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

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^

The Effects of Wettability and Interfacial Forces on the Depressurisation of Waterflooded Reservoirs

“…A mathematical model was developed to describe this behaviour (2), which was allo consistent with the results from high pressure core experiments performed by Kortekaas and van Poelgeest (3,4), and by Scherpenisse et al (5) . Subsequens visual experiments using a two component hydrocarbon mixture in a glass micromodel (6) suggested that both rock wettability and the oil spreading coefficient could substantially influenee the value of the critical gas saturation .…”

Investigation of Multiscale and Multiphase Flow, Transport and Reaction in Heavy Oil Recovery Processes

“…Pore-network simulations conducted by McDougal and Sorbie (1999) and Wang and Mohanty (1999) in the related topic of gas condensation, showed that S gc decreases as the hydrostatic pressure gradient increases, a trend also anticipated in Scherpenisse et al (1994). In a parallel study ), we h a ve analyzed the e ect of gravity and/or viscous forces on S gc , a n d developed scaling laws for the dependence of S gc on f qf and on two dimensionless parameters, the Bond and capillary numbers, de ned respectively as B = gk and Ca= q (2) Here denotes the density di erence between liquid and gas, k is permeability, the liquid-gas interfacial tension, q the liquid ow rate and the liquid viscosity.…”

“…T h e p o wer-law scaling with exponent 1, predicted by the theory in this range, is well supported by the data. Finally, a comparison of the evolution of the gas saturation, S g , as a function of the dimensionless pressure P Dl = P=P b , using the full solution for the Berea sandstone experiments of Scherpenisse et al (1994) is presented in Fig. 14.…”

“…A review of the early literature can be found in Yortsos (1995a, 1995b). Experimental work for the case of constant pressure decline rate in consolidated porous media using light o i l s w as reported by Moulu and Longeron (1989), Moulu (1989) and Scherpenisse et al (1994). Sheng et al (1999b), Wong et al (1999) and Urgelli et al (1999) conducted experiments with heavy oils.…”

Recovery Mechanisms and Oil Recovery From a Tight, Fractured Basement Reservoir, Yemen

“…The reason is that as soon as the critical gas saturation is reached, gas becomes mobile, migrates to the top of the structure and is produced resulting in fast pressure depletion and low recovery factors accordingly (e.g. Kortekaas & van Poolgeest 1991, Scherpenisse et al 1994. However, if the liquid mobility is increased compared with the gas mobility, higher recovery factors can be expected (Firoozabadi & Aronson 1999).…”