Post on 14-Oct-2020
Quantifying Gas Saturation in Tight Gas Sand Reservoirs
Behind Casing
Chiara Cavalleri
Principal Petrophysicist, Schlumberger
Aberdeen, 2-October-2019
2
▪ Introduction
▪ Gas Saturation Monitoring
▪ A New Formation Nuclear Property
▪ Case Study
▪ Evaluation Results
▪ Summary
Outline
▪ Cased-hole formation evaluation has a primary role for the proper description of the existingreservoir systems and also new players in particular conditions.
▪ Help finding that additional drop of oil
▪ Assisting completion design or intervention programs
▪ Characterization of new reserves managing costs and operational risks.
▪ Alternative saturation methods are available; with or without open hole logs
▪ Current technology is the enabler, even in tough logging conditions.
Introduction
Logging Behind Casing.…mostly with open hole logs
Sigma and Full Spectroscopy Large
Pulse neutron and Spectroscopy Slim
Pulse neutron, Inelastic GR, Full Spectroscopy Slim
0 5 10 15 20
CH res. slim
CH resistivity large
CH Density
Acoustic Wide Band Frequency
Acoustic Slim
Neutron Poro Thermal
Neutron Porosity Epithermal
Spectral Gamma Ray
Gamma Ray
Casing or hole size rangeCasing / Completions
Single Multiple
×× ×× ×××× ×× ××××× ×× ×
Pulsed Neutron Interactions
Neutron interactions Neutron pulse Measurements-ray signal
Sigma & Porosity Log• Formation sigma
• Formation porosity
• Near/Far burst and inelastic ratios
• Borehole salinity
capture
inelastic
Inelastic & Capture Log• C/O Saturation
• Three Phase Holdup
• Lithology
Water Flow Log• Water velocity
• Water flow index
1500 ms
100 ms
seconds
Energy
Time decay
Time of flight
Fast Neutron
Nucleus
Excited Nucleus
Gamma Ray
Inelastic Scattering
C,O,Si,Ca,Fe,S,Mg
Fast Neutron
Nucleus
Excited Nucleus
Gamma Ray
Inelastic Scattering
C,O,Si,Ca,Fe,S,Mg
Slow Neutron
Nucleus
Excited Nucleus
Gamma-rayNeutron Capture
Cl,H,Si,Ca,Fe,Ti,Gd,S
Slow Neutron
Nucleus
Excited Nucleus
Gamma-rayNeutron Capture
Cl,H,Si,Ca,Fe,Ti,Gd,S
gSlow Neutron
Oxygen
Gamma-ray
Oxygen Activation
Activated Nucleus
(T 1/2 = 7.1 sec)
gSlow Neutron
Oxygen
Gamma-ray
Oxygen Activation
Activated Nucleus
(T 1/2 = 7.1 sec)
Ref.: Schlumberger
0 p.u.
limestone
10 p.u. gas-
filled
limestone
10 p.u. water-
filled
limestone
Conventional CH PNL Cannot Differentiate Gas-Filled Porosity from Low Porosity
Different flavors of GR ratios
cannot differentiate gas-filled
porosity from low porosity
Why A New Independent Cased Hole Gas Measurement OH Gas Detection Based on Density/Neutron Logs
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99 102 105 108 111 114 117
Sand
ston
e /
Dol
omite
Lim
esto
ne
Anh
ydrit
e
silt
20pp
k w
ater
Sea
Wat
er
Fresh Water 255 ppk
Gas Oil Shale
Increasing sigma values
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99 102 105 108 111 114 117
Sand
ston
e /
Dol
omite
Lim
esto
ne
Anh
ydrit
e
silt
20pp
k w
ater
Sea
Wat
er
Fresh Water 255 ppk
Gas Oil Shale
water
0 p.u.
limestone
10 p.u. gas-
filled
limestone
10 p.u. water-
filled
limestone
Conventional CH PNL Cannot Differentiate Gas-Filled Porosity from Low Porosity
Sensitive to gas filled
porosity variations;
no dependence on HI
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9
Sand
ston
e
Lim
esto
ne
Shal
e
Dol
omite
Water
Gas / Co2 Oil
Increasing FNXS values
OH Gas Detection Based on Density/Neutron Logs
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99 102 105 108 111 114 117
Sand
ston
e /
Dol
omite
Lim
esto
ne
Anh
ydrit
e
silt
20pp
k w
ater
Sea
Wat
er
Fresh Water 255 ppk
Gas Oil Shale
Increasing sigma values
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99 102 105 108 111 114 117
Sand
ston
e /
Dol
omite
Lim
esto
ne
Anh
ydrit
e
silt
20pp
k w
ater
Sea
Wat
er
Fresh Water 255 ppk
Gas Oil Shale
water
Why A New Independent Cased Hole Gas Measurement
▪ Probability of fast neutrons to interact with atoms; directly sensitive to atom density and gas filled porosity, independent of hydrogen index
Gas Saturation from Fast Neutron Cross-section Measurement
0 0.2 0.4 0.6 0.8 15
5.5
6
6.5
7
7.5
8
8.5
HI
FN
XS
(1
/m)
fresh water
CH4 (0.1g/cc)
Material SIGMA (cu) TPHI FNXS (1/m)
Sandstone 4.55 0 6.84
Limestone 7.08 0 7.51
Dolomite 4.7 0 8.51
Shale 20 to 40-50 0.2-0.4 8.02
Fresh Water 22 1 7.8
25ppk Water 30.1 0.97 7.74
Diesel 23.84 1 7.85
CH4(0.1g/cc) 5 0.08 1.34
CO2(0.6g/cc) 0.03 -0.12 2.24
A Formation
Nuclear Property
Input to Mineral
Solver Model for
CHFE
Inelastic
Gamma Ray
Epi-thermal
Capture
Thermal Capture
GR Counts Rate vs Time
TR
DT
• Gas producing field
• Alternating very low-porosity sandstone and gas-filled
zones, within a tight limestone matrix.
• Mixed salinity effects
• Improved logs evaluation required to guide perforation
• Cased hole logging for formation evaluation performed
behind single and multiple casing strings
Tight Gas Field Evaluation Example
Independent Gas Measurement: FNXS
Producing field; heterogeneous rock with low porosity gas-filled and very low porosity zones alternating.
Need a new measurement to differentiate
gas filled porosity from very low porosity
Low Sigma and porosity, counts ratio change and constantly low – Gas?
Conventional methods are inconclusive due to low porosity and
contrast
Independent Gas Measurement: FNXS
FNXS deflection correlates to gas volumes changes in the formation
Producing field; heterogeneous rock with low porosity gas-filled and very low porosity zones alternating.
FNXS show liquid in this zone (previous PNL and methods might have flagged
low water-filled porosity as gas - erroneously)
FNXS vs FSNX of matrix (dry)
support the fluids identification
Prompt Gas Volume and Porosity
FNXS and Neutron porosity combined for QL crossplot porosity and gas volume as typically done with N-D logs
Increasing fluid filled porosity
Good contrast between low
porosity and gas-filled porosity
Quick gas volume and porosity estimate
with FNXS and Neutron Porosity
FNXS vs Neutron Porosity
Standalone Volumetric Formation EvaluationLinear volumetric measurements, with the addition of elemental composition and matrix calculation, allow quantitative analysis.
▪ Comprehensive and multi-mineral solver
analysis and porosity computation where
original conventional logs had difficult to
describe the lithology and porosity; hence
provide a clear understanding of storage
capacity and fluids’ dynamics.
▪ Finding quality pay in deeper reservoir,
otherwise not accessible
De-risking Deep Gas ReservoirsData recorded much deeper to characterize deeper low-porosity sandstone that could not be evaluated before
With most
porosity <5%
Open hole logging deepest evaluation depth
• Novel independent methods for saturation monitoring from logs behind casing and withincompletion extend the evaluation envelope to complex rock and wellbores.
• In the case study, fluids saturations are obtained without complex nuclear models nor theneed for formation salinity input, assumptions otherwise required for the data interpretation.
• And the cased hole data could be packaged to as if it was a conventional open holeacquisition for fast correlation and integration to other wells data.
• The information is key to define field development strategy.
In summary
Another Application: Making Informed Decision for Field Abandonment
▪ Intervention campaign in preparation to rig plugging and abandonment; gas accumulation in a well require attention and studyto exclude any field wide-challenge
▪ Pulse Neutron logging with FNXS accurately describe the wellbore environment, confirming liquid in the borehole and annuli
▪ The logging technology also measured formation despite large hole sizes, multiple casings strings, unknown annular conditions.
Correlation with open hole log
Paper # • Paper Title • Presenter Name
Slide 17