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