Measuring Pore Throat Geometry and Angularity Through Mathematical Morphology

Abstract

The geometry of pore space affects the storage and fluid flow capacity of a rock. Pore networks are typically composed of multiple pores of complex shapes connected by minuscule tubes typically showing converging-diverging geometries. Due to computer and software limitations, fluid flow models have commonly been based on simplified pore models. The classical pore-network or capillary tube models consist of spherical pores connected by tubes of constant circular or triangular section.

However, the effect of tube geometry on fluid flow is a well-known problem in hydrology and engineering. Fluid flow in converging-diverging pipes may be complex and turbulent. Turbulences often occur in the vicinity of a constriction, where zones of stagnant or reverse flow contrast with zones of rapid flow. The geometry of the constriction appears to control the flow characteristics in its vicinity. Moreover, throat morphology also controls the likeliness of fluid snap-off, an intra-pore trapping phenomenon which affects hydrocarbon production.

However, current 2D and 3D techniques such as mercury injection, computed tomography scanning and image analysis are unable to measure this geometry in rock samples. An image analysis technique called mathematical morphology has been applied to characterize porosity in laterally continuous pore networks, e.g. in sandstones. Morphology consists in studying the modification of objects such as pores through successive erosion-dilation cycles, using an expanding structuring element. This method allows the extraction of petrophysical parameters such as pore and throat diameters in 2D. This study shows that the pore throat geometry strongly affects the behaviour of the structuring element as its size increases. As a result, the diameter of an elongated, acute pore throat is more correctly measured through morphology than that of an obtuse throat. Based on this behaviour, the geometry of the pore throat can in turn be calculated.

This study introduces a new pore shape descriptor called angularity to represent the angle of opening of pore walls from the throat towards the body. This study provides the first quantification of this parameter using 2D thin section images. The novel methodology is tested on rock samples from the upper Maastrichtian Tor Formation, the most prolific chalk reservoir of the North Sea. Once adapted to 3D, it is envisioned that that the technique could improve fluid flow modelling at the pore-scale in reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018