Subsidence modelling related to Squeeze Mining Phase 1a ... · SGS Subsurface Consultancy 15 Modelling results: Match with Nedmag provided squeeze volume Mismatch between modelled

Subsidence modelling related to Squeeze

Mining – Phase 1a results (v2017)

June 2017

2 SGS Subsurface Consultancy

Presentation outline

Introduction subsidence modelling Phase 1a

Input data comparison v2016 vs v2017

Subsidence modelling

Simplified model (squeeze volume in TR-1 location)

Detailed history match (squeeze volume is allocated between

cavern locations)

Impact on forecast

Conclusions


Introduction: Squeeze modelling Phase 1a

Objective

History match the historical subsidence due to Nedmag squeeze

mining for the time period 1993-2016 using the Geertsma – van

Opstal approach with variable rigid basement

– Attempt allocation of squeeze volumes to specific wells

Data available

X, Y, Z cavern locations for 13 existing wells

Nedmag-calculated cumulative squeeze volumes

– Per individual well

– Per cluster after cavern connection

Nedmag-interpreted cavern connection times

Antea-processed subsidence at benchmark points for 1993-2016 due

to salt squeeze mining (processed from the original data by

‘objectpunt’ analysis to separate subsidence from different sources)

– 115 initial subsidence benchmark points, available from 1993

– 264 additional points that were added to the network later


Introduction: Phase 1a – original and v2017

The original Phase 1a was carried out in 2016

Conclusion: Historical squeeze volumes and subsidence

benchmark data not in line with each other

Mismatch between squeeze volume and subsidence data ~ 40%

(squeeze volume > subsidence volume)

Benchmark dataset used not properly corrected for reference point

subsidence

Phase 1a was re-done in 2017 (Phase 1a v2017) with modified

input data – focus of this presentation

Nedmag revised their squeeze volume calculations

Antea / Nedmag re-processed the benchmark subsidence dataset to

include reference point subsidence


Input data Phase 1a v2017

Data available Phase 1a v2017, changes with respect to original

Phase 1a indicated:

X, Y, Z cavern locations for the existing 13 wells

Revised Nedmag-calculated cumulative squeeze volumes

– Per individual well

– Per cluster after cavern connection

Nedmag-interpreted cavern connection times

Revised, Nedmag-processed subsidence at benchmark points due to

salt squeeze mining (processed from the original data by ‘objectpunt’

analysis)

– 115 initial subsidence benchmark points, available from 1993

– 264 additional points that were added to the network later*

* Available if required, currently not in use because modelling is performed based on cumulative subsidence, while these points

represent differential subsidence. Cumulative subsidence was chosen for modelling to reduce the influence of noise / inaccuracies

in the dataset (such as points showing uplift in certain time intervals) and for numerical efficiency.


Input data: Squeeze volume calculation

The 2017 revised squeeze volumes are the result of new mass

balance calculations carried out by Nedmag incorporating the

expansive volumetric effects of:

direct dissolution of Bischofite and

thermal expansion of the underground brine

Version 2 of the Nedmag mass balance study has been

submitted to SodM on February 27th 2017


Input data – original vs v2017: Squeeze volumes

Comparison of Nedmag-provided squeeze volumes, original

(v2016) vs revised (v2017)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

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Sq

ue

eze

vo

lum

e,

mln

sm

3

Date

v2017

v2016

Squeeze volume @ March

2016 is ~ 13% smaller in v2017


Input data: Benchmark data correction

The subsidence data

were corrected by

Nedmag for AESubs

calculated subsidence of

the 12 reference

benchmarks using the

‘objectpunt analyse’

methodology


Input data – original vs v2017: Benchmark data

Comparison of benchmark

datasets, original (v2016) vs

revised (v2017) – initial points

only

Note: In v2017 dataset, corrected

benchmark data are available for

“initial” points only

v2017 vs v2016 observations:

Smaller scatter in v2017 data at

distances larger than 2000 m

from the TR-1 cavern center

Larger subsidence in v2017 data

in the flank areas

v2017

v2016

Su

bsid

en

ce fro

m b

en

ch

mark

da

ta,

in m

m

Su

bsid

en

ce fro

m b

en

ch

mark

da

ta,

in m

m


Input data – v2017: Benchmark data (I)

Two squeeze “centers” can be

recognized by visual

inspection of the cumulative

subsidence from benchmark

data on 04-2016:

TR-1 (TR-2) location

VE-1 (VE-2) location

Main subsidence is

approximately above TR-1

(TR2) location

v2017

Subsidence data at benchmark locations

interpolated with cubic interpolation

Subsidence from benchmark data

on 04-2016 (in mm)


Subsidence modelling

Subsidence modelling in order to obtain a history match for the

time period 1993-2016 was carried out in two steps (cf. original

Phase 1a in 2016):

Simplified method: To obtain rough insight into match between total

squeeze volume and subsidence bowl volume

– Assign full squeeze volume to a single location (TR-1 cavern centre)

Detailed history match

– Allocation of squeeze volumes between the various wells / clusters


Simplified model – model specifications

Input data

One point source location – “TR-1” cavern center

Fitting data

Cumulative subsidence in the “initial” benchmark point locations from

January 1999 to March 2016.

– Data between 1993 and 1999 show scatter near the center of the bowl

and was not used

Calibration parameters and constraints

Cumulative squeeze volume (Range: 0.0 – 5.6e+6 Sm3)

Rigid basement depth (Range: 1800 – 10000 m)

Calibration was performed independently for every subsidence

measurement time step

Objective function

Root mean squared error RMSE

Minimization method – Powell algorithm


Summary of optimisation runs

The best parameter set has shifted when using the v2017 data:

The larger cumulative squeeze volume is in line with the observation that the

subsidence in larger in the flank area in v2017 data set

Deeper rigid basement depth

Sum of

errors

(RMSE)

Sum of

errors

(RMSE)

Maps indicate single global minimum

Error density map, final time step v2017 Error density map, final time step v2016


Quality of history match with simplified method

Quality of the match is good

Same for v2016 and v2017

In v2017 less scatter at the distance >2000 m from the TR-1 cavern center

v2017

Benchmark subsidence

Modelled subsidence

v2016


Modelled subsidence


Modelling results: Match with Nedmag provided squeeze volume

Mismatch between modelled and Nedmag-calculated historical squeeze volume is ~10%

Results of sensitivity study on the subsidence bowl volume are in line with Nedmag conclusions (4.33

mln. sm3 vs 4.49±0.26 mln sm3)

Date v2016 v2017

Nedmag provided

squeeze volume, sm3

Modelled squeeze

volume, sm3

Modelled / Nedmag prov.

squeeze volume, %

Nedmag provided

squeeze volume, sm3

Modelled squeeze volume,

sm3*

Modelled / Nedmag prov.

squeeze volume, %

Mar-16 5686150 3668118 64.5 4925577 4331444 87.9

Feb-14 5137462 3302366 64.3 4503023 3989265 88.6

Apr-12 4670844 2948155 63.1 4095653 3639670 88.9

Jan-10 3964964 2514562 63.4 3505141 3038207 86.7

Jan-08 3505762 2180258 62.2 3122759 2848164 91.2

Jan-06 3010418 1861517 61.8 2724744 2353722 86.4

Jan-04 2510530 1556660 62.0 2297674 2156421 93.9

Jan-02 1988924 1229853 61.8 1853106 1667802 90.0

Jan-00 1538168 935058 60.8 1456876 1443921 99.1

Jan-99 1291992 765600 59.3 1237705 1194395 96.5

Nedmag conclusions for v2017 dataset * sm3 = m3 at surface conditions


Modelling results: Squeeze volumes v2016 vs v2017

Modelled

cumulative

squeeze

volume for

v2017

subsidence

data is larger

than v2016

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

2.5E+06

3.0E+06

3.5E+06

4.0E+06

4.5E+06

5.0E+06

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Ca

lcu

late

d c

um

. sq

ue

eze v

olu

me

in T

R-1

lo

ca

tio

n,

mln

sm

3

v2017

v2016

Linear (v2017)

Linear (v2016)


Detailed history match – model specifications

Input data

Multiple source points locations in cavern centers

– With time, relevant wells are connected / added into a single cluster based on Nedmag

provided cavern connection times

100% of Nedmag calculated squeeze volume

Fitting data

Cumulative subsidence at “initial” benchmark points locations from January

1993 to March 2016

Calibration parameters and constraints

Cluster production allocation fractions

Rigid basement depth (modified TNO-AGE approach)

Calibration was performed simultaneously for all subsidence

measurement time steps

Objective function

Normalized mean squared error

Minimization method – SLSQP algorithm


Summary of optimisation runs

25 simulation runs with random initial

parameters were performed to explore

an uncertainty space

Two local minima can be identified

(highlighted in the table in green and

red color)

Parameter range

c/k(0) in a narrow range 0.701 – 0.707

D(c/k) in a wide range 0.502 – 1.0

Tau in a wide range 33.3 – 78.2 year-1

# D(c/k) c/k(0) tau Error* 22 0.502 0.707 33.3 74254.7 7 0.578 0.705 39.9 74314.5

18 0.569 0.706 39.1 74322.3 3 0.566 0.706 38.8 74322.4

21 0.568 0.706 39.0 74324.1 5 0.565 0.706 38.7 74327.1 8 0.632 0.705 44.6 74378.3

19 0.636 0.705 44.8 74383.1 13 0.634 0.705 44.7 74388.0 0 0.675 0.704 48.3 74416.8

15 0.720 0.704 52.2 74445.0 12 0.826 0.703 61.3 74504.1 14 0.830 0.703 61.7 74509.6 11 0.852 0.703 63.6 74511.5 24 0.972 0.702 74.0 74559.9 2 1.000 0.701 78.2 74615.2

16 0.006 0.001 58.8 135858.7 9 0.001 0.001 56.3 135859.0

23 0.007 0.001 77.6 135859.1 20 0.008 0.001 84.1 135859.2 1 0.003 0.001 30.4 135859.4 6 0.007 0.001 69.1 135859.4 4 0.006 0.001 59.4 135859.8

10 0.003 0.001 26.0 135859.9 17 0.003 0.001 22.8 135860.2

* sum of errors for all subsidence surveys

#22

: B

est re

sult

(sm

alle

st e

rro

r)


Observed vs calculated subsidence (I)

Subsidence

simulation run #22

based on following

parameters:

c/k(0) = 0.707

D(c/k) = 0.502

tau = 33.3 year-1

“Typical” results of subsidence simulation (for runs with green colour on the previous slide)

Run#22 (=run with smallest error)

Subsidence from benchmark data

on 04-2016 (in mm) Modelled subsidence

on 04-2016 (in mm)


Observed vs calculated subsidence (II)

Some mismatch in the modelled subsidence in VE cavern center (underprediction)

and in the flank area (overprediction)

Note the difference map is affected by interpolation of the benchmark points

Run#22

Difference Benchmark – Modelled

subsidence on 04-2016 (in mm)

Benchmark subsidence (in mm)

Mo

de

lled s

ub

sid

ence (

in m

m)

Mo

de

l u

nd

ere

stim

ate

s

Mo

de

l o

ve

restim

ate

s


Observed vs calculated subsidence (III)

Difference benchmark subsidence vs modelled subsidence, interpolated with

cubic interpolation (left) and non-interpolated as bubble map (right)

Run#22



Mo

de

l u

nd

ere

stim

ate

s

Mo

de

l o

ve

restim

ate

s



underprediction

overprediction


Difference maps @ Apr 2016

Difference maps between benchmark subsidence and modelled subsidence

are very similar for all runs

Run#22 Run#2 Run#15


subsidence on 04-2016 (in mm) Difference Benchmark – Modelled





Subsidence at benchmark location: Benchmark data

vs modelled subsidence - examples

Three benchmark

points (115, 132, 136)

at different distance

from TR-1 cavern

center


Subsidence in point #115 (near TR-1)

Run#22

Modelled subsidence


incl. error bar (± 10 mm)


Subsidence in Point # 136 (mid-distance)

Run#22

Modelled subsidence




Subsidence in Point # 132 (subsidence bowl rim)

Run#22

Modelled subsidence




Benchmark vs modelled subsidence – same scale

Run#22

Point 115 Point 136 Point 132

Modelled subsidence




Squeeze volume allocation

The largest part of squeeze volume is associated

with TR-1 cavern location (>70%)

Results of the model calibration are in line with visual

inspection of subsidence data on slide#7 (bowl

center in TR-1 (TR-2) location)

Cavern

Fraction of

cumulative cluster

squeeze volume (Jan

1997 – Apr 2016), %

TR-1 71.3%

TR-2 19.7%

TR-3 3.2%

TR-5 5.8%

Well squeeze volume fraction in the total cluster squeeze volume (Run#22)

DATE Cum. production from

clusters, mln.m3

Allocation well fraction

TR-1 TR-2 TR-3 TR-4 TR-5 TR-6 TR-7 TR-8 TR-9 VE-1 VE-2 VE-3 VE-4

Feb-1995 0.000 - - - - - - - - - - - - -

Jul-1995 0.000 - - - - - - - - - - - - -

Jan-1996 0.000 - - - - - - - - - - - - -

Jan-1997 0.017 0.00 1.00 - - 0.00 - - - - - - - -

Jan-1998 0.096 0.00 0.15 - - 0.85 - - - - - - - -

Jan-1999 0.240 0.00 0.00 - 0.00 1.00 0.00 - - - - - - -

Jan-2000 0.349 1.00 0.00 0.00 0.00 0.00 0.00 0.00 - - - - - -

Jan-2002 0.660 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 - - - - -

Jan-2004 1.089 0.73 0.00 0.27 0.00 0.00 0.00 0.00 0.00 - - - - -

Jan-2006 1.496 0.67 0.33 0.00 0.00 0.00 0.00 0.00 0.00 - - - - -

Jan-2008 1.874 0.80 0.20 0.00 0.00 0.00 0.00 0.00 0.00 - - 0.00 0.00 -

Jan-2010 2.257 0.37 0.63 0.00 0.00 0.00 0.00 0.00 0.00 - - 0.00 0.00 0.00

Mar-2012 2.847 0.61 0.39 0.00 0.00 0.00 0.00 0.00 0.00 - - 0.00 0.00 0.00

Feb-2014 3.246 0.99 0.01 0.00 0.00 0.00 0.00 0.00 0.00 - - 0.00 0.00 0.00

Apr-2016 3.625 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 - - 0.00 0.00 0.00


Detailed history match vs. simplified model results comparison

Detailed history match

simulation results are in

better agreement with

benchmark data (with

few exceptions only, but

the difference is small)

Date

Cumulative error (RMSE)

Detailed history

matching (Run#22) Simplified model

Mar-16 0.1449 0.1621

Feb-14 0.1239 0.1483

Apr-12 0.1099 0.1308

Jan-10 0.0997 0.1109

Jan-08 0.0913 0.0945

Jan-06 0.0870 0.0836

Jan-04 0.0764 0.0727

Jan-02 0.0692 0.0661

Jan-00 0.0601 0.0726


Impact of parameter range on forecast (assuming

subsidence from the single cavern center)

Model calibration Forecast

Minor

difference

Larger

difference in

the forecast


Conclusions

Good quality history match was obtained for the new

squeeze volume & subsidence data set

Results of sensitivity study are in line with Nedmag

conclusions. The calculated volume of the subsidence bowl is

within Nedmag’s confidence interval

Several parameter sets providing very similar history

match quality and different rigid basement forecast

behaviours were identified

Propose to use scenarios #22 and #2 in the forecast (dark

blue lines on previous slide)

Documents

Subsidence modelling related to Squeeze Mining Phase 1a ... · SGS Subsurface Consultancy 15 Modelling results: Match with Nedmag provided squeeze volume Mismatch between modelled