Physical Modeling of Microscopic Rock-Pore Heterogeneities
Authors: Trygstad J. C. , Ehrlich R. , Wardlaw N. C. 
Published: December 2007

Two existing technologies have been merged to investigate the heterogeneity of rock-pore geometry and its influences on pore-level, multiphase fluid movement. Rock-Pore Modeling uses Petrographic Image Analysis (PIA) to computer-generate, from thin sections, black and white images of selected parts of a rock's pore system. These images are combined or used alone as templates for fabricating glass micromodels. Multiphase fluid flow experiments in these transparent glass micromodels provide a means of assessing the effects of various types of pore structure on fluid displacement under known and controlled conditions.

Each template demonstrates in two dimensions textural heterogeneities present in the thin sections. Such heterogeneities include the geometry and size-distribution of selected parts of the pore system, including oversized pore elements, micropore elements, and pore connections. Mineral heterogeneities such as the distribution of carbonate cement are also recorded. Multiphase fluid flow in glass micromodels that are made from the templates is influenced by these pore and mineral textures.

Hexadecane, water and toluene (miscible with hexadecane) flow experiments have been performed in template-originated glass micromodels. Results have provided information on the effects of model heterogeneity and wettability on pore-level fluid movement. In a water-wet micromodel, trapping of hexadecane is stratified when the micromodel pore structure is stratified. Hexadecane desaturation during water injection is inhibited by and within intervals characterized by high aspect-ratio pores. These pores are bypassed by the water phase, which moves past them through relatively minor, very narrow, continuous pore pathways. The continuity of the trapped hexadecane is low, and subsequent injection of miscible toluene is inefficient in displacing more hexadecane. Regionally arranged "neighborhood" pore structure elements also tend to inhibit hexadecane desaturation during water injection into a water-wet system. In contrast, model heterogeneity is not a major factor in hexadecane trapping during water injection into a hexadecane-wet micromodel. High hexadecane continuity is maintained, and this promotes relatively greater hexadecane desaturation.

Mineral textures also affect fluid flow in micromodels because of their effects on pore structure heterogeneity. For example, patchy carbonate cementation in a thin section is recorded in the micromodel as regions of non-porous matrix. These regions at least partially determine hexadecane/water distribution among micromodel pores by obstructing frontal movement of the injected fluid phase through the micromodel. After hexadecane is injected into the water-saturated model, water-filled pathways are present in "shadow" zones immediately downstream of the patchy, non-porous regions. Subsequent water injection results in bypassing of hexadecane when injected water follows these pathways.

The quality of replication between thin section, template and glass micromodel is known because PIA can record the details of pore element arrangement and geometry at each modeling step. Also, time-dependent pore level fluid saturations can be measured in a glass micromodel with PIA.

Rock-pore modeling with Petrographic Image Analysis provides improved representation of complex, microscopic rock-pore heterogeneities, important to experimental work in multiphase, pore-level fluid flow. However, the extent to which replicated structures in a two-dimensional model can represent the pore system of a three-dimensional rock requires further evaluation.