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8/13/2019 Fluid Data

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2.5 INTERPRETATION OF THE FLUID DATA

The data gathered with the different techniques have to be interpreted in terms of the fluids contained in theporespace of the reservoir. The following information about the reservoir fluids are required: Fluid Type Fluid Saturation Fluid Properties

For the interpretation of the fluid data it is important to have a good understanding of the boreholeenvironment. The borehole environment is described first. Then for each tool, used to evaluate the above,the principle and evaluation techniques are discussed. The section ends with a discussion of the techniquesused to determine the fluid distribution in the reservoir.

THE BOREHOLE ENVIRONMENT

We are interested in a description of the reservoir as it is in the subsurface. When a well is drilled to gatherinformation, the natural state of the fluids in the reservoir is disturbed, at least locally around the well in theregion were data are gathered. The effect of this disturbance must be taken into account in anyinterpretation.

Mudcake and InvasionWhen a well is drilled through a porous and permeable layer, the pressure in the well has to be kept abovethe pressure of the formation fluid to prevent an influx of formation fluid into the borehole. The pressuredifferential forces drilling fluid into the formation. As it enters the formation, the suspended solids in the mudare filtered out and plaster the borehole wall to form a mudcake. The mud filtrate enters into the formation

and displaces formation fluid away from the wellbore. Figure 2.59 illustrates the process schematically.Once the mudcake has reached a certain thickness, it will effectively seal of the formation and preventfurther invasion by the filtrate. To reach this mudcake thickness, a certain amount of mud filtrate must invadethe formation, and will remain in the pores. A highly porous rock will be able to accommodate more mudfiltrate in a given rock volume than a rock with low porosity. The depth of invasion by the filtrate will be lessin a rock with high porosity.

The build up of a mudcake is a dynamic process. It may take several hours for a mudcake to fully build upif it is left undisturbed, but for much of the time the mudcake is being continually worn away by thedrillstring rubbing on the walls of the borehole while drilling or tripping. The mudcake which is worn awayis replaced by new mudcake, and more filtrate invades the formation.

The invading mud filtrate increases the pressure in the formation near to the borehole, creating a pressuregradient away from the borehole out into the formation. This pressure gradient provides a driving force todisplace formation fluids and excess mud filtrate away from the borehole. Eventually, stable saturation andpressure profiles are formed through the invaded zone.

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Invasion ProfilesIn a water bearing zone all the formation water will be replaced by mud filtrate. In an oil or gas bearingzone residual hydrocarbons will be left behind as a result of capillary effects. The diameter of an invadedzone is typically up to 1m. Figure 2.60 a & b shows typical invasion profiles for a water bearing and an

oil bearing zone.

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In a high permeability interval the formation fluids and mud filtrate can flow rapidly away from theborehole, and there is no detectable pressure gradient in the formation. The pressure differential betweenthe borehole and the formation is supported entirely by the mudcake. The pressure gradient through the

invaded zone in a low permeability zone may be quite large, because the fluids can flow only slowly awayfrom the borehole. This pressure gradient is maintained while drilling, because mud filtrate continues toinvade the formation as fresh mudcake builds up, but will decline slowly if the borehole is left undisturbed.This often complicates recording the true formation pressure in a low permeability zone by wirelinetechniques.

FORMATION RESISTIVITY

Well log information is required to quantify the hydrocarbon saturation, i.e. the fraction of the pore spacefilled with hydrocarbons. The most common logging technique for saturation determination is resistivity

logging. The resistivity of the formation is measured by passing an electrical current through it.Formation Resistivity The resistivity of a substance is defined as its electrical resistance per unit volume. It is therefore a propertyof a substance that is independent of the amount of the substance being considered.

The resistivity of reservoir rock is determined by the way in which an electric current can flow through it. Asthe formation water is the only conductive material present in the rock, it depends on: The resistivity of the formation water

Depends on temperature, and the concentration of salts dissolved in the water (i.e. its salinity). The amount of formation water

Depends on the amount of pore space, and the extent to which this pore space is filled with formationwater (i.e. water saturation). The geometry of the formation water

The more tortuous the path followed by the current, the higher will be the resistivity.




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In the case of water bearing rock it will be easier for the current to flow than in the presence of hydrocarbons. Figure 2.61 illustrates this point. The electrical resistivity of a pore system is described by theArchie equations. These equations may be used to quantify the hydrocarbon saturation from the measuredresistivity.

FIRST ARCHIE EQUATION (WATER BEARING FORMATION)

The resistivity of a water bearing formation will be proportional to the resistivity of the formation water inthe pores, and inversely proportional to the amount of pore space available to contain formation water. In

fact the formation resistivity is inversely proportional to the porosity raised to a power, because of thetortuosity of the path followed by the current. This power is expressed as the cementation exponent.

Ro = phi(-m)* Rw




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The cementation exponent varies according to rock type. Typical values range from 1.4 for unconsolidatedsand, to 2.2 or higher in some types of limestone. It can be determined on core plugs, provided theporosity can be measured, by filling them with water of known resistivity and then measuring the formationresistivity.

Second Archie Equation (Hydrocarbon Bearing Formation)When hydrocarbons are present in the pore system, their presence reduces the amount of pore spaceoccupied by conductive formation water. The formation resistivity is inversely proportional to the watersaturation. In fact the formation resistivity is inversely proportional to the water saturation raised to a power,because of the tortuosity of the path followed by the current. This power is expressed as the saturationexponent.

The saturation exponent (n) varies with rock type. Typically it has a value of about 2. It can be determinedon core plugs, by filling a sample of known porosity with water of known resistivity, and then measuring theresistivity of the sample as a function of the water saturation as the water is replaced by oil.

Combining the first and second Archie equation gives:

This can be solved for the hydrocarbon saturation

m cementation exponent from cores, or estimated for the lithology n saturation exponent from cores, or estimated for the lithology




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In any resistivity measurement, in fact in any log measurement, there will be the unwanted influence of theborehole. Logging tools are designed in such a way that these borehole influences are minimized undernormal conditions. Correction procedures, which take into account the borehole size, mud resistivity, etc.,are available to remove the remaining influences of the borehole. These procedures are described in aseries of correction charts that are available from each logging contractor.

Tool Principle: The LaterologThe laterolog emits a "measuring" current into the formation from one electrode, and "focussing" currentsfrom a series of auxiliary electrodes positioned symmetrically about the measuring current electrode. Thisfocuses the measuring current into a sheet to obtain the best tool resolution (Figure 2.63). The focussing

currents can be adjusted so that the tool simultaneously measures the "deep" resistivity (Figure 2.63 left),and the "shallow" resistivity (Figure 2.63 right). The shallow resistivity is better described as an intermediate

resistivity. This gives two of the three independent resistivity measurements. As current flows from the toolinto the formation, the laterolog is particularly suited for use with conductive borehole fluids, i.e. salty,water based muds. The tool will not work in oil based mud.

Tool Principle: The Induction LogThe induction log is based on an entirely different principle. A current is induced in the formation aroundthe borehole by electromagnetic coupling with an alternating current (~20 kHz) flowing in a coil inside thetool. The induced current flowing in the formation induces a response in a receiver coil in the tool (Figure2.64).

The response can be analyzed in terms of formation conductivity, the reciprocal of resistivity. By adjusting

the arrangement of receiver coils, the formation resistivity can be measured at a longer or shorter distancefrom the borehole. This gives two of the three independent resistivity measurements, a deep reading and anintermediate reading.




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As there is no direct flow of current from the induction tool to the formation, this tool can be used with lowconductivity borehole fluids, such as fresh water muds or oil base muds. The resolution of the tool is not asgood as that of the laterolog, because the arrangement of the coils does not allow sharp focussing of themeasurements.

Tool Principle: The Microresistivity LogBoth the laterolog and the induction log give two of the three independent resistivity measurements, a deep

and an intermediate reading. A shallow resistivity reading can be provided by a microresistivity device.The most widely used of such devices is the micro spherically focussed log (MSFL).

The measuring device is a rubber pad with rectangular electrodes on it, which is pressed against theborehole wall (Figure 2.65). Current emitted from the central electrode is focussed by means of currentsfrom the other electrodes. The resistivity of essentially only the invaded zone is obtained.




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The tool has a very good resolution as a result of the pad geometry. The tool will not work in oil basedmud, because current has to flow from the pad into the formation.

Interpretation of three resistivity logsCharts are available to determine the true formation resistivity from the three resistivity measurements atdifferent depths of investigation (e.g. in SchIumberger Chart books).




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The log shown in Figure 2.66 is an example of the three readings, recorded over a sand interval. Above833 m there is a clear separation between the curves. Close to the borehole the resistivity is low, due to thepresence of mud filtrate. Going deeper into the formation the resistivity increases. This strongly suggests thepresence of hydrocarbons. The true formation resistivity will be even higher than the deep resistivity devicereading. Below 833 m no separation is observed between the curves. They all read low resistivities. Thisindicates that even deep in the formation, where the mud filtrate did not penetrate, the resistivity is still low.We conclude that the formation is water bearing. The hydrocarbon water contact is at 833 m.

The reading of the deep resistivity tool is usually taken as the true formation resistivity for a quick lookevaluation. This ignores the influence of the invaded zone, which tends to make the reading of the deepresistivity tool lower than the true formation resistivity. The resulting interpretation is somewhat pessimistic interms of hydrocarbon saturation.

FLUID TYPES AND CONTACTS FROM TOOL COMBINATIONS

Hydrocarbon saturation can be measured by resistivity logs, but where are the fluid contacts, and is thehydrocarbon oil or gas?

Mudlogging and Sidewall SamplingThe first indications about the fluid content of a reservoir are given by the mudlog. Oil stained cuttings andoil shows in the mud indicate oil bearing formation. Gas shows in the mud, especially light components,may indicate the presence of gas. Sidewall samples may distinguish between oil and gas if the formation isknown to be hydrocarbon bearing (e.g. from interpretation of the porosity and resistivity logs) but it is notclear what type of hydrocarbons are present. Strong oil shows are expected in sidewall samples from an oilzone.

Porosity and Resistivity

The resistivity response of a reservoir with a certain formation water is determined by the porosity and thewater saturation. Simple inspection of the three resistivity logs will be sufficient to distinguish hydrocarbonsfrom water if the porosity is greater than about 20% and fairly constant, (see Figure 2.68)

If there is a significant variation in porosity, the porosity and resistivity logs are used simultaneously todistinguish between hydrocarbons and water (Figure 2.67). Across a water bearing intervals a porosity log(best to use the density log) will "tramline" with the deep resistivity log. When the porosity curve goes to theleft (indicating a higher porosity) the resistivity curve will do the same (indicating a lower resistivity) andvice versa. In doing so, both measurements follow the first Archie formula, which is an indication of waterbearing formation.

Across hydrocarbon bearing intervals the resistivity will react in the opposite manner ("Mae West"). This iscaused by an increasing hydrocarbon saturation with increasing porosity as a result of reduced capillaryeffects. In hydrocarbon bearing rock the absolute amount of water present in the formation is inverselyrelated to the porosity.




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Density and NeutronResistivity logs cannot distinguish between oil and gas, because any hydrocarbon is a nonconductivematerial and will cause the same response on a resistivity log. The combination of density and neutron logsover a hydrocarbon bearing zone can distinguish between oil and gas, as explained in section 2.4(See also Figure 2.68).




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Uncertainties in Determining Fluid ContactsThe methods described can give an accurate insight into the fluid distribution in the reservoir, provided theconditions are favourable. In less ideal conditions (for example low permeability, shaly reservoir, alternatingthin layers of reservoir and shale, insufficient well penetration), the log and pressure data may not be ableto exactly pinpoint the fluid contacts. In such cases limits on the positions of the fluid contacts can be

defined. The position of the OWC is bounded by a "water up to" (WUT), and an "oil down to" (ODT). TheGOC position is bounded by an "oil up to" (OUT), and a "gas down to" (GDT). Examples of these limitscan be seen on the logs in Figure 2.69.




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Pressure vs. DepthThe fluid distribution in a reservoir can be determined independently by analysing the pressure profileacross the reservoir interval. Pressure measurements at various depths can be obtained with a wirelineformation tester (see chapter 2.5). These are plotted on a graph of pressure versus depth to show thepressure gradients through each of the reservoir fluids (Figure 2.70). The intersection of the oil and watergradients defines the depth of the free water level (FWL). The intersection of the gas and oil gradientsdetermines the gas oil contact (GOC). The oil water contact (OWC) lies some distance above the FWL,

depending on the capillary behaviour of the rock and fluids. The difference in elevation between the OWC(or the gas water contact, GWC) and the FWL can vary between almost zero in high permeability rock totens of metres in low permeability material.




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