1. NEED FOR BETTER KNOWLEDGE OF IN-SITU
UNCONFINED COMPRESSIVE STRENGTH OF
ROCK (UCS) TO IMPROVE ROCK
DRILLABILITY PREDICTION
V.C. KELESSIDIS
Technical University of Crete
Mineral Resources Engineering
AMIREG 2009
Athens, September 7-9, 2009
2. Research aim
Optimization of drilling rates
Less expensive and safer drilling practices
Hydrocarbon, geothermal, mining, water well
drilling
Multitude of parameters affecting drilling
performance – Rock Strength
Availability of data and proper modeling
software
Optimum combination better drilling rates
22
3. The problem
Drilling allows for access to subsurface
target areas
Pythagoras saying ‘whoever digs, finds, he who
never digs, will never find’
Drilling is expensive
Optimum drilling practice arrive to targetOptimum drilling practice arrive to target
in the most economical way, but with safety
Main monitoring parameter – Penetration Rate
(m/h)
Depends on two main groups
Formation
Drilling parameters 33
4. Main parameters
FORMATION
Local stresses
Rock compaction
Mineralogical content
Fluid pore pressureFluid pore pressure
DRILLING
Weight on bit and torque
Rpm
Hydraulic parameters
Bit condition 44
5. Modeling drilling process – Teale (1965)
Rock-bit interaction
Energy to the bit
Efficiency of energy transfer
( )( )( )
ROP
AWOBDRPM
A
WOB
SE bit
t
/8 µ
+=ENERGY PER
UNIT VOLUME
55
ROPA
SE
bit
t +=
ROCK-BIT MODEL tSE
eff
UCS
=
UNIT VOLUME
( ) ( )( )
Abit
WOB
eff
UCS
AbitWOBDRPM
ROP
−
=
/)(8 µ
6. UCS - measurements
Laboratory
standard procedures (ISRM 1978, ASTM
1984)
expensive & time consuming
need coreneed core
Our analysis, petroleum & mining
Reported data
Extremely large variability & for different
rocks
Does the name of the rock IMPLY rock
strength ? 66
8. Rock Strength – Factors affecting it
Weathering
how to account for this ?
Weak and Strong rock
definitions ?
FACTORS INFLUENCING MAIN ISSUEFACTORS INFLUENCING
poor cementation
weathering
tectonic disturbance
porosity
mineral composition
particle size
… 88
MAIN ISSUE
GET FAIR
ESTIMATE OF UCS
10. Indirect UCS estimation
Measurements time consuming & expensive
Require core data
VARIETY OF ESTIMATION METHODS
From cuttings, fair successes, LAG TIME !
Schmidt hammer test
Point load test
Impact strength test
Multitude studies, R2 ~ 0.40 to 0.90
1010
Require sample
Less expensive thanLess expensive than
UCS measurment
Still time consuming
11. Estimation of UCS from sonic data
Non destructive
Sonic velocity - APPLIED ONSITE !
Use of ultrasonic pulses
Speed of sound depends onSpeed of sound depends on
rock density, stiffness, mineralogy
grain size
weathering
stress levels
water absorption, water content
temperature
1111
12. Khaksar et al., SPE 121972
2009 research, Oil Well drilling
derive Apparent Strength from porosity logs
Aim, Use the logs to estimate UCS for
improving drilling parameter values, wellbore
stability, sanding
26 correlations for sandstones26 correlations for sandstones
11 correlations for shales
7 correlations for carbonates
Poor estimates, can be improved with data an
better analysis techniques (fuzzy logic,
pattern recognition) 1212
13. UCS from sonic data
Equations of the form
b
pVaUCS ⋅=
( )
_ 1
K
K
UCSA =
1313
( ) 2
40
_ K
ct
UCSA
−∆
=
pV
eUCS
/035.0
000,143
−
⋅= Mc Nally
99.1170642.0 −⋅= pVUCS
1987
14. Data versus correlations !
USA data
UCS versus Sonic
Travel Time
Oyler et al., 2008
1414
Data from 10 wells
Apparent Rock Strength
from log data
Andrew et al., 2007
15. Predictions, Simple versus Complex
Zhou et al., 2005
Use of all available geophysical log data
3 wells
2 techniques for data processing
Plus McNally Equation
1515
R2 ~ 0.62 to 0.75
Poor prediction !
Data, site specific !
MEASURE &
CALIBRATE
17. What is the impact of UCS errors ?
With use of fairly accurate oil-well drilling
simulator
Uses UCS as main input parameter
Can be tuned with real drilling data
Once ‘tuned’ evaluate different scenariosOnce ‘tuned’ evaluate different scenarios
E.g. effect of a higher UCS than the one used
Tested on several wells - Example case here
Shale, soft sand, hard sand
SCENARIO: Increase of UCS by 50% (+/-100%)
1717
18. Increase in UCS by 50%
UCS+50%
1818
Increase may range between 58
and 96%, giving an overall
increase in total drilling time for
the sections chosen for the
simulation of 82%
UCS
19. Conclusions
Need good drilling rate models
Related to rock drillability (RD)
In their absence, RD ~ UCS
There are standard procedures for UCSThere are standard procedures for UCS
measurements
Time consuming, expensive, need cores
Indirect methods exist
1919
20. Conclusions
In situ estimation sonic velocity
measurement
Multitude of correlations, UCS – Vp
Correlation coefficients LOW!, 0.50 to 0.70Correlation coefficients LOW!, 0.50 to 0.70
Inaccuracies in UCS estimation impacts
strongly rock drillability prediction
Example case: Increase (error) of UCS by
50% decrease in ROP by 82%
2020
