# Chapter%204 - Petrophysics MSc Course Notes Fluid...

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Petrophysics MSc Course Notes Fluid Saturation and Capillary Pressure Dr. Paul Glover Page 32 4. FLUID SATURATION AND CAPILLARY PRESSURE 4.1 Fluid Saturations We have seen that the viability of a reservoir depends upon three critical parameters. The first two of these are the porosity of the reservoir rock, which defines the total volume available for hydrocarbon saturation, and the permeability, which defines how easy it is to extract any hydrocarbons that are present. The final critical parameter is the hydrocarbon saturation, or how much of the porosity is occupied by hydrocarbons. This, and the related gas and water saturations are controlled by capillary pressure. The pore space in a rock is occupied by fluids. In hydrocarbon reservoirs these fluids are hydrocarbon gasses, oil and an aqueous brine. We define the pore fraction of each of these as S g , S o and S w , respectively. Hence, S g + S o + S w = 1. The amount of each of these fluids present at a given level in the reservoir depends upon gravity (buoyancy) forces, which tend to stratify the reservoir fluids according to their density, external hydrodynamic forces such as flow from a remote aquifer, and interfacial forces that act between the various reservoir fluids and between the fluids and the rock matrix. The interfacial forces either take the form of (i) forces on the interface between two fluids, or (ii) between the fluid and the solid matrix of the rock. Both effects rely upon differences in the relative strength of inter-molecular forces between gasses, liquids and solids. Liquid-gas or liquid-vapour forces result from the differences in molecular attraction of each of the gas and liquid molecules for molecules of the same fluid compared to the molecular attraction for molecules of the other fluid. The liquid-liquid forces result from the differences in molecular attraction of each of the liquid molecules for molecules of the same liquid compared to the molecular attraction for molecules of the other liquid. The fluid-solid forces result from the preference for fluid molecules to be attracted to the solid mineral surface rather than to molecules of the same fluid. The interfacial forces give rise to what is known as a capillary pressure. Capillary pressure is the difference in fluid pressure across an interface between two fluids in a confined volume. In the discussion of capillary pressure, we will first examine surface energy and wettability, and then go on to derive a general expression for capillary pressure. 4.2 Fluid-Fluid Interactions (Surface Tension) The fundamental property of that state of matter that we call a fluid is that its molecules are free to move. In gasses the freedom is great, but in liquids the freedom is constrained by relatively strong cohesive (attractive) forces that operate between the molecules. Imagine a drop of liquid suspended in a gas. In the interior of the liquid drop, a molecule is completely surrounded by other liquid molecules. On average it is equally attracted to the molecules surrounding it in every direction. By

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## This note was uploaded on 10/30/2011 for the course PETROLEUM Short cour taught by Professor Dr.paulglover during the Winter '11 term at University of Aberdeen.

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Chapter%204 - Petrophysics MSc Course Notes Fluid...

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