Abstract

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/CH4 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.

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