The Impact of Dual Porosity on Pore-Scale Fluid Distributions During Steady State Flow

Abstract

The last 5 years have seen a rapid advance in the ability of petroleum scientists to examine reservoir processes directly at the pore scale, principally through coupling Special Core Analysis (SCAL) techniques with non-invasive x-ray micro-CT imaging. These techniques have been used to investigate processes such as trapped gas saturation, wettability changes and even dynamic multiphase displacements. Such experimental developments have, however, not typically focussed on relating flow behaviour to the pore structure of the host rock. In this study we present the first comparison of steady state core-flood experiments, conducted at reservoir conditions and imaged at the pore scale with resolutions of around 4.5 μm. Experiments were conducted in two qualitatively different pore structures; a single porosity sandstone and a dual porosity carbonate. During steady state fluid flow the single porosity sandstone showed a well-connected wetting phase flow path. The dual porosity carbonate, however, showed the wetting phase to be poorly connected through the macroporosity. Wetting phase flow (and so relative permeability) can only be explained by including connectivity through the microporous network. A multi-scale computational model was constructed which was able to reproduce the measured relative permeability of the carbonate better than when the model was constructed using macroporosity alone. The multi-scale model was created by populating microporosity from the micro-CT images with permeabilities modelled from high resolution nano-tomography images (with voxel sizes of around 50nm), proscriptively sampled from microporous regions. This work has important implications for the characterization and modelling of flow in such systems. It is often assumed that microporosity, while important for volumetric petrophysical measurements (such as fluid saturation), has only a minor contribution to displacement and flow, as the permeability of the microporous network is often much lower than the permeability of the macroporous network. For this reason it is frequently ignored. This assumption makes pore-scale modelling of petrophysical properties much more computationally efficient, however may not capture physics critical for multiphase flow in dual porosity rocks. The ability to conduct full steady state core-flooding experiments, imaged at the pore scale, shows the difficulty of such assumptions, and the value of a full multi-scale description of pore structure.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016