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For efficient and safe well operations it is important to understand the behaviour of gas influx in petroleum fluids and the impact of relevant temperature and pressure changes in the well. Drilling simulators are tools for analysing gas kick detection limits, designing well control procedures, planning the chemicals and equipment needed on the rig, and generally for planning the well design. A major weakness in current models of kick detection is the lack of experimentally verified data in the HPHT region. The data from this study will be analysed and used as an input in a computational model of two phase gas-drilling fluid flow in the well, allowing better prediction of gas absorption.This study will provide experimental measurements under HPHT conditions for oil based drilling fluids (OBDFs) and base oils mixed with methane. As a part of the DrillWell centre program, SINTEF has developed an experimental setup for studying the effects of natural gas dissolved in drilling fluids under conditions relevant for HPHT drilling operations. In this setup we are able to measure density and rheological properties of drilling fluids with different degrees of methane saturation at pressures and temperatures of up to 1000 bar and 200°C.Base oils, the major constituent of OBDFs, contribute to the mud properties. Knowing the properties of the base oils at different pressure and temperature conditions enables assessment of the influence of the other components in the drilling fluid. In this paper we present measurements of a refined mineral base oil, especially designed for deepwater operations, and a linear paraffin oil. These base oils were tested for gas absorption capacity at various pressures and temperatures, and the effect of dissolved gas on the density of the base oils was measured. Experimentally determined saturation pressures show good correlation with predictions made with PVTsim for low gas-oil-ratios (GORs), however, at higher GOR-values the deviation is significant. The temperature influence on the saturation pressure is underestimated by PVTsim, demonstrating the need for more experimental data of drilling fluid behaviour at HPHT conditions. Furthermore, there was a clear difference in the maximum saturation pressure of the two base oils, which may be of high importance for the choice of drilling fluid at high reservoir pressures.In the continuation of this study we will perform measurements of methane solubility in two OBDFs composed of the two base oils studied in this work, respectively. Density and viscosity of mixtures of OBDF/CH 4 will be measured for various amounts of methane at pressures and temperatures ranging from standard ambient to HPHT. The resulting HPHT data will be highly important for improved calculation of bottom hole pressure and prediction of gas kicks.

Drilling fluids for oil wells must meet a number of requirements, including maintaining formation integrity, lubricating the drill string, and transporting cuttings to the surface. In order to satisfy these needs, drilling fluids have become increasingly complex and expensive. To ensure safe and efficient drilling, it is vital for the drilling operator to be able to make a qualified choice of fluid appropriate for each individual well. API/ISO standards specify a set of tests for characterization of drilling fluids. However, fluids that are tested to have equal properties according to these standards are still observed to perform significantly different when used in the field. The aim of the full project is to provide a thorough comparison of drilling fluids in particular with respect to hole cleaning performance, in light of the issues presented above. As part of this investigation we here present results for two oil based drilling fluids, as well as for the corresponding base oil. The drilling fluids differ in composition by varying fraction of base oil, and thus density and water content. The fluids have been tested according to the API standard, and further, viscoelastic properties have been examined using an Anton Paar rheometer. The rheological test campaign includes determination of the linear viscoelastic range (LVER), viscosity and yield point, thixotropic time test, and temperature dependence of rheological parameters. Further, it is demonstrated how the rheological data may be used to interpret data from ongoing full scale flow loop experiments with the same fluids. In a more general context, the rheological test campaign of the drilling fluids is expected to make a crucial contribution for the petroleum industry in explaining observed differences in hole cleaning properties beyond what todays API/ISO industry standard provides.

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