Geophysical Challenges at North Sea

HPHT Wells Summit 2010, Nov 24, 2010, Aberdeen© Chevron 2010

Geophysical Challenges at the HP/HT Erskine Field, North Sea

Joerg Zaske, Caroline Pickles Chevron Upstream Europe

Stefano BagalaChevron Energy Technology Company

Presenter

Presentation Notes

Talk about a mature field. The first HPHT field developed in CNS. Acknowledgement of Co-Authors Sharing some experiences & learning’s gathered over the last years Geophysical & Geomechanical perspective


Outline

Field Background

Subsurface Challenges

Geophysics

Seismic

Geomechanics

1D Model

Full Field Model

Reservoir Monitoring

4D Seismic

Microseismics

Conclusions

2

DOC ID© Chevron 2005 3

Erskine Field: Location

Located Western side of the East Central Graben, CNS.

150 miles East of Aberdeen.

300 ft Water Depth.

50% Chevron; 32% BG and 18% BP

Presenter

Presentation Notes

Describe Location & Partnership Water Depth Seismic Surveys overlap between Erskine & Machar Fields


Erskine Field Structure

Top Erskine E70 Reservoir Structure Map 2008 Interpretation, 2007 PSDM Re-processed Seismic

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N2 kmSeveral fault blocks, formed by late Jurassic rifting.

Main Field :

Erskine & Pentland

Beta Terrace:

Erskine & Pentland

Alpha Terrace:

Heather Turbidites

Alpha Terrace

Beta Terrace

Main Field

A A’

Salt Dome

Presenter

Presentation Notes

Field consists of 3 fault blocks formed by late jurassic faulting; namely: main field, alpha and beta terrace


Exploration Wells

2STRE Northern Area

3RE - Main Field

19Y - Downthrown region adjacent to salt dome

Appraisal Wells

4 - Main Field

7 - Main Field

8 - Main Field

14 - Main Field

15 - Alpha Terrace

Producers

W1 - Main Field Pentland

W2 - Main Field Erskine


W4 - Alpha Terrace Heather


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Erskine Field: Wells

W3

Salt Dome

Presenter

Presentation Notes

Discovered in 1987; 3 Exploration wells; 5 Appraisal wells; 5 Producers placed at crest of structure; 1 well in Alpha Terrace


Seismic & Geoseismic section

Top Erskine

Top Pentland

B.C.U.

Base Chalk

A A’

B.C.U.

Chalk Group

Cromer Knoll Group

Heather Formation

Erskine Sands

Erskine Shale

Pentland Formation

Pre Jurassic

A A’

GWC

GWC

1.5km

Main Field Beta Terrace

Alpha Terrace

Presenter

Presentation Notes

Regional Seismic section and corresponding geologic correlation Approx. 4.5Km depth; 300m water depth Imaging Challenges Thick Chalk section overlaying reservoirs Significant lateral velocity variation below Base-Chalk: Wedging of the Cromer Knoll and Heather Presence of the Machar diapir in the north of the field Show Erskine and Pentland reservoirs Sands are tank like structures

DOC ID© Chevron 2005

Erskine – Some Numbers

First HPHT field developed in CNS

Discovered 1987; Field Producing since 1998

HPHT Gas Condensate; Reservoir Temp ~175 DegC;

Initial Pressure: ~14070 PSI; Today: 2760-6380 PSI Max. Drop: ~11000 PSI

Field off plateau since approx. 2004

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Erskine Historical Field Performance

Field Gas Rate Field Oil Rate Field Water Rate

Pipeline failure W2 pluback to Erskine

Export pipeline to Lomond blockage due to stuck pig (outage from 30/06/07,

Field startup on 18/08/08)

W5 shut due to downhole scale buildup

W1 back online 01/05/09 after tophole

scale removal

W5 online 26/06/09 after perforating

E50/E60 dry zones

Presenter

Presentation Notes

Erskine Pentland Heather (most depleted) Gas, Condensate, Water

DOC ID© Chevron 2005

Erskine Field – Subsurface Challenges

Significant stress redistribution

Reservoir

Overburden

Well integrity problems

Liner deformation

Sanding Problems

Scaling Issues

Drilling into depleted reservoirs

Very difficult & very expensive (>$100MM)

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Presenter

Presentation Notes

Top (Exotic Scale) & Bottom Hole Scale (CaCO3, Calcium Carbonate ?)

HPHT Wells Summit 2010, Nov 24, 2010, Aberdeen© Chevron 2010 9

Liner Deformations Can we predict failures ?

2004 2005 2006

20x magnification of lateral displacementsHere at ~15600’.

Oct 04

Nov04

2010 ?

Liner Deformation vs Depletion

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Liner Failure Strain LimitUncertainty

Data Points

Goal: Predict life of existing wells & plan for replacement wells

Feed data into DA model

Presenter

Presentation Notes

-Caliper logs taken regularly to observe deformations Here the deformations are shown at a depth of approx 15600’ Similar features have been osbserved at other HPHT fields and resulted in catastrophic well failures Prediction is very challenging – see plot on right side; (Issues are: gradient of deformation, when does it fail, measurements) Extrapolation from control points is questionable resulting in a large uncertainty Goal is to predict life of exisitng wells and include in DA for planning replacement wells


Where would we drill the next well ? How can Geophysics help ?

3D Seismic: Provide correct structure

4D Seismic: Identify unswept reserves

Geomechanics: Avoid areas with high drilling risk, predict well failures

Microseismic: Avoid drilling close to faults with subseismic resolution

Proposed Location

Existing Wellbore

W1

14

W4

W3

W2

84

W5

Presenter

Presentation Notes

- Where would the next well be placed in case of a failure of an existing well or an additional infill well ? Here is a map showing a range of infill well locations which have been analyzed - Some considerations for the new well would be: good location to maximize recovery, minimize geohazards during drilling, faults, � maximize life of new well (minimize liner deformation) How can Geophysics help? Geophysics can help by improving the subsurface understanding Seismic is used to get the depth structure; we conducted a reprocessing and looked into the option of acquiring new data Seismic can also be used as 4D to identify unswept areas Geomechanics is used to identify areas with high drilling risk and risk for well failure Microseismic monitoring can be used to identify subseismic faults


2km Thickness

Coal Surface

Machar Salt Body

Base Chalk

Cromer Knoll Wedge

BCU

Anisotropic PSDM Reprocessing

Better velocity control away from wells

Improved imaging, better event continuity and fault definition at relatively low costs.

Vint [m/s]

Seismic Reprocessing PSDM Migration Velocities


NS NErskine Main field

Two w

ay time (m

s)

Clearly improved imaging

Better interpretation product

Better event continuity, fault definition & signal/noise ratio.

Cromer Knoll wedges shown in green.

Excellent synthetic ties

Erskine Main Field

SErskine Main field

Two w

ay time (m

s)

AFTE R

2007 Anisotropic PSDM volume

Seismic Reprocessing Results 1989 vs 2007 comparisons

BEFOR E

N


EDGE Extraction Pentland horizon)

Alpha Terrace

Main Field

Good imaging of main field faults.

Poorer fault definition near the salt dome due to high degree of distortion of reflectors

Suitable as input into Reservoir modelling

Interpretation

Fault Interpretation


1D Geomechanical model: Drilling

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Erskine Mud weight window. 30 deg incl

Studied sensitivity of mud weight window as a function of well inclination

Already at 30Deg inclination mud weight window closes

Collapse Gradient in undepleted shale is higher than Fracture Gradient in reservoir

Highly challenging drilling

Undepleted Shales

Depleted Sands

Pore

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ssure

Fract

ure

Colla

pse

Min

. H

oriz.

Str

ess

Presenter

Presentation Notes

The seismic and the geological model has been the background for other studies, such as Geomechanics modeling at well location or full field. In this slide we see the results of mud weight window sensitivity as a function of well inclination for actual depletion, Erskine and Pentland reservoir. The curves shown here are the Pore Pressure Gradient (blue), the Collapse Gradient (red) the Minimum Horizontal Stress Gradient (green olive) and Fracture Gradient (grey). It is possible to see that already with a well inclination of 30 degrees, the collapse Gradient in the undepleted shale equals or is higher than Fracture Gradient in the depleted sands. This is even worse in the Pentland reservoir, where mud weight window closes in correspondence of several intra – reservoir shales. Highly deviated well drilling would prove highly challenging.


Pentland Mud weight window. 30 deg incl

Even worse in Pentland Reservoir.

Mud weight window closes in correspondence of several intra – reservoir shales.

Erskine Mud weight window. 30 deg incl

Pore

Pre

ssure

Fract

ure

Colla

pse

Min

. H

oriz.

Str

ess

1D Geomechanical model: Drilling

Presenter

Presentation Notes

The seismic and the geological model has been the background for other studies, such as Geomechanics modeling at well location or full field. In this slide we see the results of mud weight window sensitivity as a function of well inclination for actual depletion, Erskine and Pentland reservoir. The curves shown here are the Pore Pressure Gradient (blue), the Collapse Gradient (red) the Minimum Horizontal Stress Gradient (green olive) and Fracture Gradient (grey). It is possible to see that already with a well inclination of 30 degrees, the collapse Gradient in the undepleted shale equals or is higher than Fracture Gradient in the depleted sands. This is even worse in the Pentland reservoir, where mud weight window closes in correspondence of several intra – reservoir shales. Highly deviated well drilling would prove highly challenging.


1D Geomechanical Model: Sand Failure Analysis

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Sanding

Safe

Sanding

Safe

Presenter

Presentation Notes

Another application of geomechanical modeling is given by the sand failure analysis. This is shown for Erskine and Pentland reservoirs. In the chart , in X – axis there is reservoir pressure with maximum value corresponding to pre – production pressure and lower values corresponding to several depletion scenarios. In Y – axis is bottom hole flowing pressure. The green domain is the domain of production with no sand failure risk. The red domain is the domain of production that may trigger sand failure. Sanding analysis can be regarded as a very useful tool for production, since it can predict the sand failure at several stages of depletion, so it can suggest the appropriate drawdown for each production phase ( life of the field). Erskine and Pentland Reservoir are heavily depleted. Erskine : 13981 psi reservoir pressure down to 6381 (current) Pentland: 14024 psi reservoir pressure down to 4931 psi (current) It is possible to see from blue dots that represents current production conditions, that this geomechanical model predicts sand failure for both formations. Sand production is currently experienced primarily at Erskine well W3. In this case, the safe drawdowns suggested by the geomechanical are considered, but the production team use Clampon sand detectors to guide production limits. In the field: Sanding varies between different wells. What are we doing about it ? – Choke back.


1. Build full field model to investigate production induced stresses in overburden

2. Overburden grid starts top reservoir & extends up top Chalk.

3. Assume homogeneous, uniform elastic material.

4. Use different simulation scenarios to calculate reservoir compaction (see below)

5. Reservoir compaction used to calculate overburden stresses - Nucleus-of-strain equations (Geertsma)

Full Field Geomechanical – Model

20052018 & No New Well 2018 & New Well

Input Data: Total vertical compaction based on reservoir simulation scenarios.

Overburden Model

Presenter

Presentation Notes

Built a full field geomechanical model to better understand stresses observed - Use this model to compare with existing liner deformations & optimize well placement


Full Field Geomechanics Overburden Coulomb Failure Function

Even in 2005 there are large regions predicted to slip (CFF > 0)

Slip-prone region expands due to continued reservoir depletion (e.g. 2018)

Additional well production worsens slip

Highest risk: close to major faults;

Consistent with liner deformations

2005

2018 / No New Well

2018 / With New WellExtract CFF

(Above Reservoir) New well Target

Beta Terrace

Main fieldAlpha terrace

CFF0 +100-100

Presenter

Presentation Notes

Describe results at Layer above the Erskine reservoir - CFF function is shown here. He color scale is explained here. CFF lower than zero means stable surface, CFF higher than zero means slip occur along the surface (for example bedding plane surface or a fault). CFF is a RELATIVE measurement of slip, not ABSOLUTE. So, different positive values of CFF mean that surfaces with higher (positive) CFF are more prone to slip and red areas are areas of highest risk. - Considering the depletion effect, the increase of CFF with depletion means that the risk of slip increases with the depletion. This can be seen here from 2005 to 2018. Results show highest stresses along the crest where the producers are located and not surprisingly close to the key bounding faults - Slipping risk increases in the area of the infill well location due to additional production� - Results may be used to optimize potential location any new well Note: Slip is driven by the development of shear stress (due to overburden bending) as well as � the reduction in normal stress (due to overburden stretching) BACKGROUND: The vertical compaction strain in each reservoir cell has been performed by the Erskine Team, from dynamic simulation. Then this has been the input for the calculation of compaction induced strains and stress changes in the overburden from Russ & Robert. But yes Russ and Robert, with the Nucleus of Strain theory, investigated only compaction in the overburden. The model in the overburden is assumed homogeneous, it is like the overburden would be constituted by a unique rock type. What is changing point by point in the GoCad grid is the state of stress. The medium is modelled as linear elastic, so cells interactions are linear elastic. The Coulomb Failure Function (CFF) is a measure of the slip tendency (or risk) of a potential failure surface (for example bedding plane surface or a fault). CFF has units of bars and it is expressed by: CFF = Tau – mu* (Sigma N) where CFF = Coulomb Failure Function Tau = maximum shear stress along the surface of potential failure (for example bedding plane surface or a fault) mu = coefficient of sliding friction, taken equal to 0.4, appropriate value for slip in Chalk and shale. Sigma N = Normal Stress applied to the surface of potential failure (for example bedding plane surface or a fault) CFF lower than zero means stable surface, CFF higher than zero means slip occurs along the surface (for example bedding plane surface or a fault). CFF is a RELATIVE measurement of slip, not ABSOLUTE. So, different positive values of CFF mean that surfaces with higher (positive) CFF are more prone to slip. Considering the depletion effect, the increase of CFF with depletion means that the risk of slip increases with the depletion. The infill (replacement) well would worsen the slip in certain zones. The Slip is driven by the development of shear stress (due to overburden bending) as well as the reduction in normal stress (due to overburden stretching).


Reservoir Monitoring

4D Seismic

Timelapse 3D Surveys

Observe amplitude and travel time changes

Draw conclusions from changes

Microseismic

Deploy Geophones in Boreholes or Ocean-Bottom

Record “passive” Seismicity

High Potential at HPHT due to Production related stress re-distribution

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4D Seismic Monitoring (It works) Erskine 1989 vs Machar4D 2001

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4D seismic response detected (after 3yrs production)

4D Time Effect observed

Shows connection of Beta Terrace

Would need new 4D survey to detect later production effects

Value considered small due to late stage in field life

Beta Terrace clearly Connected to Main Field

Main Field

Triassic Surface

Tim

e Shifts

[m

s]

Presenter

Presentation Notes

4D analysis was performed based on two seismic data sets available over the Erskine field. - The Erskine 1989 survey was the baseline which covers also the neighbouring Machar field operated by BP. - A 4D monitor survey was shot in 2001 over the Machar field operated by BP. This survey also extends over the Erskine area and has been used to identify any production effects early in field life – after approx. 3 yrs of production. - Significant time shifts have been observed at the Triassic level indicating production effects and clearly showing a connection between the Beta Terrace and Main Field area. - While this result shows that 4D would work in principal it is only a snapshot early in field life and a new 4D seismic would need to be acquired to observe any production effects since 2001. This would be very difficult to justify economically.


Microseismic Monitoring

Conducted Feasibility Study

Motivation

Identify 3D distribution of stress changes

Detect re-activation of major faults (seismic resolution)

Detect compaction and movements (sub-seismic faults)

Impact

Constrain Geomechanical model

Help predict well failures

Reduce drilling risks

Optimize well placement

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Microseismic Monitoring Feasibility Study

Workflow

Modelled microseismic source events

Used different layouts of Geophones

Ocean-Bottom Nodes & Borehole deployed sensors

Investigated detectability of events

Results:

Seismic signal very likely detectable.

Significantly better sensitivity for borehole deployed sensors

Deployment in production wells would be very challenging.

Comments

Value too low at stage of field life

May have been very useful early in field life, i.e. deployment in appraisal or exploration wells

Focal Mechanism based on Geomechanical Modelling

Presenter

Presentation Notes

Conducted a microseismic feasibility study for Erskine Modelled microseismic source events based on the Geomechanical Modelling. Here you can see the Hypothetical Microseismic Evenets placed at the key fault locations of the field. The focal mechanism is based on Geomechanical Modelling and the Wavefield was then calculated. Different recording geometries have been modelled: borehole deployed in one or more boreholes, and also ocean bottom deployed, i.e. Some nodes on the seabed which could record a few months before beeing picked up again. The goal was to investigate detectability of the microseismic seismic events. RESULTS: Results showed that successful seismic signal detection would be very likely. However, the sensitivity of the geophones would be much higher than at the sea floor (factor 100). DEPLOYMET: The deployment of sensors in existing wells would be very challenging, especially considering the well integrity problems we have. Deployment in appraisal or exploration wells would have been ideal if it had been done early in field life. HYPOTHETICAL MICROSEISMIC EVENT FOCAL MECHANISMS BASED ON THE PREDICTED FAILURE MECHANISMS FROM THE GEOMECHANICAL ANALYSIS. BLUE BEACHBALLS ARE NORMAL FAULTING MECHANISMS, WHERE THE WHITE QUADRANTS ARE DILATIONAL FIRST MOTION. THE RED MECHANISMS ARE REVERSE FAULTING MECHANISMS.


ConclusionsGeophysical & Geomechanical work lead to improved understanding of subsurface

3D Reprocessing improved seismic data quality significantly.

Geomechanical modelling improved understanding of stress regime and highlights zones of increased risk for well failure. Can be used to mitigate risk and optimize well placement.

Reservoir monitoring techniques such as Seismic 4D and Microseismic monitoring applicable to Erskine and probably other HP/HT fields

Maturity of field makes it very difficult to justify any new seismic or other surveillance methods

Recommend to consider Seismic 4D & Microseismic Monitoring techniques early on in field life for other HPHT fields.

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Presenter

Presentation Notes

Geophysical & Geomechanical work lead to improved understanding of subsurface; 3D Reprocessing improved seismic data quality significantly. This was a very cost effective way to improve data quality. Geomechanical modelling improved understanding of stress regime and highlights zones of increased risk for well failure. Can be used to mitigate risk and optimize well placement. Reservoir monitoring techniques such as Seismic 4D and Microseismic monitoring applicable to Erskine and probably other HP/HT fields Maturity of field makes it very difficult to justify any new seismic or other surveillance methods Recommend to consider Seismic 4D & Microseismic Monitoring techniques early on in field life for other HPHT fields.


Acknowledgements

We wish to thank our management and co- venturers BG and BP for permission to present this work.

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Geophysical Challenges at North Sea

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Transcript of Geophysical Challenges at North Sea