When a waterflooded oil reservoir is to be depressurised, it is important to predict the production of solution gas from the by-passed and trapped oil. To quantify the gas saturation build-up and the relation between gas mobility and gas saturation, we have carried out a large number of experiments on core samples. These pressure-depletion experiments, which were conducted at various depletion rates and with both model fluids and actual reservoir fluids, began from one of two starting conditions: reservoir rock at initial conditions (i.e. containing oil and connate water) or watered-out reservoir rock (i.e. containing brine and residual oil). Relating the experimental results to field conditions was a major concern for two reasons:The experimental results were sensitive to the depletion rate (and the field depletion rate is two orders of magnitude lower than what was possible in the laboratory).The experimental results were sensitive to the fluid/rock system used. In this paper we discuss the interpretation of the experimental results and the approach we have adopted for extrapolating them to field conditions, so that the reservoir engineer can assess their practical consequences for field development. We found that when the experiments were run starting under initial reservoir conditions, the nucleation properties of the fluid/rock/pressure combination, together with the depletion rate, determine the gas saturation build-up. To extrapolate such laboratory results to the pressure-decline rate of the field, it is essential that the experiments be conducted with reservoir fluids at reservoir pressure and temperature. When the experiments were run under watered-out conditions, no such dependency on nucleation properties was found. Instead, the mobility of the gas could be correlated with the total hydrocarbon saturation, the same correlation applying to different fluid/rock/pressure combinations. Introduction The Brent Field in the North Sea is currently producing oil under pressure maintenance by waterflooding. A future development phase involves a reservoir depressurisation to recover most of the reservoir gas and to increase oil recovery. To properly plan this development phase so that the hydrocarbon recovery is optimised, it is important to know the critical gas saturation above which liberated solution gas becomes mobile enough to be produced. A considerable effort has been put into laboratory experiments to determine this critical gas saturation and, in fact, to plot the complete gas saturation build-up versus declining pressure. P. 361^

Critical Gas Saturation During Depressurisation and its Importance in the Brent Field. Abstract After some 20 years of pressure maintenance by waterflooding it is planned to extend the Brent Field life by depressurisation to recover the remaining gas and to increase the oil recovery. During depressurisation the gas from the expanding gascap as well as gas which is in solution in bypassed and residual oil will be produced. In this way the life of the field will be extended to around 2010. The liberation and subsequent mobility of solution gas as a function of reservoir pressure will be a major factor in the management of gas production from the Brent Field during depressurisation. While the depressurisation process is controlled via the quantity of back produced water, the rate of production and ultimate recovery of this gas is controlled by the gas saturation at which the liberated solution gas becomes sufficiently mobile to be transported to the production wells and by the ultimate gas saturation at the end of the pressure decline. These so-called "mobile" and "ultimate" gas saturations follow from upscaling of the basic property governing the gas liberation process during depressurisation: the "critical gas saturation" at which the gas becomes fully mobile within the oil. The application of these parameters, via a gas/hydrocarbon relative permeability model, in the Full Field simulation Model and the impact of these parameters on the predictions for the oil and gas recovery during depressurisation of the Brent Field, are described. Two conditions have to be fulfilled to allow the gas to be produced to the wells:The gas saturation within the oil must exceed the critical gas saturation;During depressurisation the hydrocarbon phase has to expand to a continuous network and needs sufficient mobility. The model was calibrated on laboratory core experiments carried out under virgin oil as well as waterflooded conditions. Using this relative permeability model, fine-grid cross-sectional simulations were carried out for various types of reservoir architectures with respect to sand permeability, averaged distance between shales, and the oil saturation at the start of depressurisation. These simulations provide a range of numbers for the mobile and ultimate gas saturations which are used in thickness-averaged ("pseudo") relative permeability and capillary pressure curves in the various layers of the Full Field simulation Model. According to the model, gas becomes mobile at a higher pressure than previous models predicted. This has a substantial impact on the requirement for the back produced water facilities, used to depressurise the aquifer. Introduction After some 20 years of pressure maintenance by waterflooding it is planned to extend the Brent Field life by depressurisation to recover the remaining gas and to increase the oil recovery. During depressurisation the gas from the expanding gascap as well as gas which is in solution in bypassed and residual oil will be produced. In this way the life of the field will be extended to around 2010. The liberation and subsequent mobility of solution gas as a function of reservoir pressure will be a major factor in the management of gas production from the Brent Field during depressurisation. While the depressurisation process is controlled via the quantity of back produced water, the rate of production and ultimate recovery of this gas is controlled by the gas saturation at which the liberated solution gas becomes sufficiently mobile to be transported to the production wells and by the ultimate gas saturation at the end of the pressure decline. P. 127^

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