RESERVOIR PETROPHYSICS

RESERVOIR PETROPHYSICS LABORATORY DETERMINATIONOF POROSITY

POROSITY DEFINITION Porosity: The fraction of a rock that is occupied by pores • Porosity is a static property – it can be measured in the absence of flow • Determining effective porosity requires fluid flow to determine if pores are interconnected

matrix pore space ROCK MATRIX AND PORE SPACE

MEASUREMENT OF POROSITY • Core samples(measure two of: Vb, Vp, or Vm) • Openhole wireline logs

LABORATORY DETERMINATIONOF POROSITY • Most methods use small samples (core plugs) • multiple samples must be analyzed to get statistically representative results • sampling technique is important • often all samples are taken from “sweet spots” skewing analysis • To determine porosity, measure 2 of 3 • volumetric parameters: • Bulk volume, Vb • Matrix volume, Vm (also called grain volume) • Pore volume, Vp

Volume is an extensive property Fraction of volume consisting of pores or voids Fraction of volume consisting of matrix

MATRIX DENSITIES (m) OF TYPICAL PURE COMPONENTS OF RESERVOIR ROCK

LABORATORY METHODS OFPOROSITY DETERMINATION • Bulk volumedeterminations • Direct calculation • Fluid displacement methods • Gravimetric • Volumetric – mercury pycnometer ( a precisely calibrated bottle)

BULK VOLUME BY DIRECT MEASUREMENT • Applicable for regularly shaped cores or core plugs • Calculate from core dimensions • For example; volume of right circular cylinder

LABORATORY METHODS OFPOROSITY DETERMINATION • Bulk volumedeterminations • Direct calculation • Fluid displacement methods • Gravimetric (Archimedes) methods • Volumetric – in pycnometer

Wdry Wsat Wsub Vp = Wsat - Wdry fluid Vm = Vb = Wdry - Wsub fluid = Wsat - Wdry Wsat - Wsub Wsat - Wsub fluid ARCHIMEDES METHOD

EXAMPLE 1Bulk Volume Calculated by Displacement A core sample coated with paraffin immersed in a container of liquid displaced 10.9 cm3 of the liquid. The weight of the dry core sample was 20.0 g, while the weight of the dry sample coated with paraffin was 20.9 g. Assume the density of the solid paraffin is 0.9 g/cm3. Calculate the bulk volume of the sample.

SOLUTION - Example 1 Weight of paraffin coating, Wparaffin = Weight of dry core sample coated with paraffin - Weight of dry core sampleWparaffin = 20.9 g = 20.0 g = 0.9 g Volume of paraffin coating = Weight of paraffin / density of paraffin Vparaffin = 0.9 g / 0.9 g/cm3 = 1.0 cm3 Bulk volume of core sample = (Bulk volume of core coated with paraffin) – (volume of paraffin) Vb= 10.9 cm3 – 1.0 cm3 = 9.9 cm3 (V = m/ρ)

(Vb) (Vm) (Vp) LABORATORY METHODS OF POROSITY DETERMINATION • To determine porosity, measure 2 of 3 • basic parameters: • Bulk volume • Matrix volume • Assume matrix (grain) density • Displacement method • Boyles Law • Pore volume

LABORATORY METHODS OFPOROSITY DETERMINATION • Matrix(Vm) • Assume rock density based on lithology and measure dry mass • Displacement methods • volumetric • gravimetric (see previous description) • Boyle’s Law:

MATRIX VOLUME FROM MATRIX DENSITY • Known or assumed matrix density

APPLICABILITY AND ACCURACY OF MATRIX MEASUREMENT TECHNIQUES • Known or assumed matrix density • Accurate only if matrix density is known and not assumed • Core samples are often mixtures of several components with varying matrix densities, so density must be measured

(Vb) (Vm) (Vp) LABORATORY METHODS OF POROSITY DETERMINATION • To determine porosity, measure 2 of 3 • basic parameters: • Bulk volume • 2. Matrix volume • Assumed matrix (grain) density • Displacement method • Boyles Law • 3. Pore volume

MATRIX VOLUME FROM DISPLACEMENT METHOD • Reduce sample to particle size • Measure matrix volume of particles by • Volumetric method • Archimedes method (gravimetric measurement)

EXAMPLE 2SOLUTIONCalculating the Matrix Volume and Porosity of a Core Sample Using the Displacement Method

9.9 cm3 – 7.7 cm3 = 9.9 cm3 SOLUTION - Example 2Calculate the Porosity of a Core Sample Using the Displacement Method and Matrix Volume The core sample from Example 1 was stripped of the paraffin coat, crushed to grain size, and immersed in a container with liquid. The volume of liquid displaced by the grains was 7.7 cm3. Calculate the matrix volume and the core porosity. Is this effective porosity or total porosity? (It is total porosity) Bulk Volume, Vb = 9.9 cm3 Matrix Volume, Vma = 7.7 cm3 = 0.22

(Vb) (Vm) (Vp) LABORATORY METHODS OF POROSITY DETERMINATION • To determine porosity, measure 2 of 3 • basic parameters: • Bulk volume • 2. Matrix volume • Assumed matrix (grain) density • Displacement method • Boyles Law (Gas Expansion) • 3. Pore volume

MATRIX VOLUME FROM GAS EXPANSION METHOD • Involves compression of gas into pores • Uses Boyle’s law

GAS EXPANSION METHOD TO CALCULATE Vma • Initial conditions, with volumes of 2 cells known • Place core in second cell, evacuate gas (air) from second cell • Open valve

GAS EXPANSION METHOD TO CALCULATE Vma P1 Initial conditions Core V1 Valve closed Evacuate Cell 2 Cell 1

GAS EXPANSION METHOD TO CALCULATE Vma Final conditions P1 P2 Core Valve open Cell 1 Cell 2

GAS EXPANSION METHOD TO CALCULATE Vma • Vf = Volume of Cell 1 + Volume of Cell 2 - Matrix Volume of Core • Vt = Volume of Cell 1 + Volume of Cell 2 • Vm = Vt - Vf

APPLICABILITY AND ACCURACY OF MATRIX MEASUREMENT TECHNIQUES • Displacement method - Very accurate when core sample is crushed without destroying individual matrix grains • Gas expansion method - Very accurate, especially for samples with low porosities Neither method requires a prior knowledge of core properties

LABORATORY METHODS OF POROSITY DETERMINATION • To determine porosity, measure 2 of 3 • basic parameters: • Bulk volume • Matrix volume • Pore volume (Vb) (Vm) (Vp)

LABORATORY METHODS OF POROSITY DETERMINATION • Pore volume determination (Effective) • Gravimetric (Archimedes) • Wsat - Wdry • fluid • 2. Boyle’s Law: • (Gas expansion) Vp =

PORE VOLUME FROM SATURATION METHOD • Measures the difference between the weight of a core sample saturated with a single fluid and the dry weight of the core • Pore volume,

EXAMPLE 3Archimedes MethodofCalculatingPorosity a Core Sample Using the gravimetric method with the following data, calculate the pore and bulk volumes and the porosity. Is this porosity total or effective? Dry weight of sample, Wdry = 427.3 g Weight of sample saturated with water, Wsat = 448.6 g Density of water (f ) = 1.0 g/cm3 Weight of saturated sample submerged in water, Wsub = 269.6 g

Wsat – Wdry = f 448.6 – 427.3 g 1.0 g/cm3 Vp = = 21.3 cm3 Wsat – Wsub = f 448.6 – 269.6 g 1.0 g/cm3 Vb = = 179.0 cm3 21.3 cm3 = = 0.12 179.0 cm3 EXAMPLE 3SolutionArchimedes MethodofCalculatingPorosity a Core Sample

Applicability and Accuracy of Pore Volume Measurement Techniques • Saturation (Archimedes) method • Accurate in better quality rocks if effective pore spaces can be completely saturated • In poorer quality rocks, difficult to completely saturate sample • Saturating fluid may react with minerals in the core (e.g., swelling clays)

LABORATORY METHODS OF POROSITY DETERMINATION • Pore volume determination (Effective) • Gravimetric (Archimedes) • Wsat - Wdry • fluid • 2. Boyle’s Law: • (Gas expansion) Vp =

PORE VOLUME FROM GASEXPANSION METHOD Initial conditions P1 Core V1 Valve closed Cell 1 Cell 2

PORE VOLUME FROM GASEXPANSION METHOD Final conditions P1 P2 Core Valve open Cell 1 Cell 2

PORE VOLUME FROM GAS EXPANSION METHOD • Very accurate for both high-quality (high ) and low-quality (low ) core samples • Should use low-molecular-weight inert gases (e.g., helium) • Measures effective (connected) pore volume

SUMMARY To determine porosity, measure 2 of 3 basic parameters: • Bulk volume • Matrix volume • Pore volume

CORES • Allow direct measurement of reservoir properties • Used to correlate indirect measurements, such as wireline/LWD logs • Used to test compatibility of injection fluids • Used to predict borehole stability • Used to estimate probability of formation failure and sand production

SOME KEY FORMULAS

CROSS BEDDING, CARRIZO SANDSTONE

RESERVOIR PETROPHYSICS