paar MCR-302 compact rheometer (1,000 bar–14,500 psi, 300°C capacity) is preferred rather than using HPHT viscometers due to exact frictionless air bearings in the laboratory experiments. Controlled shear rate (CSR) and rate sweep test methodologies are used in the rheological tests. Tests are performed from ambient surface conditions to 12,000 psi pressure at constant temperature. To get a better understanding of flow properties and to have a more accurate hydraulic design, rheological characterization is simulated under in-situ conditions. Test results are analyzed to understand the compressibility and pressure effects on rheological parameters in constitutive equation (yield stress, apparent viscosity, and flow behavior index) and deformation behavior.

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