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Core Inspection
• Why? Cores offer the best idea of the formation next to the reservoir.
• They also help correlate the open hole logs, giving valuable guidance on pay identification, formation strength, permeability, fluid type, bed tilt, reworking, etc.
8/25/2015 1 George E. King Engineering
GEKEngineering.com
Pay Zone and Cores
• BHP – 4990 psi
• Perm – 200 – 700 md
• Porosity – 26%
• Oil – 36 API
• Cloud Pt – 70 F
What does the formation really look like?
Where are the permeability streaks, barriers, lithology changes, permeability variance and weak zones?
From which zones is the produced fluid likely to flow?
What core is missing?
What does the log really tell you?
8/25/2015 2 George E. King Engineering
GEKEngineering.com
H145
H150
H160
TOPS.TOPS
GR
GAPI0 150
AO90
OHMM0.2 200
2950
2975
3000
DE
PT
HM
ET
RE
S
RHO8
G/C31.95 2.95
HTNP
V/V0.45 -0.15
Log Responses – Pay Zone Identification
Take your logs along and correlate them with the logs. Get a Geologist’s comments as well to help understand the rock.
Correlating the log with the core makes the log “come alive” if you can visually identify the types of rock that correspond to the log readings and understand how each rock type will eventually influence completion or production.
8/25/2015 3 George E. King Engineering
GEKEngineering.com
Pay Zone Variance – What would oil stains in some of the core indicate?
• BHP – 5000 psi
• Perm – Clean sand 100 – 1000 md mixed with claystone
• Porosity – 25%
• Oil – 36 API
• Cloud Pt – 68 F
And: Bed tilt to well deviation? Mineralogy changes and barriers? Kh and Kv differences? Soft or reactive zones? Reworking? Minerals like salts, anhydrite, calcite? Natural fractures?
8/25/2015 4 George E. King Engineering
GEKEngineering.com
Mineralogy of Sands
CALCULATED WHOLE ROCK COMPOSITION RELATIVE CLAY ABUNDANCE
(Weight %) Normalized to 100%
Dolomite / Total Mixed-Layer
Quartz Plagioclase K-Feldspar Calcite Fe-Dolomite Siderite Pyrite Clay Illite Kaolinite Chlorite Illite/Smectite *
Core
Core 1 88.77 2.10 2.13 0.27 0.00 0.25 1.08 5.40 49.63 18.47 20.40 11.50
Core 2 88.9 2.4 2.1 0.0 0.1 0.3 0.8 5.4 48.1 17.6 18.8 15.5
Core 3 85.2 3.4 2.4 0.0 0.0 1.2 0.5 7.4 42.7 19.8 20.7 16.8
Core 5 -145 81.6 4.5 2.2 0.0 0.0 2.5 0.5 8.7 46.3 18.0 20.3 15.5
Core 5-150 86.3 3.3 2.1 0.0 0.0 0.8 1.2 6.3 41.3 26.3 20.9 11.5
Cores with similar mineralogy may have sharply different permeabilities. The difference is in depositional energy, reworking, tectonic forces that create fractures, water flow that creates cementation and/or mineral growths in the pores, etc.
8/25/2015 5 George E. King Engineering
GEKEngineering.com
SEM Photos are needed in high clay wells – Just a gross mineralogy analysis may be very misleading
SEMs (scanning electron microscope) photos are needed to determine clay location and type. Clay are most problematic when they occupy the pores (authogenic), rather than just emdedded in the matrix (detridal). The form of the clay may also be very important – clays have a million times more surface area than the rock in authogenic form and can be very reactive to specific liquids.
The kaolinite on the left is in a weathered form, and reactivity may range from highly reactive to near totally inert.
8/25/2015 6 George E. King Engineering
GEKEngineering.com
SEM Photomicrographs – Near Unconsolidated Sands
The strength of the rock depends on the extent and stability of the cementation between the grains.
Cementation types include: calcite, quartz overgrowth, clay, heavy oil, cohesion by oil phase and some precipitated minerals.
The level of cementation can be significantly affected by presence (or absence) of pore water, mineral forming reactions, reworking, arching of upper formations (overburden presence or absence), pressure development, etc.
Core testing is very helpful in understanding the rock, but care is
needed to get representative samples.
8/25/2015 7 George E. King Engineering
GEKEngineering.com
Lessons from core
• Take the log – see what the rock looks like for the various log readings.
• See how the core changes: the location of the barriers, channels, fractures and mineralogy variance.
• What is missing from the recovered core? Why?
• Oil stains, bedding planes, tilt, mineral streaks, layered and laminated core….
8/25/2015 8
George E. King Engineering GEKEngineering.com