The efficiency with which oil or gas can be displaced by water in the pores of a reservoir rock is affected by the properties of the fluids and the properties of the pore spaces which contain those fluids.

Pore casts provide the most effective way of viewing pore structures and are prepared by impregnating pore systems with resins and subsequently removing the host rock by solution in acid.

Unsteady state relative permeability tests, counter-current imbibition tests, and drainage-imbibition capillary pressure tests can be used to estimate the displacement efficiency for particular fluid conditions in selected cores of reservoir rocks. The results of these tests can be compared with visual observations of pore structure made from pore casts and, in this way, it is possible to suggest which attributes of pore systems in reservoir rocks are critically important in influencing displacement efficiency. These variables include: pore to throat size ratio, the average number of throats connecting with pores, the types, abundance, and arrangement of non-random heterogeneities, and the roughness of surfaces.

However, pore systems in any rock are a complex of variables and, to further understand and define the interaction of fluid and pore variables in trapping oil or gas during displacement, it is necessary to create physical models of pore systems which incorporate the characteristics of real systems but in simplified and controlled forms. The fluid and pore geometric attributes can be varied singly and displacement tests can be viewed under the microscope in transparent micromodels

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to gain a better understanding of displacement processes.

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