Rock Properties Rock Properties MeasurementsMeasurements

Adrian C Todd

Heriot-Watt UniversityHeriot-Watt University

DEPARTMENT OF PETROLEUM ENGINEERING

Heriot-Watt UniversityHeriot-Watt University

DEPARTMENT OF PETROLEUM ENGINEERING

Core Analysis

Cores recovered from formation contain considerable information about the nature of the rocks and related properties.

Give information on their reaction to fluids displacement.

Core recovery can be influenced by the friable and or unconsolidated nature of the section.

The core provides a time record of the depositional process.

Core Analysis

Routine Core Analysis

Porosity

Permeability

Fluid Saturations

Special Core Analysis Detailed mineral description

Relative permeability studies

steady state/ unsteady state

Capillary pressure

Resistivity measurements

Studies under reservoir conditions

Core Analysis

Core Definitions

Fresh Core

Any newly recovered core material preserved as quickly as possible to prevent evaporative losses.

Fluid used for coring should be noted/

– fresh state ( oil based drilling fluid or water based drilling fluid

Core Definitions

Preserved Core

Similar to fresh core BUT some storage is implied.

Core is protected by various techniques.

Film wrap,

freezing

heat sealable plastic, dips and coatings

Core Definitions

Cleaned core

Core from which fluids have been removed by solvents

Core cleaning process, solvents, sequence and temperatures should be specified.

Special cleaning procedures may be required for sensitive materials

Friable clays - critical point drying.

Illite in sandstone rock

Critical Point Drying

Used to prepare delicate specimens

After conventional core cleaning some rock had higher than expected permeabilities.

Water injectivity test on well gave considerably lower permeability than conventional core cleaned rock.

Critical point drying reduces damage to rock

Critical Point Drying-Procedure Rock immersed in ‘formation water’ at well

sight.

Core recovered, prepared and loaded into core holder under formation water.

Critical Point Drying-Procedure

Water in core displaced with miscible fluid - alcohol.

Alcohol in core displaced with miscible fluid - high pressure carbon dioxide.

Pressure and temperature in core holder changed to go round critical point of carbon dioxide.

Gaseous carbon dioxide exists in core holder at end of test.

Critical Point Drying-Procedure

Core Definitions

Restored - State Core

Core which has been cleaned and re-exposed to reservoir fluids to reestablish reservoir wettability conditions.

Exposure to crude oil, initial water saturation, temperature and time can effect ultimate wettablility.

Core Definitions

Pressure - Retained Core

Core which has been kept as far as possible at the pressure of the reservoir.

Objective to avoid change in fluid saturations during recovery process.

Whole Core Scanning

A feature of modern core analysis.

Can relate whole core characteristics to indirect down hole measurements.

Purpose to recognise, lithological, depositional, structural and diagenetic features.

Can recognise features which might go undetected and generate unrepresentative information in subsequent analysis.

Whole Core Scanning Gamma or X-Ray, Computer Tomography,CT.

Nuclear Magnetic Resonance, NMR

Natural Gamma Scan

X-Ray or Gamma Computer Tomography,CT

Advances in medical applications

Resolution depends on beam thickness and pixel size used to reconstruct the image.

For fluid imaging in pores difficult to get density contrast.

X-Ray or Gamma Computer Tomography,CT

hoI I e

hoI I e

X-Ray or Gamma Computer Tomography,CT

Industrial CAT Scanner

Medical CAT Scanner

Radiograph of the core assembly

Transverse slices

Sample of a porosity slice

What will fluid flow be in this case?

Radiograph

Conglomerate(Adriatic, Italy)

Hole

Crystal

3 cm

Tomograph

Natural fracture

Induced fracture

CAT Scan of Fractured Core

Progressive views though a core

Core Cleaning Sample preparation is important

in core analysis.

Prior to conventional porosity and permeability measurement all oil and water extracted from cores.

Exception saturation measurements of porosity

Conventional cleaning-Soxlet reflux solvent extraction

Porosity Measurements

Porosity MeasurementsPorosity Measurements

Porosity Measurements

Porosity Measurements-Bulk Volume

Archimedes mercury immersion apparatus

Porosity Measurements-Bulk Volume

Volumetric mercury displacement pump

Porosity Measurements-Summation of fluids

Porosity Measurements- Gas Transfer

Porosity Measurements- Resaturation

Pores are filled with a liquid of known density

Whole core versus sidewall samples

It is possible to recover cores with wireline tools.

A core barrel is either exploded into the formation.

This is only suitable for mineral description

Or a side wall corers , these cut into the formation

Little mechanical damage.

Permeability Steady state method

Pressure drop for a fixed flow is measured.

Generally a gas.

Unsteady state method

Flow in transient regime is measured.

Pulse-decay method

pressures set up and downstream of core

Pressure fall off method

low pressure set upstream and time for release through core to atmosphere is measured.

Permeability- Impact of stress Recent years reservoir stresses of greater interest.

Permeability- Impact of

stress orientation

Stress orientation for horizontal core plug

Stress orientation for vertical core plug

Important to examine if the stresses applied

represent those that the rock would be subjected

to in formation

Types of stress - Triaxial stress

Unequal stress is applied to the three major axes.

Strains different on each axis.

A cube or rectangular prism shaped sample used

Steady State Permeability Methods Conventional measurement is to measure pressure

drop associated with a fixed flow rate.

Permeability calculated from Darcy’s Law

Hassler type core holderstress is low, sufficient to ensure flow of

gas does not by-pass core

Permeability measurements to simulate stresses

Isostatic stress condition

Slideable inlet tube enables strain to be taken up.

Permeability measurements to simulate stresses

Sample

High pressure core holder for biaxial loading

Permeability measurements to simulate stresses-True Triaxial Cell

Tubes running parallel along confining rubber sleave are pressured to simulate the stress distribution of interest.

True Triaxial Cell

Probe Permeameter

Recent innovation

Useful to give indications of outcrop permeabilities.

Also being used to exam different levels of permeability measurement. Upscaling

Radial Flow Permeameter

Flow is from outer to inner radius.

Preparation not easy and axial stresses are not balanced by radial stresses

Unsteady state permeability measurement High speed

computers and data acquisition systems enabling unsteady state measurement.

Principles are similar to well testing.

Analysis if pressure draw down or build up.

Pressure - fall off gas permeameter

Unsteady state permeability measurement Pressure decay method

Pressure time behaviour between two initial pressures determined.

Fluid Saturations Because of large variations between formation and

surface core based fluid saturations are not too representative.

Gas Saturation

– Measured by injecting mercury into uncleaned core. Mercury compresses gas.

Oil Saturation

– Measured in retort by distilling off oil.

– Temperatures up to 650oC

Water Saturation

– Measured by atmospheric distillation with the oil content

– Can also be measured using Dean Stark Method

Capillary Pressure

General procedure is to saturate a core with wetting phase and measure how much wetting phase is displaced when subjected to a given pressure of non wetting phase.

Displacement occurs when capillary pressure just exceeds pressure corresponding to largest pore.

Volume of fluid displaced represents volume of all pores of that particular size.

Plot of water volume versus pressure represents capillary pressure versus percentage of the pores with a capillary pressure greater than the subject pressure

Capillary Pressure Measurement Techniques

Desaturation or displacement through porous diaphragm.

Centrifuge method

Dynamic capillary pressure method

Mercury injection method

Capillary Pressure Measurement - Porous diaphragm.

Permeable membrane containing pores much smaller than those of the sample in contact with sample.

Pressure applied to displacing phase

Volume of displaced fluid measured at each pressure step.

Takes considerable time.

Capillary Pressure Measurement - Centrifuge method

High centrifugal force gives higher gravitational force than in natural state.

Plug mounted in centrifuge tube.

Using strobe lighting volume of fluid is measured at each speed.

Increased speed of measurement

Capillary Pressure Measurement - Dynamic method A simultaneous steady

state flow of two fluids is established

Saturation varied by regulating flow of each fluid

Pressure difference between two fluids is measured

Capillary Pressure Measurement - Mercury Injection

Most common procedure

Developed to speed up measurement

Mercury is non wetting

Mercury forced into dry sample under pressure

Volume of mercury injected at each pressure determines non wetting phase saturation

Capillary Pressure Measurement - Mercury Injection-Conversion of data

Necessary to convert laboratory data to fluids of field interest.

Capillary pressure

c

w o

Ph

g

c

w o

Ph

g

Need to consider interfacial tension and contact angle properties

Surface tension of water = 70 dynes/cmSurface tension of mercury = 480 dynes/cmContact angle water/solid= 0 degreesContact angle mercury/solid=140 degrees

c

2 CosP

r

Capillary Pressure Measurement - Mercury Injection-Conversion of data-contd.

c

2 CosP

r

air/mercury

air/water

oc

oc

P 480Cos1405

P 70Cos0

air/mercury air/waterc cP 5P

Interfacial tension and wettability ( contact angle ) depend on fluids. The relationship between mercury/air and

oil/water is usually taken as 10:1

air/mercury water/oilc cP 10P

Capillary Pressure Measurement - Mercury Injection-Conversion of data-contd.

The equations enable the height saturation profile for a reservoir to be generated from laboratory based Pc vs.

saturation capillary pressure data.

cL R

cRL

w h w h

P Cos

Cos Ph

g g

cL R

cRL

w h w h

P Cos

Cos Ph

g g

h = height above free water level corresponding to zero capillary pressure.

R & L denote reservoir & laboratory conditions

Calculation of saturation in layered reservoir

Step 1: conversion of Pc to height scale

Step 2: determine Free Water LevelStep 3: Determine saturation at

each formation change and positions of 100% Sw

Calculation of saturation in layered reservoir

Calculation of saturation in layered reservoir

Positions of 100% water saturation

Averaging Capillary Pressure Data Capillary pressure not a routine core analysis

measurement.

Comprehensive set of Pc curves not always available

Leverett 1941, generated a function to relate Pc with porosity and permeability.

Leverett J Function

Used to generate Pc information when laboratory value not available.

Leverett J Function Based on flow through a core as represented

by a bundle of capillary tubes.

4

cap

r PPoiseuille's equation q

8 L

4

ncap

n r PFor n tubes q

8 L

Leverett J Function

2n rPorosity of bundle of tubes

A

A

coreq LPermeability k

A P

Lcore

Combining equations gives:

cap2

core

L8kr

L

cap

core

L is the 'tortuosity' of the bundle of tubes

L

If in the reservoir rock the tortuosity remains constant then

Leverett J Functioncap2

core

L8kr

L

k

r constant

Substituting in capillary pressure equation c

2 CosP

r

c

2 CosP

kconstant

c

kP

1J

constant Cos

Sometimes J function written without Costerm

Set of Capillary Curves

Leverett J Function

c

kP

ws

Leverett J Function

Data set may not give a good curves

Alternative to plot against specific connate water saturation.

This is considered to reduce the impact of the constant assumed tortuosity

w wcwc

wc

S S where S is connate water saturation

1 S

Modified Leverett J Function

* w wcw

wc

S SS

1 S

cP k

Effective Permeability Two approaches to measuring effective permeabilities

Unsteady state Displacement process where one phase displaces the other.

Flow rates and pressure drops measured as a function of time for a fixed rate process.

Saturations calculated on basis of remaining volumes.

Steady state Range of constant rate tests of coinjected fluids.

Pressure drop measured when equilibrium achieved

Steady State Effective Permeability

Three phase relative permeability a big challenge.

Very important in gas condensate considerations.