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.
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