Usual practice in drilling engineering is to determine drilling fluid density at surface conditions assuming that drilling fluid density does not change with changing downhole conditions. This assumption may result in inaccuracy while estimating static and dynamic pressures in the wellbore especially when synthetic based drilling fluids are used. Inaccurate estimation of pressure profile in the annulus can lead to severe well problems such as kicks, drilling fluid losses and wellbore instability. In addition, inaccurate pressure profile estimation can affect the success of Managed Pressure Drilling (MPD) operations which require real time knowledge of wellbore pressures in order to keep it between formation pore and fracture pressures using a control choke placed on the return line of the annulus.

Effects of pressure and temperature on volumetric behavior of two olefin based synthetic oils are investigated in this study using a mercury free PVT system. The olefin based synthetic oils used in this study are C16C18 internal olefin and C12C14 linear alpha olefin. In order to simulate deep offshore situations the temperature is ranged between 25 °C and 175 °C, while the pressure is ranged between 0 psig and 14,000 psig.

In addition, volumetric performances of olefin based synthetic oil systems under investigation are compared with those of water, brine solution, mineral oil, diesel oil and n-paraffin based oil under similar conditions.

The study shows that synthetic based oils' volumetric properties are more sensitive to pressure and temperature conditions compared to water, brine solutions, mineral oil and diesel oil. Once emulsion systems containing synthetic based oils are used, density change with respect to downhole conditions should be modeled to increase the reliability of pressure profile calculations.

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