Abstract
Confinement by tight pores can influence fluid phase behaviour according to some recent investigations. Most of these studies are theoretical and the experimental investigations are relatively scarce. In this work, we have employed the calorimetric approach to study the equilibrium of n-alkanes confined in two synthetic porous materials with a narrow pore size distribution, not only at atmospheric conditions but also at elevated pressures. In addition, we also measured two chalk samples from the Danish North Sea in order to shed light on whether the tight chalk formation will influence the phase behaviour. A shift in the saturation temperature of the confined fluids was observed when confined in the synthetic porous materials, whereas no temperature shift was observed in the fluid confined in the chalk powders. By using phase equilibrium calculation incorporating capillary pressure difference between the gas and liquid phases, we also predicted the temperature shifts and compared them with the experimental values.
Introduction
The influence of porous media on phase behaviour is a topic discussed in the oil industries for decades with revived interests and investigations in recent years due to its potential impact on production from tight shale. For the tight chalk formation in the North Sea, a similar question on the role of porous media in phase behaviour has been frequently asked. Some recent experimental and modelling methods developed for shale may help answer the question.
The Lower Cretaceous chalks in the Danish Central Graben have porosities in the range from 15 to 45% with a very small average permeability, lower than 1 mD. The current production from these reservoir rocks in the Danish Central Graben happens in the Valdemar field, which is a very complex and heterogeneous reservoir with a low and uneven production (Jakobsen et al. 2005). It is expected that in this tight rock the reservoir fluids are subjected to a high degree of confinement. The interaction between the fluid and the pore wall is significant, and there is always a question whether the phase behaviour in the tight formation will be dramatically changed. In principle, the phase behavior of confined fluid can be altered due to large capillary pressure differences, selective adsorption, and extreme reduction in the free space of movement. These effects result in changes of fluid physical properties and shifts in saturation pressures/temperatures. When such differences are taken into account into reservoir simulation studies, well performance, gas-oil ratio, oil and gas production rate, and ultimate recovery may be affected. (Wang et al. 2014, Teklu et al. 2014, Dong et al. 2016, Firincioglu et al. 2012)
