Predicting Gas Saturation Buildup During Depressurisation of a North Sea Oil Reservoir

Abstract

Abstract 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^