Saturation irreducible

The key to understanding dewatering by air displacement is the capillary pressure diagram. Figure 6 presents an example typical for a fine coal suspension there is a minimum moisture content, about 12%, called irreducible saturation, which cannot be removed by air displacement at any pressure and a threshold pressure, about 13 kPa. 

Vertical interval in the reservoir, whose length depends on porosity and permeability, in which the water saturation changes from 100 per cent at the bottom to irreducible water saturation at the top. In the transition zone, two phases (water or oil, and gas) are movable. 

Accurate interpretation of the formation properties (porosity, permeability and irreducible water saturation) requires reliable estimates of NMR fluid properties or the relationship between diffusivity and relaxation time. Estimation of oil viscosity and solution-gas content require their correlation with NMR measurable fluid properties. These include the hydrogen index, bulk fluid relaxation time and bulk fluid diffusivity [8]. 

If the NMR response is capable of estimating the pore size distribution, then it also has the potential to estimate the fraction of the pore space that is capable of being occupied by the hydrocarbon and the remaining fraction that will only be occupied by water. The Free Fluid Index (FFI) is an estimate of the amount of potential hydrocarbons in the rock when saturated to a given capillary pressure. It is expressed as a fraction of the rock bulk volume. The Bulk Volume Irreducible (BVI) is the fraction of the rock bulk volume that will be occupied by water at the same capillary pressure. The fraction of the rock pore volume that will only be occupied by water is called the irreducible water saturation (Siwr = BVI/cj>). The amount of water that is irreducible is a function of the driving force to displace water, i.e., the capillary pressure. Usually the specified driving force is an air-water capillary pressure of 0.69 MPa (100 psi). 

Interpretation for irreducible water saturation assumes that the rock is water-wet or mixed-wet (water-wet during drainage but the pore surfaces contacted by oil becomes oil-wet upon imbibition). If a porous medium is water-wet and a nonwetting fluid displaces the water (drainage), then the non-wetting fluid will first occupy the larger pores and will enter the smaller pores only as the capillary pressure is increased. This process is similar to the accumulation of oil or gas in the pore space of a reservoir. Thus it is of interest to estimate the irreducible water saturation that is retained by capillarity after the hydrocarbon accumulates in an oil or gas reservoir. The FFI is an estimate of the amount of potential hydrocarbon in... 

Interpretation of NMR well logs is usually made with the assumption that the formation is water-wet such that water occupies the smaller pores and oil relaxes as the bulk fluid. Examination of crude oil, brine, rock systems show that a mixed-wet condition is more common than a water-wet condition, but the NMR interpretation may not be adversely affected [47]. Surfactants used in oil-based drilling fluids have a significant effect on wettability and the NMR response can be correlated with the Amott-Harvey wettability index [46]. These surfactants can have an effect on the estimation of the irreducible water saturation unless compensated by adjusting the T2 cut-off [48]. 

NMR has proven to be a valuable tool for formation evaluation by well logging, downhole fluid analysis and laboratory rock characterization. It gives a direct measure of porosity as the response is only from the fluids in the pore space of the rock. The relaxation time distribution correlates with the pore size distribution. This correlation makes it possible to estimate permeability and irreducible water saturation. When more than one fluid is present in the rock, the fluids can be identified based on the difference in the fluid diffusivity in addition to relaxation times. Interpretation of NMR responses has been greatly advanced with the ability to display two distributions simultaneously. 

J. Chen, G. J. Hirasaki, M. Flaum 2004, Effects of OBM Invasion on Irreducible Water Saturation Mechanisms and Modifications of NMR Interpretation, SPE 90141 presented at SPE ATC E, Houston, TX, 26-29 September, 2004. 

Conventionally, the sample is initially saturated with one fluid phase, perhaps including the other phase at the irreducible saturation. The second fluid phase is injected at a constant flow rate. The pressure drop and cumulative production are measured. A relatively high flow velocity is used to try to negate capillary pressure effects, so as to simplify the associated estimation problem. However, as relative permeability functions depend on capillary number, these functions should be determined under the conditions characteristic of reservoir or aquifer conditions [33]. Under these conditions, capillary pressure effects are important, and should be included within the mathematical model of the experiment used to obtain property estimates. 

Procedure. Core floods were carried out in horizontally mounted Berea sandstone cores of length 61 cm and diameter 5 cm. Porosity varied from 18 to 25% and brine permeability from 100 to 800 Jim2. The cores were coated with a thin layer of epoxy and cast in stainless steel core holders using molten Cerrobend alloy (melting point 70°C). The ends of the cores were machined flush with the core holder and flanges were bolted on. Pore volume was determined by vacuum followed by imbibition of brine. Absolute permeability and porosity were determined. The cores were initially saturated with brine (2% NaCl). An oil flood was then started at a rate of lOm/day until an irreducible water saturation (26-38%) was established. 

In Equation 6.22, Sa at Zow is maximum with the limiting value of 1 - Sr, where ST is the irreducible aqueous-phase saturation in the porous medium under study. 

If the dependence of Ar on saturation is known, then it can be used in eq 49 (via eq 50) directly. Nguyen and co-workers and Berning and Djilali assume a linear dependence of Ar on saturation, and most of the other models use a cubic dependence the model of Weber and Newman yields close to a cubic dependence. This last model differs from the others because it obtains an analytic expression for Ar as a function of the capillary pressure (the independent variable). Furthermore, they also calculated and used residual or irreducible saturations, which are known to but have only been... 

The liquid saturations in foam flow are typically close to irreducible liquid saturation. As a result, the liquid saturation in a foam filled medium is generally not a good measure of the quality of the in situ foam, but rather the fraction of pore segments completely filled with liquid. More permeable media, such as unconsolidated media, generally have smaller residual liquid saturations (32,33) and thus tend to have higher gas saturations when foam is flowing. 

When gas alone is injected into a porous medium where foam had been flowing, the liquid saturation can be reduced below the irreducible liquid saturation. It would be expected that the liquid phase becomes discontinuous at this point and the further reduction of the liquid saturation occurs as a result of liquid flowing from the core as bubble train lamellae. This does not occur in conventional gas-liquid displacement, and the lower limit of the liquid saturation corresponds to the irreducible value. 

One important and as yet not fully understood feature of CO2 foam concerns delineation of the length of the foam bank. Multiple pressure tap measurements indicated that the largest pressure drop occurred within 3 to 6 inches around the bulk CO2 phase front. These and additional studies suggest that the foam bank can be relatively short compared to the length of the core. The pressure drop decreased to very low values as brine approaches its irreducible saturation. This is reasonable, since aqueous surfactant lamella cannot form or propagate when there is no mobile brine present. 

Wettability. Wettability of the porous medium controls the flow, location, and distribution of fluids inside a reservoir (7, 28). It directly affects capillary pressure, relative permeability, secondary and tertiary recovery performances, irreducible water saturations, residual oil saturations, and other properties. 

With impermeable woods - and heartwood - the supply of moisture from the interior eaimot keep paee with evaporation of water vapour from the surface, because mass flow of water is not possible and diffusion is a much slower process. Thus the surfaee moisture eontent quiekly falls below fibre saturation and the evaporative front starts reeeding into the wood. Figure 8.6 shows the parabolic moisture content profile for a slowly air-dried impermeable hardwood. Similarly for permeable softwoods that have been dried below the irreducible moisture content, Stamm (1964, 1967b) reported parabolic moisture profiles that are consistent with diffusion of both water vapour and bound water. 

Even with a permeable wood diffusion assumes increasing importance as the average moisture content approaches the irreducible moisture content indeed, in every part of the board where the moisture eontent approaehes this value drying is diffusion controlled. Permeable and impermeable timbers of similar densities should dry from fibre saturation at about the same rate. The behaviour of mixed heart/sapwood boards is eomplieated sinee, at first, there is both an evaporative interface near the sapwood surfaee and one in the interior at the zonal boundary between heart and sapwood. For a board with only a slither of heartwood along one face, mass flow can only move to the sapwood faee so in effeet the board appears to be twice the width than it aetually is. Pang et al. (1994) predieted that such a 50 mm thick board would dry from green to 6% moisture eontent using a 140°C/90°C schedule in 14 hours, compared to 10 hours for sapwood and 11 hours for heartwood. 

Figure 18. Relative permeability to water at residual oil saturation in clean and wettability-modified Berea cores containing different oils, calculated relative to the effective permeability to oil at irreducible water saturation.

For our laboratory work with aqueous foams, oil was generally present at SQIV/. This value was chosen because it can be achieved in a reproducible manner. Oil saturation is largely unaffected by foam generation, because most foamers do not reduce the interfacial tension between oil and water, irreducible water, to capture any sensitivity to water (which, until now, has not been observed) as well as to maintain the porous medium in contact with its preferred wetting liquid. 

In the presence of oil at high temperature, it is harder to obtain accurate saturation data, but three experiments on 0.5% Fluowet OTN in sea water with crude oil at 70 °C in 8-darcy beads gave Sw between 0.04 and 0.08 (21). Comparable residual nonwetting-phase saturations for the same 8-darcy medium in the absence of foam were 0.13 for the residual water saturation to gas, Swrg, in two gas floods and an average of 0.11 for residual water saturation to oil, Swro, in 24 oil floods with a standard deviation of 0.02 (20). Values of Sw below irreducible were also observed by other investigators (39, 40) in studying foams at comparable conditions. In contrast, steady-state foam flow is consistently associated with 5W values above connate (2—4,18, 34). 

Porous Medium. Ottawa sand was used as the porous medium for flow experiments. The porosity and absolute permeability to the water were determined prior to the start of flow experiments (Table I), and the core was then resaturated with live oil by displacing the water. During the resaturation process, almost 98% of the water was displaced. This value of irreducible water saturation (2%), although much lower than the field values, is not exceptional in laboratory tests of this nature. 

SnR irreducible saturation ratio of the nonwetting phase, dimensionless... 

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