Abstract

In strongly wetted reservoir rocks, the extent of non-wetting phase trapping is strongly dependent on the geometric and topologic properties of the pore system. It has been demonstrated that pore system. It has been demonstrated that pore-throat size ratio and certain types of heterogeneity pore-throat size ratio and certain types of heterogeneity affect displacement efficiency adversely. Throat to pore coordination number or connectivity and the pore coordination number or connectivity and the condition or roughness of pore surfaces are other important factors affecting displacement efficiency.

Under conditions of intermediate wetting, pore geometry appears to be of reduced significance as a cause of trapping and, under these conditions, displacement efficiency is greater.

Introduction

Trapping of oil and gas on a microscopic scale in a petroleum reservoir rock is affected by the geometric and topologic properties of the pores, by the fluid properties and by properties related to fluid-rock interaction such as wettability.

Pore properties of importance are pore to throat size ratio, the types and degrees of heterogeneity in the arrangement of pores, throat to pore coordination number or connectivity and the properties of the pore surfaces which include composition and degree of roughness. Fluid properties of importance include viscosity, density, interfacial tension and phase behaviour. Contact angle or wettability is affected by both fluid properties and the nature of the rock surface.

Because of hysteresis effects during drainage-imbibition cycles, displacement efficiency is also a function of the previous saturation history of the rock-fluid system.

The major purpose of this paper is to summarize some effects of pore structure on trapping in strongly wetted systems and to show that wettability changes have pronounced effects on the extent of trapping.

PORE STRUCTURE AND DISPLACEMENT EFFICIENCY IN PORE STRUCTURE AND DISPLACEMENT EFFICIENCY IN STRONGLY WETTED SYSTEMS

Strongly wetted systems are defined as those with wetting phase contact angles of less than 40 degrees.

Pore-throat Size Ratio Pore-throat Size Ratio A pore network is made up of larger spaces, referred to as pores, which are connected by smaller spaces referred to as throats. It has been demonstrated both for rocks and for artificial models that the amount of residual non-wetting phase, following imbibition, is related to pore-throat size ratio.

Average pore size was measured directly on random traverses of pore casts by measuring the diameters of inscribed circles in successive pores. Pore casts are prepared by impregnating pore systems Pore casts are prepared by impregnating pore systems with resin and, after solidification, dissolving the host rock so that the resin filled pores stand in relief. Average throat size can be obtained from mercury injection capillary pressure curves. Since the injection pressure of mercury is controlled by the sizes of the throats in the systems and not by the sizes of the pores. The average equivalent throat size was obtained from the pressure required to saturate 50 per cent of the pore volume of the sample using;

(1)

This expression is for parallel plates separated by a distance d and is more appropriate than the expression for tubes because, in reservoir rocks, throats are mainly sheet-like.

Average pore-throat size ratios were obtained from these measurements and, for a group of 27 sandstones studied, ranged from 3 to 94 with an average of 14.

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